Deposit-feeding in benthic macrofauna: Tracer studies from the Baltic Sea

Deposit-feeding in benthic macrofauna:

Tracer studies from the Baltic Sea

Lars Byrén

Department of Systems Ecology Stockholm University SE – 106 91 Stockholm

Sweden

Stockholm 2004

Doctoral Dissertation 2004

Lars Byrén

Department of Systems Ecology Stockholm University

SE – 109 91 Stockholm Sweden

byren@system.ecology.su.se

© 2004 Lars Byrén ISBN 91-7265-824-X

Printed by DocuSys Cervice Center in Stockholm

Original cover photo by Gunilla Ejdung and reworked by Per Westergård

Abstract

A low content of organic matter, which is largely refractory in nature, is characteristic of most sediments, meaning that aquatic deposit-feeders live on a very poor food source. The food is derived mainly from sedimenting phytodetritus, and in temperate waters like the Baltic Sea, from seasonal phytoplankton blooms. Deposit-feeders are either bulk-feeders, or selective feeders, which preferentially ingest the more organic-rich particles in the sediment, including phytodetritus, microbes and meiofauna.

The soft-bottom benthos of the Baltic Sea has low species biodiversity and is dominated by a few macrobenthic species, among which the most numerous are the two deposit-feeding amphipods Monoporeia affinis and Pontoporeia femorata, and the bivalve Macoma balthica.

This thesis is based on laboratory experiments on the feeding of these three species, and on the priapulid Halicryptus spinulosus.

Feeding by benthic animals is often difficult to observe, but can be effectively studied by the use of tracers. Here we used the radioactive isotope

C to label food items and to trace the organic matter uptake in the animals, while the stable isotopes

C and

N were used to follow feeding on aged organic matter in the sediment.

The abundance of M. balthica and the amphipods tends to be negatively correlated, i.e., fewer bivalves are found at sites with dense populations of amphipods, with the known explanation that newly settled M. balthica spat are killed by the amphipods. Whether the postlarvae are just accidentally killed, or also ingested after being killed was tested by labelling the postlarvae with

C and Rhodamine B. Both tracer techniques gave similar evidence for predation on and ingestion of postlarval bivalves. We calculated that this predation was likely to supply less than one percent of the daily carbon requirement for M.

affinis, but might nevertheless be an important factor limiting recruitment of M. balthica.

The two amphipods M. affinis and P. femorata are partly vertically segregated in the sediment, but whether they also feed at different depths was unknown. By adding fresh

C- labelled algae either on the sediment surface or mixed into the sediment, we were able to distinguish surface from subsurface feeding. We found M. affinis and P. femorata to be surface and subsurface deposit-feeders, respectively.

Whether the amphipods also feed on old organic matter, was studied by adding fresh

14

C-labelled algae on the sediment surface, and using aged, one-year-old

C- and

N-labelled sediment as deep sediment. Ingestion of old organic matter, traced by the stable isotopes, differed between the two species, with a higher uptake for P. femorata, suggesting that P.

femorata utilises the older, deeper-buried organic matter to a greater extent.

Feeding studies with juveniles of both M. affinis and P. femorata had not been done previously. In an experiment with the same procedure and treatments as for the adults, juveniles of both amphipod species were found to have similar feeding strategies. They fed on both fresh and old sediment, with no partitioning of food resources, making them likely to be competitors for the same food resource.

Oxygen deficiency has become more wide-spread in the Baltic Sea proper in the last half-century, and upwards of 70 000km

are now devoid of macrofauna, even though part of that area does not have oxygen concentrations low enough to directly kill the macrofauna. We made week-long experiments on the rate of feeding on

C-labelled diatoms spread on the sediment surface in different oxygen concentrations for both the amphipod species, M.

balthica and H. spinulosus. The amphipods were the most sensitive to oxygen deficiency and

showed reduced feeding and lower survival at low oxygen concentrations. M. balthica

showed reduced feeding at the lowest oxygen concentration, but no mortality increase. The

survival of H. spinulosus was unaffected, but it did not feed, showing that it is not a surface

deposit-feeder. We conclude that low oxygen concentrations that are not directly lethal, but

reduce food intake, may lead to starvation and death in the longer term.

Table of contents

List of papers ...5

Introduction

6

The soft bottom benthic environment...6

Deposit-feeders and their food sources...6

Oxygen depletion and benthic community response...7

Isotopes as tracers in studies of feeding...8

Aims of the thesis...9

Study area ...9

The study organisms...10

The amphipods Monoporeia affinis and Pontoporeia femorata...10

The bivalve Macoma balthica...11

The priapulid Halicryptus spinulosus...11

Experimental methods...12

Results and discussion...13

Predation and ingestion of newly settled Macoma balthica by the amphipod Monoporeia affinis (I)...13

Differences in the rate and depth of feeding between the amphipods Monoporeia affinis and Pontoporeia femorata (II)...14

Differences in use of new and old organic matter in the sediment by the amphipods Monoporeia affinis and Pontoporeia femorata (III) ...15

Differences in feeding between juveniles of Monoporeia affinis and Pontoporeia femorata (III)...15

Effects of oxygen deficiency on feeding (IV)...16

Final remarks...16

References...18

Tack och erkännanden...24

List of papers

I Ejdung G., Byrén L., Elmgren R. 2000. Benthic predator-prey interactions: evidence that adult Monoporeia affinis (Amphipoda) eat postlarval Macoma balthica (Bivalvia).

J Exp Mar Biol Ecol 253:243-251.

II Byrén L., Ejdung G., Elmgren R. 2002. Comparing rate and depth of feeding in benthic deposit-feeders: a test on two amphipods, Monoporeia affinis (Lindström) and Pontoporeia femorata Kröyer. J Exp Mar Biol Ecol 281:109-121.

III Byrén L., Ejdung G., Elmgren R. The use of sedimentary organic matter by two deposit-feeding amphipods, studied using three isotopic tracers. Manuscript.

IV Ejdung G., Byrén L., Eriksson Wiklund A.-K., Sundelin B. Effects of hypoxia on the feeding of macrobenthos. Submitted manuscript.

Papers I and II are reproduced with permission from the publisher Elsevier

Introduction

This thesis is based on four papers and focuses on different aspects of deposit-feeding (feeding on sediments), studied using isotopes of carbon and nitrogen as tracers. The studied aspects include deposit-feeders as predators (Paper I), feeding depth including food quality and competition for food (Paper II and III) and the effect of oxygen depletion on feeding (Paper IV). Common to all studies is the use of tracers, mainly

C (all papers), but also

C and

N (Paper III).

The soft bottom benthic environment

Soft bottoms cover the main portion of the ocean floor (Sverdrup et al., 1942). Common to most deposit-feeding organisms living on or in soft bottoms is the usually low nutritional quality of their food and a pulsed supply of higher-quality food. Soft bottoms below the photic zone are almost entirely dependent on imported organic matter, while those in the photic zone also have an internal source through benthic primary production.

