In highly seasonal systems, the emergence of planktonic resting stages from the sediment is a key driver for bloom timing and plankton community composition. The termination of the resting phase is often linked to environmental cues, but the extent to which recruitment of resting stages is affected by climate change remains largely unknown for coastal environments. Here we investigate phyto- and zooplankton recruitment from oxic sediments in the Baltic Sea in a controlled experiment under proposed temperature and light increase during the spring and summer. We find that emergence of resting stage differs between seasons and the abiotic environment. Phytoplankton recruitment from resting stages were high in spring with significantly higher emergence rates at increased temperature and light levels for dinoflagellate and cyanobacteria than for diatoms, which had highest emergence under cold and dark conditions. In comparison, hatching of copepod nauplii was not affected by increased temperature and light levels. These results show that activation of plankton resting stages are affected to different degrees by increasing temperature and light levels, indicating that climate change affects plankton dynamics through processes related to resting stage termination with potential consequences for bloom timing, community composition and trophic mismatch.

At temperate latitudes where seasonal changing environmental conditions strongly affect the magnitude, duration and species composition of pelagic primary production, macrobenthic organisms living below the photic zone rely on the sedimentation of organic matter as their primary energy source. The succession from nutritious spring blooms to summer cyanobacteria is assumed to reduce food quality for benthic primary consumers and their fatty acid (FA) profiles. In contrast, we find low seasonal variability in FA content of five benthic macroinvertebrates spanning two trophic levels in the Baltic Sea, a system with high seasonal variation in phytoplankton species composition. However, levels of the major FA groups vary greatly between benthic species. The results suggest that benthic macroinvertebrates have evolved FA metabolism adapted to degraded sedimenting material. Moreover, our study shows that species composition of benthic macrofauna rather than seasonal changing conditions affect availability of essential nutrients to higher trophic levels.

The neurotoxic secondary metabolite β-N-methylamino-L-alanine (BMAA) and its structural isomer 2,4-diaminobutyric acid (DAB) are known to be produced by various phytoplankton groups. Despite the worldwide spread of these toxin producers, no obvious role and function of BMAA and DAB in diatoms have been identified. Here, we investigated the effects of biotic factors, i.e., predators and competitors, as possible causes of BMAA and/or DAB regulation in the two diatom species Phaeodactylum tricornutum and Thalassiosira pseudonana. DAB was specifically regulated in T. pseudonana by the presence of predators and competitors. The effects of DAB on both diatoms as competitors and on the copepod Tigriopus sp. as predator at individual and at population levels were examined. The toxic effects of DAB on the growth of T. pseudonana and the population of Tigriopus sp. were significant. The effect of DAB as a defensive secondary metabolite is assumed to be environmentally relevant depending on the number of the copepods. The results show a potential function of DAB that can play an important role in defense mechanisms of T. pseudonana.

Pelagic primary production is the main input of organic energy for benthic production below the photic zone. In the Baltic Sea, spring phytoplankton blooms are dominated by diatoms that sink out rapidly and export nutritionally favorable matter to benthic secondary production, while the summer blooms have more variable sedimentation rates and nutritional profile. Changes in phytoplankton species composition and bloom dynamics, as a consequence of climate change and eutrophication are reducing high quality diatoms reaching the benthic fauna, while promoting cyanobacteria. Here, we test uptake and assimilation of changing phyto- plankton composition for three common benthic invertebrates, a clam, an amphipod and a polychaete under varying degrees of spring-bloom associated diatoms (Skeletonema costatum) and summer-bloom associated cyano- bacteria (Nodularia spumigena). The phytoplankton were labeled with stable isotopes (15N and 13C, respectively) in order to trace assimilation in consumers’ tissues. We found that all three macrofauna species fed on both dia- toms and cyanobacteria. A linear pattern was found for all three species in assimilation of carbon and nitrogen from diatoms, with increasing assimilation associated with higher proportion of diatoms. There was no clear pattern found between proportion of cyanobacteria and assimilation of carbon and nitrogen for any of the spe- cies. This study shows that the investigated macrofaunal species display a selective feeding behavior with prefer- ence for spring-bloom associated diatoms. Thus, changes in phytoplankton bloom composition are likely affecting benthic species composition and production. 

