We report on methane enrichments that were observed during summer in the upper water column of the Gotland Basin, central Baltic Sea. In the eastern part of the basin, methane concentrations just below the thermocline varied between 15 nM and 77 nM, in contrast to the western part where no methane enrichments could be detected. Stable carbon isotope ratios of methane (d 13 C-CH 4 of 267.6&) indicated its in situ biogenic origin from CO 2 reduction, which was supported by clonal sequences that clustered with Methanomicrobiaceae, a family of methanogenic Archaea. Incubation experiments with a Temora longicornis dominated seston fraction obtained from the relevant depth showed a positive correlation between seston concentration and methane production rates. Our results, in combination with previous literature outcomes, suggest that the methane enrichment in the eastern basin might be sustained by a diet-consumer relationship between the dinoflagellate Dinophysis norvegica and the copepod T. longicornis. However, our mass balance indicates that a local methane production of 110 pmol L 21 d 21 was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH 4 d 21 per adult T. longicornis (about 1 pmol L 21 d 21 ) was too low to maintain the methane enrichment by zooplankton associated methane production only. These calculations also showed that methane was consumed below the thermocline and not transported into the upper-ocean, suggesting that other sources in the mixed layer in the range of 95 pmol L 21 d 21 are needed to maintain the observed methane air-sea flux.

Knowledge of how zooplankton can utilize cyanobacteria to sustain their nitrogen (N) demand for essential compounds like amino acids (AAs) is a key to predicting responses of higher trophic levels in terms of production and food web structure to future enhanced water column stratification. We explored the natural abundances of bulk N and AA‐specific nitrogen isotopes (δ15N) in particulate organic matter and mesozooplankton size‐fraction samples from three vertically separated water bodies in the central Baltic Sea during two summertime cyanobacteria blooms. The combination of plankton community and isotope data together with environmental variables helped to identify a mechanism of diazotrophic AA supply (synthesized during N2 fixation) for mesozooplankton, that largely depended on the sea surface temperature which regulated the access to the diazotrophic N‐based food web in the surface water (SW). We found that in the warmer summer, thermophilic cladocerans (e.g., Bosmina spp.) benefited most from diazotrophic AAs in the SW (19.8°C), while only in the colder summer, temperate copepods (e.g., Temora longicornis) ascended from the subjacent winter water into the SW (16.2°C) and incorporated diazotrophic AAs. Trophic position estimates based on the phenylalanine and glutamic acid δ15N signatures revealed that the diazotrophic AA supply into mesozooplankton was mainly indirect via feeding on mixo‐ and heterotrophic diets. Significantly enriched δ15N signatures in phenylalanine in the deep mesozooplankton (mainly copepods of Pseudo‐ and Paracalanus spp.) from the bottom water (BW) that was a region of the suboxic zone point to a reliance on a local food web. Mesozooplankton in the BW was feeding on diets of heterotrophic origin and probably profited from the heterotrophic re‐synthesis of AAs originating from sinking organic matter, as well as from the indirect incorporation of de novo synthesized AAs that most likely originated from chemoautotrophic bacteria or archaea communities in the suboxic zone. Our findings suggest that indirect feeding on diazotrophs and chemoautotrophs will be principal ways of amino acid supply for zooplankton in future enhanced stratified aquatic systems. Only a relatively small increase in temperature may restrict temperate key species from diazotrophic N‐based food webs in the mixed layer.

Changes in the complexity of planktonic food webs may be expected in future aquatic systems due to increases in sea surface temperature and an enhanced stratification of the water column. Under these conditions, the growth of unpalatable, filamentous, N2‐fixing cyanobacterial blooms, and their effect on planktonic food webs will become increasingly important. The planktonic food web structure in aquatic ecosystems at times of filamentous cyanobacterial blooms is currently unresolved, with discordant lines of evidence suggesting that herbivores dominate the mesozooplankton or that mesozooplankton organisms are mainly carnivorous. Here, we use a set of proxies derived from amino acid nitrogen stable isotopes from two mesozooplankton size fractions to identify changes in the nitrogen source and the planktonic food web structure across different microplankton communities. A transition from herbivory to carnivory in mesozooplankton between more eutrophic, near‐coastal sites and more oligotrophic, offshore sites was accompanied by an increasing diversity of microplankton communities with aging filamentous cyanobacterial blooms. Our analyses of 124 biotic and abiotic variables using multivariate statistics confirmed salinity as a major driver for the biomass distribution of non‐N2‐fixing microplankton species such as dinoflagellates. However, we provide strong evidence that stratification, N2 fixation, and the stage of the cyanobacterial blooms regulated much of the microplankton diversity and the mean trophic position and size of the metabolic nitrogen pool in mesozooplankton. Our empirical, macroscale data set consistently unifies contrasting results of the dominant feeding mode in mesozooplankton during blooms of unpalatable, filamentous, N2‐fixing cyanobacteria by identifying the at times important role of heterotrophic microbial food webs. Thus, carnivory, rather than herbivory, dominates in mesozooplankton during aging and decaying cyanobacterial blooms with hitherto uncharacterized consequences for the biogeochemical functions of mesozooplankton.

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Eurytemora affinis (Copepoda) were fed 15 N-labeled Rhodomonas salina (Cryptophyta) or 15 N-labeled Nodularia spumigena (Cyanobacteria) in excess under controlled laboratory conditions. Zooplankton collected from the Baltic Sea were fed natural phytoplankton amended with 15 N-labeled N. spumigena. We quantified the direct incorporation of 15 N tracer from N 2 -fixing N. spumigena (diazotroph nitrogen) and ammonium-utilizing R. salina into the amino acid nitrogen (AA-N) of zooplankton using complementary gas chromatographycombustion-isotope ratio mass spectrometry, gas chromatography-mass spectrometry, and elemental analysisisotope ratio mass spectrometry approaches. Specific and mass-specific TN and AA-N incorporation rates of the 15 N tracers were calculated for zooplankton. Highest incorporation of 15 N was found in field zooplankton relying on N. spumigena and in E. affinis relying on R. salina. Lowest incorporation was found in E. affinis relying on N. spumigena. Decreasing specific and mass-specific rates during field experiments possibly were due to food shortage, whereas decreasing rates in E. affinis grazing on R. salina were more likely due to satiation. Specific and mass-specific rates were consistently low in E. affinis when exposed to N. spumigena, suggesting that these animals were reluctant to feed on N. spumigena. Essential isoleucine received most of the diazotroph nitrogen in field zooplankton, while nonessential amino acids received most 15 N tracer in E. affinis. N. spumigena was clearly an important amino acid nitrogen source for Baltic Sea zooplankton.