In temperate waters, deeper benthic communities depend mostly on phytodetritus derived from seasonal phytoplankton blooms (Elmgren, 1978; Levinton and Bianchi, 1981;

Nixon, 1981; Bianchi and Levinton, 1984; Graf, 1987; Fitzgerald and Gardner, 1993;

Goedkoop and Johnson, 1996). When these blooms settle to the bottom, the mineralization process often responds rapidly (Graf, 1987; Goedkoop et al., 1997). This mineralization is carried out by benthic micro-organisms as well as by meio- and macrofauna and regenerates nutrients into inorganic forms, which become available for biological primary production after transport to the photic zone (Nixon, 1981; Hylleberg and Riis-Vestergaard, 1984;

Blackburn, 1988; Jensen et al., 1995). Due to bioturbation, i.e. the reworking of sediment by deposit-feeders, some of the settling material is buried deeper in the sediment, where the utilisation rate is slower. As shown by Levin et al. (1997) the subduction of utilisable organic matter into the sediment can be both deep and rapid. Surface deposit-feeders can exploit fresh phytodetritus as soon as it has settled on the sediment surface (Aljetlawi et al., 2000; van de Bund et al., 2001). In contrast, sub-surface deposit-feeders generally ingest material that has been buried in the sediment for some time and thus has been exposed to a greater degree of microbial activity (Tenore et al., 1982). Detritus found deeper in the sediment is therefore often considered to be old, nutrient-poor and more refractory. Bioturbation may, however, make fresh material available also to sub-surface deposit-feeding infauna.

In addition to providing food for benthic organisms, abundant settling phytodetritus may during its decomposition deplete the oxygen of the bottom water, thus harming or killing benthic animals (Diaz et al., 1995; Johansson, 1997).

Deposit-feeders and their food sources

Like all heterotrophic organisms, deposit-feeders require food to supply the chemical

building blocks for their bodies and energy used in metabolism. The origin of their

sedimentary food is varied, including riverine and sewage input, drifting macroalgal debris, and direct sedimentation of phytoplankton and seston produced in the overlying water column. The pool of non-living organic matter in the sediment is thus derived from a variety of sources, of different nutritional value. Its chemical composition is continually modified by physical, chemical and biological processes, with deposit-feeders affecting it by feeding, defecation and burrowing. Deposit-feeders use sedimentary food either non-selectively, as bulk feeders, which indiscriminately ingest large volumes of sediment, or selectively, by ingesting selected particularly food-rich particles from the sediment (Lopez and Levinton, 1987). Most of a settling phytoplankton bloom is directly digestible, while some is more refractory, as is much macrophyte material (Twilley et al., 1986). Microbial decomposition can make refractory organic matter available to deposit-feeders (Lopez and Levinton, 1987).

Thus faecal matter produced by deposit-feeders is often recolonized by microbes (Tenore, 1988) and then reingested (Newell, 1965; Frankenberg and Smith, 1967; Taghon et al., 1984).

Since most sediment consists mainly of mineral grains, with typically only a low percentage of organic matter, it is a very poor food source. Despite living on this poor substrate, deposit-feeders can survive and often even grow rapidly, and dominate the benthos of freshwater and marine sediment. To accomplish this, many deposit-feeders process massive volumes of sediment by bulk feeding, assimilating some of the organic matter, while all the inorganic particles end up in faecal pellets (Lopez and Levinton, 1987).

There is no doubt that bacteria and micro-organisms are generally well absorbed in the gut of deposit-feeders, but they are generally not abundant enough in sediments to provide a large fraction of the assimilated organic matter, most of which must come from non-living organic matter in the sediment Lopez and Levinton 1987). Bacteria lack polyunsaturated fatty acids (components of lipids that serve as storage compounds), and certain sterols (essential to growth in crustaceans and in many bivalves) (Phillips, 1984), thus deposit-feeders probably cannot survive on a diet consisting of bacteria only. However, it is most likely that bacteria are an important source of some B-complex vitamins and certain amino acids. The importance of diatoms as a source of polyunsaturated fatty acids is discussed by Phillips (1984), and has been shown for both marine (Lopez and Levinton, 1987) and fresh water environments (Johnson and Wiederholm, 1992). According to Lopez and Levinton (1987), most deposit- feeders seem to require both detritus and microbes in their diet. Deposit-feeders can also expand their nutrient intake by feeding on animal tissue, which probably is the most nutritious food for marine invertebrates (Phillips, 1984).

Oxygen depletion and benthic community response

Oxygen deficiency can develop naturally when vertical stratification of the water

column effectively inhibits exchange with the water surface and the photic zone. Vertical

stratification in combination with oxygen-consuming decomposition of organic material can

lead to hypoxia or anoxia. Anoxia is defined as the complete absence of oxygen, while

hypoxia is generally used as a term for conditions of low dissolved oxygen. According to Diaz (2001), oxygen values below 2 mg O

l

affect most aquatic animals negatively, but some have adapted to extremely low oxygen concentrations (Levin, 2002). Hypoxic and anoxic environments have existed throughout geological time, but in recent decades oxygen- deficient areas have expanded throughout the world in shallow coastal and estuarine areas, as a consequence of anthropogenic eutrophication (Diaz and Rosenberg, 1995; Diaz, 2001; Gray et al., 2002; Karlson et al., 2002). Excessive nutrient loadings enhance primary production, and the resulting increase in oxygen demand creates areas of hypoxia and anoxia, which may persist for shorter or longer periods.

The response of the benthic community to oxygen depletion involves structural changes in species diversity, abundance and biomass (Diaz and Rosenberg, 1995; Karlson et al., 2002). In severe cases animals will die (Diaz and Rosenberg, 1995; Diaz, 2001), to an extent dependent on both species- and life-stage-specific ability to escape from affected areas and tolerance to low oxygen levels (Wang and Widdows, 1991; Gray et al., 2002).

Isotopes as tracers in studies of feeding

Radioactive isotopes. Natural elements exist as both stable and unstable (radioactive) isotopes, the nucleus of which have the same number of protons, but differ in the number of neutrons. To study feeding selectivity, assimilation and overall mineralization of different elements, isotope tracer techniques are very useful. The radioactive carbon isotope

C is commonly used in biological and ecological research to study feeding mechanisms (Lopez and Crenshew, 1982; Lopez et al., 1989) and community responses to settling organic matter (Andersen, 1996; Goedkoop et al., 1997). This isotope is particularly useful, since it is quite safe (low specific activity), and allows labelling of specific positions in organic compounds.

In addition, the half-life of

C is long (5570 years), making it suitable as an experimental tracer, since time corrections will be negligible. The specific activity of a radioisotope defines its radioactivity related to the amount of material, usually expressed as Bq/mol, Ci/mmol or dpm/µmol.

