Activities 2005-2007: Activities 1990-2004:

Fish and mysid studies

Significant changes in the primary and secondary production of the Baltic Sea have taken place during the past 40 years. The eutrophication of the coastal and the pelagic areas may have far-reaching consequences to the biota. Drifting algal mats, deep-water hypoxia, turbidity and cyanobacterial blooms are typical symptoms of increased production.

Trophic interactions in the Baltic Sea
Although the primary and secondary production have increased individual growth and population sizes of the commercially important fish stocks, Baltic herring, sprat and cod, have not shown a linear increase, but been more closely connected to the salinity variations of the Baltic Sea. We hypothesise that this discrepancy can be explained by a better understanding of the predatory interactions between individual fish and their prey species. The project aims to (1) reveal the degree of zooplankton patchiness in the field, (2) quantify how cyanobacteria blooms influence the feeding and occurrence of zooplankton and fish in the field, and (3) build an individual based foraging and growth model to simulate how variations in zooplankton community structure affect the growth of pelagic fish.

The effect of mysids on herring recruitment and population dynamics
Predation and competition between species have major effects on populations at the higher trophic levels of the northern Baltic Sea. Mysid shrimps are predators, competitors and food for different age classes of herring. They influence herring populations both in the littoral areas after hatching and in the pelagial after fish migrate from the shallow areas to outer sea. This project investigates the extent of the effect of mysid shrimps on Baltic herring stocks, through competition and predation. The change in distribution and abundance of mysids has probably a major influence on herring through changed competition pressure, decreased food during autumn and winter and increased predation pressure after hatching. The objective is to clarify the present influence of mysids on herring recruitment and stock development by intensive field sampling, experiments and modelling.

Eutrophication and the behavioural ecology of the Pike (Esox lucius): conservation implications
The pike (Esox lucius) is a freshwater species that has decreased alarmingly, or totally disappeared in the outer archipelago. The idea of the present work is to reveal the factors that have affected the decrease of the pike in the Baltic coastal waters by doing experimental work and fieldwork with larvae, on the behavioural ecology and conservation biology of this species. The aim is to analyse larval growth and condition, survival, inter-specific competition, foraging success, as well as predation risk as responses to environmental changes, such as turbidity caused by microalgae, decreases in habitat complexity caused by filamentous algae, and decreases in oxygen concentrations.

Ecosystem consequences of cyanobacteria in the Baltic Sea

Harmful algal blooms, especially those of cyanobacteria (mainly Nodularia spumigena and Aphanizomenon flos-aquae), seem to be increasing in frequency in the Baltic Sea. These blooms are a major esthetic nuisance, and many of the species are directly toxic to humans and various terrestrial and marine organisms. The mechanisms underlying the formation of the blooms in the Baltic Sea are nowadays relatively well known, but, in contrast, very little is known about their consequences to the rest of the Baltic Sea pelagic ecosystem, including phytoplankton, zooplankton, mysid shrimps and planktivorous fish.


The fate and effects of Nodularia spumigena and its toxin, nodularin, in the Baltic Sea planktonic food webs
This study attempts to measure the uptake and accumulation of nodularin to zooplankton, evaluate the magnitude of vectorial transport of nodularin from zooplankton to predators, and observe the effects of vectorially transported toxins on the predators, comparing these effects with crude extracts obtained from Nodularia spumigena –cultures. This is achieved by using sensitive methods in toxin analyses: ELISA assay and a novel application of radiolabelled toxin in accumulation and vectorial transport studies. Nodularin was detected in copepods after feeding on toxic N. spumigena, but only a small fraction compared to the estimated ingested toxin (paper 43). However, a negative relationship was found between the accumulated toxins in copepods and their gross growth efficiency, indicating either higher metabolic costs when coping with cyanobacterial toxins, or that cyanobacteria are a poor quality food source for copepods. Toxin accumulation in copepods also occurred both in cultured N. spumigena and natural community containing this toxic species, but only a minor fraction of this toxin was detected in their faecal pellets (paper 39). In addition, direct uptake and accumulation of toxin from water to the animals was observed (paper 42).