In coastal aphotic sediments, organic matter (OM) input from phytoplankton is the primary food resource for benthic organisms. Current observations from temperate ecosystems like the Baltic Sea report a decline in spring bloom diatoms, while summer cyanobacteria blooms are becoming more frequent and intense. These climate-driven changes in phytoplankton communities may in turn have important consequences for benthic biodiversity and ecosystem functions, but such questions are not yet sufficiently explored experimentally. Here, in a 4-week experiment, we investigated the response of microeukaryotic and bacterial communities to different types of OM inputs comprising five ratios of two common phytoplankton species in the Baltic Sea, the diatom Skeletonema marinoi and filamentous cyanobacterium Nodularia spumigena. Metabarcoding analyses on 16S and 18S ribosomal RNA (rRNA) at the experiment termination revealed subtle but significant changes in diversity and community composition of microeukaryotes in response to settling OM quality. Sediment bacteria were less affected, although we observed a clear effect on denitrification gene expression (nirS and nosZ), which was positively correlated with increasing proportions of cyanobacteria. Altogether, these results suggest that future changes in OM input to the seafloor may have important effects on both the composition and function of microbenthic communities.

Changing abiotic factors, like temperature and light are important drivers of seasonality, affecting aquatic ecosystems and organisms annually in a predictable pattern, structuring species composition, growth, reproduction and trophic interactions. As a consequence of human induced climate change, coastal ecosystems are seeing a disruption in the predictability of annual cycles. This in turn is affecting species composition at the base of the food web, possibly causing effects for higher trophic levels. Although the benthic and pelagic environments are coupled, still very little is known about the effect that changes in species composition will have on this interaction. In Study I, three species of common macrofauna from Baltic Sea soft sediment bottoms were for the first time subjected to a gradient of spring bloom related diatoms and summer bloom related cyanobacteria as food resource. We found a clear differentiation between the two food types regarding preference of the consumers. The results highlight that diatoms were consumed by all species with a strongly positive linear relationship with available food. Cyanobacteria was consumed, but with no clear pattern relating to amount available. In Study II, we investigated if seasonal phytoplankton succession affects the composition of essential compounds in benthic macrofauna, focusing on transfer of fatty acids (FAs) from phytoplankton via sediment to benthic consumers. The outcome showed for the first time large interspecific variation in FA composition and concentration, but low seasonal variation within species for five major invertebrate taxa over three seasons. We found only few convincing links between seasonal phytoplankton production and its succession of FAs in the seston, sediment and the animals. This suggests that demands for FAs differ substantially between taxa and that FAs of sedimenting organic material is not clearly reflected in the investigated species. In Study III, we quantified recruitment of phyto- and zooplankton from oxic sediment to the pelagic environment, under proposed changes to light and temperature caused by climate change. We found clear indications of both light and temperature having an effect on phytoplankton (mainly cyanobacteria and dinoflagellates) recruitment in spring. We found no effect on recruitment of phyto- or zooplankton in summer, but large differences in zooplankton recruitment in summer compared to spring. The phytoplankton taxa that showed strong recruitment responses in spring, could with proposed climate change scenarios potentially impact the species composition during spring, thus affecting food web dynamics. In Study IV we used advanced molecular tracing techniques of compound specific stable isotope analysis of carbon and nitrogen in amino acids in five taxonomic groups of phytoplankton, with the goal to advance methods of differentiating between groups in mixed samples of phytoplankton. By applying this method to amino acids, we were able to differentiate between all five taxonomical groups. The key in revealing the group specific finger printing on particular amino acids was based on amino acid synthesis pathways and thus depletion or enrichment of isotopes. The novel findings presented in this thesis further advances our knowledge about responses of benthic-pelagic coupling to environmental change and how to further develop tracing of food webs in order to predict the effects of a changing environment on aquatic organisms. 

Fish larvae abundances, diversity and trophic position across shallow seagrass, coral reef and open water habitats were examined to characterize their distribution in coastal East Africa. Larvae were identified to family and analysed for abundance differences between sites and habitats, trophic level using stable-isotope analysis and parental spawning mode. Abundances differed greatly between sites with the highest numbers of larvae occurring in the open-water and seagrass habitats. Larval fish diversity was high across habitats with 51 families identified with small differences between sites and among habitats. Notably, larvae of abundant large herbivorous fishes present in reef and seagrass habitats were almost completely absent at all sampling locations. In the seagrass, demersal spawned larvae were more abundant compared with the reef and open-water habitats. Stable-isotope analysis revealed that fish larvae have a varied diet, occupying trophic level two to three and utilizing planktonic prey. This study offers new insights into distributional aspects of fish larvae along the East African coast where such information is sparse.