Stable isotopes. Most elements of biological interest have two or more stable isotopes, although one isotope, generally the lighter one, is often present in far greater abundance, as is the case with carbon (

C and

C) and nitrogen (

N and

N). The isotopic composition is expressed as the deviation in ‰ from a reference standard, which for nitrogen is atmospheric nitrogen gas (N

), and for carbon a limestone, Vienna Peedee belemnite (VPDB). The standard equation for determining the isotope ratio is:

δ R (‰) = (R

/R

– 1) x 10

where δ R is the deviation in ‰ in the ratio (R) between the heavy and light isotopes (

C /

C

or

N/

N) relative to the standard (Owens, 1987).

The use of stable isotopes, e.g. δ

C and δ

N, is well established in biological research, with natural variations in isotopic ratios being used for food web studies (Owens, 1987; Rolff and Elmgren, 2000; Guiguer and Barton, 2002), and manipulated ratios for experimental studies (Levin et al., 1999, Paper III), including nutrition studies (Aoki et al., 1995; Kreeger et al., 1996; Preston et al., 1996).

Aims of the Thesis

The overall aim of this thesis was to improve understanding of deposit-feeding in soft- bottom invertebrates. The main goals were to:

• Test whether the amphipod Monoporeia affinis really ingests, or just accidentally kills, newly settled postlarvae of the bivalve Macoma balthica (Paper I)

• Test if there are differences in the rate and depth of feeding between the amphipods Monoporeia affinis and Pontoporeia femorata (Paper II)

• Test if aged phytodetritus buried below the surface layer of the sediment is really assimilated by the amphipods, and whether the two amphipod species differ in the use of old and new organic matter in the sediment (Paper III)

• Test whether there are differences in feeding between juveniles of the two amphipods (Paper III)

• Study the effect of oxygen deficiency on the feeding rate of deposit-feeding benthic macrofauna (Paper IV)

Study area

The Baltic Sea is one of the largest brackish water areas in the world (373 000 km

). As such, it has very low species diversity, mainly due to the low salinity, but also due to the short evolutionary time, some 7000 years, since its last freshwater phase. The Baltic Sea has a strong salinity gradient, from 15-20 in the Danish sounds down to 2-3 in the Bothnian Bay at the northern end, making salinity a barrier for a wider distribution in many species (Elmgren and Hill, 1997). The northernmost part of the Baltic Sea is thus, for natural reasons, inhabited by species of fresh water origin. Conversely, the southern part has more marine species. A halocline is found at depths around 60-80 m in the Baltic proper, under which oxygen is normally depleted, in the deepest areas even causing anoxia (Laine et al., 1997).

In the Baltic Sea, benthic deposit-feeders are usually food-limited for most of the year

(Cederwall, 1977; Elmgren, 1978; Sarvala, 1986; Uitto and Sarvala, 1991; Lehtonen and

Andersin, 1998). However, after the spring phytoplankton bloom has settled to the bottom,

there is often a temporary food surplus, creating the main growth period of the year (Cederwall, 1977).

All the experiments in this thesis were performed at the Department of Systems Ecology of Stockholm University, the Askö Field Station (58º49’N, 17º38’E) or the Studsvik Laboratory, Institute of Applied Environmental Research of Stockholm University. Animals and sediment were collected near the Askö Field Station.

The study organisms

The amphipods Monoporeia affinis and Pontoporeia femorata

Over large areas of the Baltic Sea, the benthic amphipods, Monoporeia affinis (Lindström) (syn. Pontoporeia affinis, see Bousfield, 1989) and Pontoporeia femorata Kröyer (Fig 1), are two of the most abundant species on deeper soft-bottoms (Ankar and Elmgren, 1976; Ankar, 1977; Elmgren, 1978; Laine et al., 1997), where they commonly co-occur below 30 m. M. affinis is a glacial relict of fresh water origin, while P. femorata has a marine arctic origin (Segerstråle, 1950). Due to its salinity requirement, P. femorata is not regularly found north of the southern Bothnian Sea, while M. affinis is found from the southern Baltic Sea to the Bothnian Bay and in deep lakes below the highest postglacial shore line. M. affinis is more active than P. femorata, with a higher respiration rate (Cederwall, 1979) and higher fecundity (Cederwall, 1977). The two amphipods are normally found in the upper 5 cm of the sediment, with M. affinis closest to the sediment surface (Ankar, 1977; Hill and Elmgren, 1987), and juveniles of both species are found even closer to the sediment surface than the adults

Figure 1. Monoporeia affinis (left) and Pontoporeia femorata (right).

(Hill, 1984). Although P. femorata is found deeper in the sediment than M. affinis, Lopez and Elmgren (1989) found no evidence of spatial food partitioning between the two species when sympatric, since both seemed to be surface deposit-feeders.

Both amphipods swim actively at night and stay buried during daytime (Cederwall,

1979; Lindström and Lindström, 1980). The mating period is during October to December,

and the juveniles are released in March or April, around the time the spring bloom settles. The

male dies shortly after mating and the female after releasing the young from its brood pouch.

The food source of the two amphipod species studied here has been considered to be mostly settled phytoplankton and detrital organic matter, but bacteria, meiofauna and temporary meiofauna are also included in the diet (Elmgren, 1978; Elmgren et al., 1986;

Goedkoop and Johnson, 1994; Lehtonen, 1996; Ejdung and Elmgren, 1998).

The bivalve Macoma balthica

Macoma balthica (L.) (Fig 2) is a small (up to 30 mm) tellinid bivalve found in coastal waters in north-temperate regions, including Europe and North America. This species is euryhaline and is found both in fully marine and in brackish water conditions, down to a salinity of about 3 in the southern Bothnian Bay (Haahtela, 1974). Depending on food availability, M. balthica switches feeding mode between suspension- and deposit-feeding (Ólafsson, 1986). M. balthica is vertically distributed from very shallow water to at least 60 m depth in the open Baltic Sea (Leppäkoski, 1969). In the Baltic Sea, M. balthica often dominates the biomass and sometimes also the abundance of macrobenthic fauna (Ankar and Elmgren, 1976; Laine et al., 1997).

Figure 2. Macoma balthica

M. balthica spawns in late spring in the Baltic Sea and its planktotrophic larvae settle after six to eight weeks, at a length of about 250 µm (Ankar, 1980; Ólafsson and Elmgren, 1997). Small individuals (< 2 mm) using their foot for deposit-feeding, while larger individuals develop siphons, with which they can also suspension-feed (Caddy, 1969). M.

balthica is food for fish, diving ducks, e.g. eider (Somateria mollissima), and the large benthic isopod Saduria entomon (L.) (Ejdung and Bonsdorff, 1992).