Allelopathy in the Baltic cyanobacteria
Interactions between cyanobacteria and the rest of the Baltic Sea phytoplankton community are still largely unknown. Cyanobacteria are able to produce a variety of bioactive chemicals that can inhibit or retard the growth of various organisms. A part of the good competitive ability of cyanobacteria is probably based on the excretion of such chemicals. On the other hand, cyanobacteria may also positively affect the plankton community, through the release of nutrients from decaying cells (paper 34). The aim of this study is to examine allelopathy in the Baltic cyanobacteria, i.e., their effects on other phytoplankton species, mediated through chemicals excreted into the environment. To do this, we have performed experiments with species pairs and a natural plankton community, and will statistically analyse long-term plankton data. In a study where cell-free filtrates of the cyanobacteria Nodularia spumigena, Aphanizomenon flos-aquae and Anabaena lemmermannii were added to cultures of co-occurring phytoplankton species, all cyanobacteria inhibited the growth of the cryptophyte Rhodomonas sp., whereas none of them affected the prymnesiophyte Prymnesium parvum. No evidence was found for the involvement of the hepatotoxin, nodularin, in the allelopathic effects of N. spumigena (paper 50).

Bioturbation studies

Bioturbation by benthic macrofauna modifies the physical and chemical properties of the seafloor sediments, and affects the abundance and diversity of other compartments of the benthic biota. Bioturbation changes the rate and pathways of organic matter mineralization and thus the functioning of the whole ecosystem. Beyond truly benthic species, marine sediments provide a temporal refuge for a variety of planktonic organisms; long-lived resting stages of plankton accumulate in the seafloor where they are subject to various benthic processes. Functional differences in the feeding and burrowing modes between the various macrofaunal species are reflected to the outcome of bioturbation, and consequently, the composition of the macrofaunal community is a key factor determining the properties of soft sediment habitats. We aim to compare the influence of four functionally distinctive taxa, the amphipod Monoporeia affinis, the bivalve Macoma balthica, the polychaetes Marenzelleria spp. and the mysid shrimps Mysis spp. on the chemical and physical properties of organic rich soft sediments and on the benthic propagules of plankton. We have carried out laboratory experiments on zooplankton emergence, particle transport and benthic nutrient fluxes in the presence of various macrofaunal species. The zooplankton species studied include the cladocerans Bosmina longispina maritima, Cercopagis pengoi, the copepods Acartia spp., Eurytemora affinis, Temora longicornis, and the rotifers Synchaeta spp. The results demonstrate that both sediment-dwelling (paper 68) and epibenthic organisms (paper 52 ) feed on zooplankton benthic eggs, and that the various zoobenthic species transport zooplankton eggs and other particles in sediment in species-specific patterns. Moreover, benthic recruitment of zooplankton as well as the cycling of nutrients between sediment and water is strongly affected by the species composition of macrozoobenthos.

Previous activity

1. PREDATION STUDIES

We have mainly investigated prey selection and the components of the predatory interaction between Baltic herring, threespined sticklebacks, and their main prey, the crustacean zooplankton. The main aim of the studies is to reveal which species are easy and which species are difficult prey for the different predators, and why. This will eventually allow predicting what will happen to the feeding and, hence, growth rates of various predators, when zooplankton communities change.

1. Bottom-up processes also influence herring growth in the Baltic. The weight-at-age of Baltic herring declioned by 50% during the 1980s. It has been suggested that this was caused by the disappearance of cod predation and intraspeciffic competition between herring individuals. However, we present data showing that the quality of the food regime for hering has also declined during the same period: during the stagnation of the Baltic Sea, watre slainity has lowered, which induced a shift from a copepod-dominated to a cladoceran-(Bosmina) -dominated mesozooplankton community. These small plankters are less favourable food for herring than the large copepods, and thus herring have got less energy from their food than previously (paper 15).

2. A vulnerability ranking of the Baltic Sea mesozooplankton. We obtained prey selectivity indices for 2 different predators, Baltic herring and Mysis mixta from previous studies. We then quantified the escape performance of the 5 most abundant crustacean zooplankton species in the northern Baltic Sea by videofilming the plankters escaping from an artificially created water flow (siphon). As hypothesized, herring behaved as size-selective predators, wheras in M. mixta, prey selection was not clearly based on size, as the small cladocerans were not as negatively selected than expected from §their size. For the most abundant copepod prey, the selectivity rank of mysids was however exactly the same as that of herring, which suggests that similar factors lie behind prey selectivities in these 2 predator species. Comparing these results with those obtained with the artificial flow allowed us to identify cases where the predators appeared to show ‘true’ selection for a certain prey, and where the selection was ‘truly’ negative (Paper 32). On the basis of these results we provide a hypothesis on how potential changes in zooplankton community structure may affect the food gain of herring and mysids in the Baltic Sea. In a previous study (Paper 19) we have also built a hydrodynamic model describing the positive relationship between fish approaching speed and copepod reaction distance, as well as the difference between the 2 copepod species. This suggests that, by determining the hydrodynamic sensitivity of various copepod species, we can predict their vulnerability to predation by small fish with different sizes and swimming speeds.