The priapulid Halicryptus spinulosus

The priapulid Halicryptus spinulosus (von Siebold) (Fig 3) is one of only about 15

known species in the phylum Priapulida (Meglitsch and Schram, 1991). It occurs in sediments

in the boreal and arctic waters of the Northern Hemisphere, where it burrows to a depth of

approximately 30 cm in the sediment (Powilleit et al., 1994).

Figure 3. Halicryptus spinulosus

H. spinulosus has been described variously as a subsurface deposit-feeder (Arntz, 1978), as a predator on other infauna (Ankar and Sigvaldadottir, 1981), and as an omnivore (Aarnio et al., 1998). In gut analyses (Aarnio et al., 1998), 68% of the stomach content was characterized as detritus, and 24% as algal remains. The remaining 8% consisted of crustaceans, polychaetes, oligochaetes, nematodes and chironomid larvae, but whether these were alive or already dead when ingested could not be determined from the gut analyses.

Experimental methods

This thesis is based entirely on feeding studies by means of isotope tracers.

C-labelled diatoms were used to label Macoma balthica spat, in order to identify bivalve ingestion by Monoporeia affinis (Paper I). In addition, we used Rhodamine-B stain for fluorescent labelling of M. balthica. Living M. balthica spat were then offered to M. affinis, and a resulting

C signal, or strong orange fluorescence, in M. affinis provided evidence of prey ingestion. In Paper II

C-labelled diatoms, were used as food and distributed on the sediment surface or mixed into the sediment, making it possible to separate surface feeding from sub- surface feeding. In Paper III a multiple tracer approach was used, in order to separate feeding on high quality surface organic matter (

C-labelled diatoms) from low quality subsurface organic matter (

C- and

N-labelled diatoms, aged for 1 year). Finally,

C-labelled diatoms were used to study the effect of oxygen deficiency on deposit-feeding by benthic macrofauna (Paper IV).

When culturing the algae, artificial seawater was used in order to control total carbon content in the water. Replacing a part (25%) of NaHCO

with NaH

CO

in artificial seawater will enhance the labelling efficiency compared to adding NaH

CO

to natural seawater. In the procedure for labelling with

C and

N, artificial seawater was used for the same reason, with all of the NaHCO

and NaNO

replaced by NaH

CO

(98%) and Na

NO

(99.6%), respectively, in order to get the strongest possible δ-signals.

Individual variability between the amphipods in

C-uptake (all papers) and in

C-/

N-

uptake (Paper III) has been very high (see Fig 4 for example). This high variability may be

due to an uneven addition or to later redistribution of

C-labelled algae, and may also reflect

differences between individuals, the latter emphasised by Taghon and Jumars (1984). An uneven addition of algae on the sediment surface will create local spots of high activity.

Animal movements may also resuspend labelled surface sediment, which will then preferentially settle into holes dug by the amphipods. Animals that feed in such spots can become very radioactive.

Monoporeia affinis

10 100 1000 10000 100000

0 0.2 0.4 0.6 0.8 1 1.2

Weight in mg

DPM mg-1

Figure 4. Variability in DPM (disintegrations per minute) between individuals of Monoporeia affinis fed ¹⁴C- labelled diatoms (Skeletonema costatum) added on top of the sediment surface for 20 days; data from Byrén and Cederwall (unpublished). n = 128. Highest value is over 45100 DPM and lowest is 126 DPM, lower by a factor of 359. Mean = 3800 ± 6100 DPM (± SD). Note log scale on the y-axis.. Dashed line indicates the background level of 56 ± 3 DPM (± SD).

It is also possible that reduced feeding during moulting has reduced label uptake in some individuals. Lower feeding during pre- and post-moult has been shown for the common shrimp Crangon crangon (Oh et al., 2001). In more long-term experiments the effect of moulting should be reduced, but the studies in this thesis lasted between 7 and 32 days. We have tried to counter the problem of highly variable individual uptake of label by using many replicates with several individuals per replicate.

Results and discussion

Predation and ingestion of newly settled Macoma balthica by the amphipod Monoporeia

Mortality is usually very high in the younger stages of benthic invertebrates, and crucial

for recruitment success (Thorson, 1966; Bachelet, 1990). For species with planktonic larval

stages, the most critical benthic stage is the postlarval, immediately after settling. During this time, both the postlarvae and the predators occupy the surface layer of the sediment, resulting in a high abundance of available prey for the predator, and high early juvenile mortality (Thorson, 1966; Gosselin and Qian, 1997).

Field observations from deeper areas in the Baltic Sea have shown that Macoma balthica is often scarce or absent at sites with dense populations of the amphipods Monoporeia affinis and/or Pontoporeia femorata (Segerstråle, 1962). Earlier studies by Elmgren et al. (1986) and Ejdung and Elmgren (1998) had shown that both amphipods M.

affinis and P. femorata were able to kill M. balthica spat, but had provided no evidence for subsequent ingestion. To test whether the amphipods ate the crushed bivalves, we offered

C- labelled and Rhodamine-B-stained postlarvae to M. affinis, and demonstrated that some of the postlarvae had been ingested. During their settling period, M. balthica spat are locally very abundant (Ankar, 1980; Ólafsson and Elmgren, 1997), offering a potential addition to the normal diet. Crushing the shells will cost the amphipods some energy, but the reward in high quality food may be worthwhile. As proposed by Phillips (1984), animal tissue is likely to contain all the amino acids and other essential nutrients required by marine invertebrates. If the animals are non-selective bulk feeders, the carbon content in one unit M. balthica is about 50 times greater than in a corresponding dry weight unit of sediment (for estimations, see discussion in Paper I). However, we found that the feeding rate was low; on average each amphipod consumed one M. balthica every fourth day, probably covering less than one percent of its daily energy requirement (Elmgren et al., 1986).

Under these experimental conditions, M. balthica was not a major food source for M.

affinis, but such predation may nevertheless drastically reduce the recruitment of M. balthica.

The methods of labelling postlarvae with

C and Rhodamine B proved useful for demonstrating ingestion of postlarval soft tissue by the predator.

Differences in the rate and depth of feeding between the amphipods Monoporeia affinis and Pontoporeia femorata (II)

The two amphipods Monoporeia affinis and Pontoporeia femorata are found at different

depths in the sediment (Hill and Elmgren, 1987), and it seemed likely that they also feed at

different depths as suggested by Ankar (1977), and hence on partly different food sources. A

short-term study by Lopez and Elmgren (1989) could not confirm the existence of a

difference in feeding depth between the two species. Later, van de Bund et al. (2001) found

much lower

C-uptake in P. femorata than in M. affinis, when fed labelled algae on the

sediment surface, indicating that P. femorata was mainly feeding elsewhere. We made

experiments where

C-labelled algae were added either on top of the sediment (indicating

surface feeding), or mixed down into the sediment (indicating sub-surface feeding). The

amphipods were studied both in single- and mixed-species treatments. The results showed a

clear difference in feeding depth between the two species, with P. femorata predominantly a

sub-surface deposit-feeder, and M. affinis showing a wider vertical feeding range, but concentrating mostly on surface material (Fig. 1 and 2 in Paper II).

Differences in use of new and old organic matter in the sediment by the amphipods

The most obvious advantage conferred by feeding deeper in the sediment should be a lower risk of detection by predators active on the sediment surface. There should be a trade- off between predator avoidance and the lower food quality of deep sediment. If the sediment food quality allows, it may be a good strategy for a species with low fecundity, like P.

femorata, to feed deeper in the sediment, where it is less vulnerable to surface-active predators (Hill and Elmgren, 1992).

In Paper III we used three different tracers,

C,

C and

N, to test the hypothesis that Pontoporeia femorata relies on older organic matter in the sediment, as suggested in Paper II.

We added

C- and

N-labelled diatoms to sediment, mixed thoroughly, and aged it for one year. The sediment thus contained old, stable-isotope-labelled organic matter and was used as deep sediment, overlaid by a thin layer of unlabelled fresh sediment, on top of which fresh

14

C-labelled diatoms were added. As indicated by its higher C/N-ratio, the old sediment should be of lower food quality than the fresh algae added on top of the sediment.

Similar treatments were used as in Paper II, i.e. both species either single or mixed.

Monoporeia. affinis took up 5 times as much

C as Pontoporeia femorata, but significantly less

C and

N. In Paper II, where the added deep organic matter was fresh material, M.

affinis fed about as much on deep sediment as P. femorata, but in this study, where the deeper sediment had been aged for a year, M. affinis fed significantly less on deep sediment. We therefore suggest that subsurface feeding by M. affinis is regulated both by food quality and by depth in the sediment.

Despite the lower food quality of the old sediment, a mixing model for carbon estimated that the old sediment contributed about 84% of new body carbon in Pontoporeia femorata and about 45% for Monoporeia affinis.

Differences in feeding between juveniles of Monoporeia affinis and Pontoporeia femorata (III)

The adults of Monoporeia affinis and Pontoporeia femorata showed different feeding

rates and fed at different depths in the sediment. We made an experiment with juveniles of

both species to test if their feeding also differed in a similar way (Paper III). The same

treatments as in the adult experiment, i.e., single and mixed with

C-labelled algae on top of

the sediment and

C-/

N-labelled old sediment, was used. In contrast to the adults, juveniles

of both species fed on both surface and subsurface sediment, i.e., more like generalists,

suggesting that interspecific food competition should be stronger for juveniles than adults.

Both species fed equally on the old sediment, and the only difference in uptake was in surface feeding, where single-species treatments had higher

C-uptake than mixed treatments, again suggesting feeding interference, or food competition at higher density. The overall

C-uptake was several times that of the adults, for M. affinis c. 5 and P. femorata c. 17 times.

Effects of oxygen deficiency on feeding (IV)

Numerous studies have found lethal effects of low oxygen concentrations in benthic animals (e.g. Johansson, 1997), while only a few (e.g. Das and Stickle, 1993) have studied the effect of poor oxygen conditions on food intake by deposit-feeders.

We tested how oxygen deficiency influences feeding in the two amphipods Monoporeia affinis and Pontoporeia femorata, the bivalve Macoma balthica, and the priapulid Halicryptus spinulosus by exposing them for a week to oxygen concentrations ranging from 0.8 to 10.6 mg l

(control treatment). The lowest concentration (0.8 mg l

) killed all P. femorata, while 15% of M. affinis survived. In the second lowest oxygen concentration, 1.6 mg l

, 58% of P.

femorata and 65% M. affinis survived. Similar results were obtained in a longer experiment (24 days) by Johansson (1997). No significant difference in survival between oxygen levels was found in M. balthica or H. spinulosus. Feeding, measured as uptake of radioactivity from

14

C-labelled algae, was significantly reduced at 1.6 mg O

l

1, by about half for P. femorata and about 20% for M. affinis. In the lowest oxygen concentration (0.8 mg l

), feeding by M.

balthica was reduced by about half.

For the amphipods, both survival and feeding were reduced by oxygen deficiency, while only feeding was affected in Macoma balthica. However, in the longer term it seems likely that reduced feeding would cause M. balthica to starve, and thus eventually die, even at low oxygen concentrations above those acutely lethal. Halicryptus spinulosus did not feed on the algae offered, demonstrating that it is not a surface deposit-feeder.

Final remarks

Although many benthic invertebrates are classified as deposit-feeders, they are often also omnivores. When offered Macoma balthica spat, Monoporeia affinis readily eats them.

Whether they actively search for such food or ingest just by accident is not clear, but their

functional response curve (Ejdung and Elmgren, 1998) suggests an active modification of the

amphipods foraging behaviour. It is likely that M. affinis is a predator not only on M. balthica

spat, but also on other meiofauna in the sediment, as indicated by Elmgren (1978) and

(Sundelin and Elmgren, 1991). This may, to some extent, be the result of bulk feeding, but

could nevertheless have a great influence on the meiofauna community structure, and also

provide a limited quantity of high-quality food for the amphipods.

The demonstration of segregation in feeding depths between Monoporeia affinis and Pontoporeia femorata means that the two amphipods will affect mineralization and bioturbation differently, depending on their local abundance. Although M. affinis feeds preferentially at the sediment surface and assimilates more organic matter, it probably also contributes more than P. femorata to the downward transport of surface organic matter, through its greater activity. Due to their high feeding rates, juveniles of both species will probably also have a considerable influence on the mineralization process.

At very low oxygen concentrations, most animals will die, while at somewhat higher oxygen levels, they will survive as long as they feed enough to maintain a balanced or positive mass balance. Normally an oxygen concentration of about 2 mg l

is considered lethal to most organisms in the long term (Diaz, 2001). However, different groups of animals and species respond differently to these factors. For instance, in the deep ocean some organisms, like foraminifera, polychaetes and mysids, have adapted to very low oxygen concentrations, even far below 2 mg l

(Levin, 2002). Our studies demonstrated differences in tolerance to oxygen deficiency between and within animal groups.

Since feeding is the most important life supporting mechanism, it is crucial to

understand where, how, when and on what animals feed. In aquatic systems, feeding by

benthic animals is often difficult to study. However, isotopic tracers are excellent tools for

tracking nutrients, and hence animal feeding.

References

Aarnio, K., Bonsdorff, E., Norkko, A., 1998. Role of Halicryptus spinulosus (Priapulida) in structuring meiofauna and settling macrofauna. Mar Ecol Prog Ser 163, 145-153.

Aljetlawi, A.A., Albertsson, J., Leonardsson, K., 2000. Effect of food and sediment pre- treatment in experiments with a deposit-feeding amphipod, Monoporeia affinis. J Exp Mar Biol Ecol 249, 263-280.