3. Pelagic mysids feed actively on both phyto- and zooplankton. The diet changes along growth of mysids. At first months the diet contains mainly phytoplankton and benthic material and after attaining a threshold size mysids shift feeding on zooplankton and pelagic material (Paper 27). Carnivorous feeders have advantage to grow faster compared to phytoplankton grazers and thus food quality seems to be an important factor influencing mysid growth. We investigated also the effect of light on the carnivorous feeding of both pelagic and littoral mysids. Highest feeding rate of pelagic mysids was observed in total darkness but littoral mysids were not affected by light at all. This may be explained by different habitats and the prevailing light conditions in these environments. Pelagic mysids are used to migrate to deep water when light increases to avoid fish predation, thus they are not used to feed in light. In the littoral zone, on the other hand, mysids are used to bright light and their feeding is not hampered by it (Paper 31).

Antipredator behaviour of Baltic planktivores
Predation is an important source of mortality for most aquatic animals. Thus being able to avoid being eaten brings substantial fitness benefits to individuals. Detecting predators and modifying behaviour accordingly are of central importance in escaping predation. The project investigates the different behavioural responses of Baltic planktivores to predation risk. Predator detection abilities and antipredator behaviour is studied in different planktivores, i.e. littoral mysids Neomysis integer and Praunus flexuosus (paper 45), three-spined stickleback Gasterosteus aculeatus, pelagic mysids Mysis mixta and M. relicta, and the predatory cladoceran Cercopagis pengoi (paper 59), with cues from their respective predators, European perch Perca fluviatilis and Baltic herring Clupea harengus membras. Also, the use of different aquatic macrophytes as predation refuges by the littoral planktivores is studied (paper 52). The project includes experimental laboratory studies as well as field experiments and sampling.

2. EFFECTS OF CYANOBACTERIAL BLOOMS ON THE PELAGIC FOOD WEB

1. Cyanobacteria are poor food for calanoid copepods. Egg production was very low, mortality very high and hatching of eggs prohibited when Eurytemora affinis were fed toxic Nodularia sp. However, with non-toxic Nodularia the mortality remained at a moderate level, and was lowered with increasing cyanobacteria concentration (Paper 22). This indicates that Nodularia lacks essential nutrients for production, irrespective of toxicity. However, toxins of Nodularia sp. are directly harmful for copepod production and survival.

2. The effect of cyanobacteria varies with copepod species. Eurytemora affinis fed actively on non-toxic Nodularia sp. strain and moderately actively on a toxic strain, whereas Acartia bifilosa totally avoided feeding on both strains (Paper 28). Since ingestion of toxic cyanobacteria is harmful for copepods (Paper 22), this suggests that the occurrence of Nodularia sp. blooms in the Baltic Sea favours copepod species capable of selective feeding, such as Acartia spp.

3. A senescing cyanobacteria bloom indirectly favours zooplankton production. The consequences of a senescing cyanobacteria bloom to the pelagic ecosystem were studied in a mesocosm experiment. Cultured Nodularia sp. were added into 150-l enclosures filled with 100-mm-filtered seawater, thus launching a cyanobacteria bloom. The enclosures were regularly sampled for various parameters and experiments were made where mesocosm water was provided to calanoid copepods and their egg production and grazing were measured. Despite high concentrations of toxins, bacteria, ciliates and several mesozooplankton species thrived in the mesocosms. In the feeding experiments, copepod mortality was low and egg production higher than in controls, especially when the ciliates increased in abundance.

4. Mysid shrimps are not harmed by cyanobacteria. In a laboratory experiment, omnivorous Mysis mixta fed cyanobacteria strains in the following ‘preference’ order: non-toxic Aphanizomenon flos-aquae > non-toxic Nodularia sp. > toxic Nodularia sp. Furthermore, the predation of mysids on copepods was not impaired by the presence of toxic Nodularia sp., nor was the long-term mortality of mysids affected by the presence of toxic cyanobacteria (Paper 29). This suggests that mysids can avoid feeding on toxic cyanobacteria and feed on other available food instead.


3. LIFE CYCLE STUDIES

Zooplankton may adapt to harsh environmental conditions by developing life stages that survive for long periods in the sediment. Well known examples are the sexually produced resting eggs of otherwise asexually reproducing rotifers and cladocerans. Similarly, it has been shown that certain zooplankton eggs may pass the intestines of planktivorous fish.

Viability of copepod eggs in Baltic sediments. We were the first to show that there are literally millions of viable calanoid eggs in the sediments of the Baltic Sea (Papers 8, 14). Also, by using Cs137 and soot particle dating we have shown that some of the viable eggs are >10 years old (Paper 11).