Andersen, F.Ø., 1996. Fate of organic carbon added as diatom cells to oxic and anoxic marine sediment microcosms. Mar Ecol Prog Ser 134, 225-233.

Ankar, S., 1977. The soft bottom ecosystem of the northern Baltic proper with special reference to the macrofauna. Contrib Askö Lab Univ Stockholm 19, 1-62.

Ankar, S., 1980. Growth and production of Macoma balthica (L.) in a Northern Baltic soft bottom. Ophelia Suppl 1, 31-48.

Ankar, S., Elmgren, R., 1976. The benthic macro- and meiofauna of the Askö-Landsort area (northern Baltic proper). A stratified random sampling survey. Contrib Askö Lab Univ Stockholm 11, 1-115.

Ankar, S., Sigvaldadottir, E., 1981. On the food composition of Halicryptus spinulosus Von Siebold. Ophelia 20, 45-51.

Aoki, S., Kanda, J., Hino, A., 1995. Measurements of the nitrogen budget in the rotifer Brachionus plicatilis by using

N. Fish Sci 61, 406-410.

Arntz, W., 1978. The "upper part" of the benthic food web: the role of macrobenthos in western Baltic. Rapp P-v Reun Cons int Explor Mer 173, 85-100.

Bachelet, G., 1990. Recruitment of soft-sediment infaunal invertebrates: the importance of juvenile benthic stages. La Mer 28, 199-210.

Bianchi, T.S., Levinton, J.S., 1984. The importance of microalgae, bacteria and particulate organic matter in the somatic growth of Hydrobia totteni. J Mar Res 42, 431-443.

Blackburn, T.H., 1988. Benthic mineralization and bacterial production. In: Blackburn, T.H., and Sørensen, J.(Eds.), Nitrogen Cycling in Coastal Marine Environments. John Wiley and Sons, New York, pp. 175-190.

Bousfield, E.L., 1989. Revised morphological relationships within the amphipod genera Pontoporeia and Gammaracanthus and the "glacial relict" significance of their postglacial distributions. Can J Fish Aquat Sci 46, 1714-1724.

Caddy, J.F., 1969. Development of mantle organs, feeding, and locomotion in postlarval Macoma balthica (L.) (Lamellibranchiata). Can Jour Zool 47, 609-617.

Cederwall, H., 1977. Annual macrofauna production of a soft bottom in the northern Baltic

proper. In: Keegan, B.F., Céidigh, P.Ò., and Boaden, P.J.S.(Eds.), Biology of benthic

organisms. Pergamon Press, Oxford, pp. 155-164.

Cederwall, H., 1979. Diurnal oxygen consumption and activity of two Pontoporeia (Amphipoda, Crustacea) species. In: Naylor, E., and Hartnoll, R.G.(Eds.), Cyclic phenomena in marine plants and animals. Pergamon Press, Oxford, pp. 309-316.

Das, T., Stickle, W., 1993. Sensitivity of crabs Callinectes sapidus and C. similis and the gastropod Stramonita haemastoma to hypoxia and anoxia. Mar Ecol Progr Ser 98, 263- 274.

Diaz, R.J., 2001. Overview of hypoxia around the world. J Envir Qual 30, 275-281.

Diaz, R.J., Rosenberg, R., 1995. Marine benthic hypoxia: A review of its ecological effects and the behavioural responses of benthic macrofauna. Oceanogr Mar Biol Ann Rev 33, 245-303.

Ejdung, G., Bonsdorff, E., 1992. Predation on the bivalve Macoma balthica by the isopod Saduria-Entomon - Laboratory and field experiments. Mar Ecol Prog Ser 88, 207-214.

Ejdung, G., Elmgren, R., 1998. Predation on newly settled bivalves by deposit-feeding amphipods: a Baltic Sea case study. Mar Ecol Prog Ser 168, 87-94.

Elmgren, R., 1978. Structure and dynamics of Baltic benthos communities, with particular reference to the relationship between macro- and meiofauna. Kiel Meeresforsch Sonderheft 4, 1-22.

Elmgren, R., Ankar, S., Marteleur, B., Ejdung, G., 1986. Adult interference with postlarvae in soft sediments: The Pontoporeia-Macoma example. Ecology 67, 827-836.

Elmgren, R., Hill, C., 1997. Ecosystem function at low biodiversity - the Baltic example. In:

Ormond, R., Gage, J., Angel, M., (eds) Marine biodiversity. Cambridge University Press, Cambridge, p 319-336.

Fitzgerald, S.A., Gardner, W.S., 1993. An algal carbon budget for pelagic-benthic coupling in Lake Michigan. Limnol Oceanogr 38, 547-560.

Frankenberg, D., Smith, K.L., Jr., 1967. Coprophagy in marine animals. Limnology and Oceanography Limnol Oceanogr 12, 443-450.

Goedkoop, W., Gullberg, K.R., Johnson, R.K., Ahlgren, I., 1997. Microbial response of a freshwater benthic community to a simulated event: interactive effects of benthic fauna.

Microb Ecol 34, 131-143.

Goedkoop, W., Johnson, R., 1994. Exploitation of sediment bacterial carbon by juveniles of the amphipod Monoporeia affinis. Freshwat Biol 32, 553-563.

Goedkoop, W., Johnson, R.K., 1996. Pelagic-benthic coupling: Profundal benthic community response to spring diatom deposition in mesotrophic Lake Erken. Limnol Oceanogr 41, 636-647.

Gosselin, L.A., Qian, P.Y., 1997. Juvenile mortality in benthic marine invertebrates. Mar Ecol Prog Ser 146, 265-282.

Graf, G., 1987. Benthic energy flow during a simulated autumn bloom sedimentation. Mar

Ecol Prog Ser 39, 23-29.

Gray, J.S., Wu, R.S.-s., Or, Y.Y., 2002. Effects of hypoxia and organic enrichment on the coastal marine environment. Mar Ecol Prog Ser 238, 249-279.

Guiguer, K., Barton, D.R., 2002. The trophic role of Diporeia (Amphipoda) in Colpoys Bay (Georgian Bay) benthic food web: A stable isotope approach. J Great Lakes Res 28, 228-239.

Haahtela, I., 1974. The marine element in the fauna of the Bothnian Bay. Hydrobiol Bull 8, 232-241.

Hill, C., 1984. Spatial niche separation in the co-occuring amphipods Pontoporeia affinis and Pontoporeia femorata. Master of Science Thesis, Dept of Zool, Stockholm Univ.

Hill, C., 1992. Interactions between year classes in the benthic amphipod Monoporeia affinis : Effects on juvenile survival and growth. Oecologia 91, 157-162.

Hill, C., Elmgren, R., 1987. Vertical distribution in the sediment in the co-occurring benthic amphipods Pontoporeia affinis and P. femorata. Oikos 49, 221-229.

Hill, C., Elmgren, R., 1992. Predation by the isopod Saduria entomon on the amphipods Monoporeia affinis and Pontoporeia femorata : Experiments on prey vulnerability.

Oecologia 91, 153-156.

Hylleberg, J., Riis-Vestergaard, H., 1984. Marine environments; the fate of detritus.

Akademisk Forlag, Copenhagen.

Jensen, H.S., Mortensen, P.B., Andersen, F.O., Rasmussen, E., Jensen, A., 1995. Phosphorus cycling in a coastal marine sediment, Aarhus Bay, Denmark. Limnol Oceanogr 40, 908- 917.

Johansson, B., 1997. Tolerance of the deposit-feeding Baltic amphipods Monoporeia affinis and Pontoporeia femorata to oxygen deficiency. Mar Ecol Prog Ser 151, 135-141.

Johnson, R.K., Wiederholm, T., 1992. Pelagic-benthic coupling – The importance of diatom interannual variability for population oscillations of Monoporeia affinis. Limnol Oceanogr 37, 1596-1607.

Karlson, K., Rosenberg, R., Bonsdorff, E., 2002. Temporal and spatial large-scale effects of eutrophication and oxygen deficiency on benthic fauna in Scandinavian and Baltic waters - A review Oceanography and Marine Biology, 40, 427-489.

Kreeger, D.A., Hawkins, A.J.S., Bayne, B.L., 1996. Use of dual-labeled microcapsules to discern the physiological fates of assimilated carbohydrate, protein carbon, and protein nitrogen in suspension-feeding organisms. Limnol Oceanogr 41, 208-215.

Laine, A.O., Sandler, H., Andersin, A.-B., Stigzelius, J., 1997. Long-term changes of macrozoobenthos in the Eastern Gotland Basin and the Gulf of Finland (Baltic Sea) in relation to the hydrographical regime. J Sea Res 38, 135-159.

Lehtonen, K., 1996. Ecophysiology of the benthic amphipod Monoporeia affinis in an open-

sea area of the northern Baltic Sea: Seasonal variations in body composition, with

bioenergetic considerations. Mar Ecol Prog Ser 143, 87-98.

Lehtonen, K., Andersin, A.-B., 1998. Population dynamics, response to sedimentation and role in benthic metabolism of the amphipod Monoporeia affinis in an open-sea area of the northern Baltic Sea. Mar Ecol Prog Ser 168, 71-85.

Leppäkoski, E., 1969. Transitory return of the benthic fauna of the Bornholm Basin, after extermination by oxygen insufficiency. Cah Biol Mar X, 163-172.

Levin, L., 2002. Deep-ocean life where oxygen is scare. Am Sci 90, 436-444.

Levin, L., Blair, N., DeMaster, D., Plaia, G., Fornes, W., Martin, C., Thomas, C., 1997. Rapid subduction of organic matter by maldanid polychaetes on the North Carolina slope. J Mar Res 55, 595-611.

Levin, L.A., Blair, N.E., Martin, C.M., DeMaster, D.J., Plaia, G., Thomas, C.J., 1999.

Macrofaunal processing of phytodetritus at two sites on the Carolina margin: in situ experiments using

C-labeled diatoms. Mar Ecol Prog Ser 182, 37-54.

Levinton, J.S., Bianchi, T.S., 1981. Nutrition and food limitation of deposit-feeders. I. The role of microbes in the growth of mud snails (Hydrobiidae). J Mar Res 39, 531-545.

Lindström, M., Lindström, A., 1980. Swimming activity of Pontoporeia affinis (Crustacea, Amphipoda). Seasonal variations and usefulness for environmental studies. Ann. Zool.

Fenn. 17, 213-220.

Lopez, G., Elmgren, R., 1989. Feeding depths and organic absorption for the deposit-feeding benthic amphipods Pontoporeia affinis and Pontoporeia femorata. Limnol. Oceanogr.

34, 982-991.

Lopez, G.R., Crenshew, M.A., 1982. Radiolabelling of sedimentary organic matter with

C- formaldehyde: Preliminary evaluation of a new technique for use in deposit-feeding studies. Mar Ecol Prog Ser 8, 283-289.

Lopez, G.R., Levinton, J.S., 1987. Ecology of deposit-feeding animals in marine sediments. Q Rev Biol 62, 235-260.

Meglitsch, P., Schram, F., 1991. Invertebrate Zoology. Oxford University Press, New York.

Newell, R., 1965. The role of detritus in the nutrition of two marine deposit feeders, the prosobranch Hydrobia ulvae and the bivalve Macoma balthica. Proc Zool Soc Lon 144, 25-45.

Nixon, S.W., 1981. Remineralization and nutrient cycling in coastal marine ecosystems. In:

Nielson, B.J., and Cronin, L.E. (Eds.), Estuaries and Nutrients. Humana Press, New Jersey, pp. 111-138.

Oh, C.W., Hartnoll, R.G., Nash, R.D.M., 2001. Feeding ecology of the common shrimp Crangon crangon in Port Erin Bay, Isle of Man, Irish Sea. Marine Ecology-Progress Series 214, 211-223.

Ólafsson, E., Elmgren, R., 1997. Seasonal dynamics of sublittoral meiobenthos in relation to

phytoplankton sedimentation in the Baltic Sea. Estuar Coast Shelf Sci 45, 149-164.

Ólafsson, E.B., 1986. Density dependence in suspension-feeding and deposit-feeding populations of the bivalve Macoma balthica: A field experiment. J Anim Ecol 55, 517- 526.

Owens, N., 1987. Natural variations in

N in the marine environment. Adv Mar Biol 24, 390- 451.

Phillips, N.W., 1984. Role of different microbes and substrates as potential suppliers of specific, essential nutrients to marine detritivores. Bull Mar Sci 35, 283-298.

Powilleit, M., Kitlar, J., Graf, G., 1994. Particle and fluid bioturbation caused by the priapulid worm Halicryptus spinulosus (V. Seibold). Sarsia 79, 109-117.

Preston, N.P., Smith, D.M., Kellaway, D.M., Bunn, S.E., 1996. The use of enriched

N as an indicator of the assimilation of individual protein sources from compound diets for juvenile Penaeus monodon. Aquaculture 147, 249-259.

Rolff, C., Elmgren, R., 2000. Use of riverine organic matter in plankton food webs of the Baltic Sea. Mar. Ecol. Prog. Ser. 197, 81-101.

Sarvala, J., 1986. Interannual variation of growth and recruitment in Pontoporeia affinis (Lindström) (Crustacea: Amphipoda) in relation to abundance fluctuations. J Exp Mar Biol Ecol 101, 41-59.

Segerstråle, S.G., 1950. The amphipods on the coasts of Finland - some facts and problems.

Soc Scient Fenn, Commentat Biol X 14, 2-28.

Segerstråle, S.G., 1962. Investigations on Baltic populations of the bivalve Macoma balthica (L.). Part II. What are the reasons for the periodic failure of recruitment and the scarcity of Macoma in the deeper waters of the inner Baltic? Commentat Biol Soc Sci Fenn 24, 1-26.

Sundelin, B., Elmgren, R., 1991. Meiofauna of an experimental soft bottom ecosystem - effects of macrofauna and cadmium exposure. Mar Ecol Prog Ser 70, 245-255.

Sverdrup, H., Johnson, M., Fleming, R., 1942. The Oceans, their Physics, Chemistry and General Biology. Prentice-Hall, New York.

Taghon, G.L., Jumars, P.A., 1984. Variable ingestion rate and its role in optimal foraging behavior of marine deposit feeders. Ecology 65, 549-558.

Taghon, G.L., Nowell, A.R.N., Jumars, P.A., 1984. Transport and breakdown of fecal pellets:

Biological and sedimentological consequences. Limnol Oceanogr 29, 64-72.

Tenore, K.R., 1988. Nitrogen in benthic food chains. In: Blackburn, T.H., and Sørensen, J.(Eds.), Nitrogen Cycling in Coastal Marine Environments. John Wiley and Sons, New York, pp. 190-206.

Tenore, K.R., Cammen, L., Findlay, S.E.G., Phillips, N., 1982. Perspectives of research on detritus: Do factors controlling the availability of detritus to macroconsumers depend on its source? J Mar Res 40, 473-490.

Thorson, G., 1966. Some factors influencing the recruitment and establishment of marine

benthic communities. Neth J Sea Res 3, 267-293.

Twilley, R.R., Ejdung, G., Romare, P., Kemp, W.M., 1986. A comparative study of decomposition, oxygen consumption and nutrient release for selected aquatic plants occurring in an estuarine environment. Oikos 47, 190-198.

Uitto, A., Sarvala, J., 1991. Seasonal growth of the benthic amphipods Pontoporeia affinis and P. femorata in a Balthic archipelago in relation to environmental factors. Mar Biol 111, 237-246.

van de Bund, W.J., Olafsson, E., Modig, H., Elmgren, R., 2001. Effects of the coexisting Baltic amphipods Monoporeia affinis and Pontoporeia femorata on the fate of a simulated spring diatom bloom. Mar Ecol Prog Ser 212, 107-115.

Wang, W.X., Widdows, J., 1991. Physiological responses of mussel larvae Mytilus edulis to

environmental hypoxia and anoxia. Mar Ecol Prog Ser 70, 223-236.

Tack och erkännanden

Jag har många att tacka för att jag äntligen ”landat”. Först och främst vill jag tacka min handledare Ragnar Elmgren. Det har varit en stor förmån att ha dig som handledare under min doktorandtid. Du har haft ett stort tålamod med alla mina frågor, läst igenom och förbättrat mina inte alltid lättbegripliga engelska texter. Trots att du oftast varit upptagen med annat, så har du tagit dig tid att diskutera experimentuppställningar, läsa manuskript, ansökningar, tillhandahållit referenser...ja, nära nog allt man kan tänka sig. Tack Ragnar för allt stöd.

Gunilla Ejdung, vi har haft många trevliga samtal om både forskning och livets alldagliga frågor. Vi har upplevt mycket tillsammans, allt ifrån att sitta och sålla sediment i timtal, till att plocka tusentals små musslor på Askö. Det har varit ett sant nöje att jobba med dig under dessa år.

Hans Cederwall, jag kommer ihåg alla intressanta diskussioner vi haft om sediment och bottenfauna. Det har varit mycket trevligt och lärorikt att följa med dig på den årliga provtagningen.

Carl Rolff, ingen jag känner behärskar statistik som du, och du har givit mig mycket hjälp. Det har varit mycket stimulerande och givande att undervisa på kursen om marin miljöövervakning.

Sven Blomqvist, du har alltid tagit dig tid att diskutera och ge klok vägledning i olika frågor. Jag har uppskattat att undervisa med dig på kursen i akvatisk ekologi, tack för allt, Sven.

Sture Hanson, många goda pratstunder har vi haft. Du har kommit med bra råd och synpunkter på mina manuskript, och dessutom bidragit med trevligt cykelsällskap hem.

Elsina Flach-Lundgren, tack för gott samarbete när vi gjort experiment, och för trevlig vistelse i Nederländerna.

Ulf Larsson, tack för du gav mig extra tid att slutföra avhandlingsarbetet.

Personalen på Kemiska laboratoriet vid Institutionen för Systemekologi, som varit så tillmötesgående på alla sätt och vis, både när det gäller kemiska frågor och rent praktiska analyser, tack till er alla, inte minst Anders Sjösten.

Dessutom vill jag rikta ett tack till alla på institutionen, särskilt mina rumskamrater genom åren, Sigrid Ehrenberg, Candida Savage, Tomas Didrikas och Elena Gorokhova, för många glada skratt och trevliga stunder. Lars Gustafsson, du har långt utöver dina skyldigheter hjälpt till med datatekniska problem, för vilket jag är dig tacksam.

Olle Svanberg, du är en sann vän och medmänniska. Tack för din tid och ditt stöd som mentor.

Alf Ekblad, utan dig hade mitt sista manuskript aldrig blivit färdigt. Du var faktiskt den enda som kunde analysera mina prover trots att de var svagt radioaktiva.

Vi ”benthosfolk” är en liten skara människor som är utspridda på olika universitet och

institutioner. På Zoologiska institutionen finns några likasinnade som jag vill tacka för ett

mycket trevligt samarbete och heta diskussioner. Tack, Johan Wenngren, Rasmus Neideman

och Emil Òlafsson. Emil, vi har inte alltid varit eniga, men tänk om hela världens befolkning skulle ha samma uppfattning och tycka likadant om allt, vad tråkig världen skulle te sig.

Ann-Kristin Eriksson Wiklund, Brita Sundelin och Eva Håkansson, det var mycket trevligt att samarbeta med er på laboratoriet i Studsvik.

Ett stort tack till personalen på SMF, som gjort det möjligt för mig att vistas och genomföra försök på Askö. Eddie Eriksson, Susann Ericsson, Stefan Andersson och Mikael Karlsson, tack för insamling av djur och sediment, samt hjälp på laboratoriet.

Sist men inte minst ett stort tack till min familj: Anna, Hanna, Oskar och Alva för att ni varit tålmodiga och förstående under de gågna år jag fyllt min tid med forskning. Min mor Jane, mina systrar Lena och Lotta, tack för allt stöd och uppmuntran längs vägen.

Jag har tacksamt mottagit pengar från Stockholm Marina Forskningscentrum, Alice och

Lars Siléns fond, Gustaf Lindströms minnesfond och Hierta-Retzius fond.