No abstract

Cercopagis pengoi, a recent invader to the Baltic Sea and the Laurentian Great Lakes, is a potential competitor with fish for zooplankton prey. We used stable C and N isotope ratios to elucidate trophic relationships between C. pengoi, zooplankton (microzooplankton, 90-200 microm, mostly copepod nauplii and rotifers; mesozooplankton, >200 microm, mostly copepods), and zooplanktivorous fish (herring, size range 5-15 cm and sprat, 9-11 cm) in a coastal area of the northern Baltic Sea. The isotope ratios in C. pengoi and fish were much higher than those of zooplankton, showing general trends of enrichment with trophic level. Young-of-the-year (YOY) herring had a significantly higher (15)N/(14)N ratio than C. pengoi, suggesting of a trophic linkage between the two species. To evaluate the possible relative importance of different food sources for C. pengoi and YOY herring, two-source isotope-mixing models for N were used, with micro- and mesozooplankton as prey for C. pengoi and mesozooplankton and C. pengoi as prey for YOY herring. These models indicate that mesozooplankton was the major food source of both species. However, microzooplankton may be important prey for young stages of C. pengoi. Comparative analyses of the herring trophic position before and after the invasion by C. pengoi showed a trophic level shift from 2.6 to 3.4, indicating substantial alterations in the food web structure. Our findings contribute to a growing body of evidence, showing that C. pengoi can modify food webs and trophic interactions in invaded ecosystems.

We studied the vertical distribution and settling of dominant diatoms and dinoflagellates during the 1996 spring phytoplankton bloom in the offshore NW Baltic Sea proper. We sampled phytoplankton at 11 depths (to 80 m) and collected settling cells in sediment traps at 25, 50 and 100 m depth, every week from March 26 to May 7. Phytoplankton were counted and sized from both water and trap samples, to estimate the share of phytoplankton in bulk settling carbon. Diatoms, mainly Chaetoceros spp. and Achnanthes taeniata, dominated the early bloom, but were replaced by the dinoflagellates cf. Scrippsiella hangoei and Peridiniella catenata when inorganic nitrogen was depleted above the seasonal pycnocline at ca. 10 m depth. By late April, vertically migrating dinoflagellates had depleted inorganic nitrogen down to 30 m, well below the seasonal pycnocline. We found clear species-specific sedimentation patterns. Scrippsiella hangoei and Chaetoceros spp., which dominated in the water column, were clearly underrepresented in the traps, while Thalassiosira baltica and T. levanderi, which were sparse in the water column, were overrepresented in sediment traps. Only 4, 3 and 0.5 g C m -2 (or 16, 12 and 2% of phytoplankton primary production) settled as intact phytoplankton cells at 25, 50 and 100 m, respectively, although these numbers may be overestimated due to migrating P. catenata. The settling bulk carbon was ~3 g C m -2 or 12% of the primary production at all depths. This is low compared to other estimates from coastal waters and suggests additional loss mechanisms, e.g. disintegration in the water column and grazing by zooplankton overwintering in the permanent halocline area.

No abstract

Ocean climate change strongly affects organisms and ecosystems, and the causes, consequences, and underlying mechanisms need to be documented. In the Baltic Sea, a marginal sea under severe eutrophication stress, a longer productive season, and changes in the phytoplankton community over the last few decades have likely impacted diet and condition of keystone species, from individual to population level. This study uses stable isotopes (δ 13 C, δ 15 N, and derived isotope niche metrics) to trace energy and nutrient flows in archived samples of blue mussel (Mytilus edulis trossulus) spanning 24 yr (1993-2016). We test if long-term changes in isotope and elemental composition in mussels, as well as population abundance and biomass, can be explained by changes in abiotic and biotic variables, using partial least square regressions and structural equation modeling. We found decreasing trends in δ 13 C and δ 15 N as well as in mean size and total biomass of mussels, but no unidirectional changes in their stoichiometry or condition index. Changes in isotope composition were best explained by nitrogen-fixing cyanobacteria, by increased terrestrial organic carbon from land runoff (reflecting precipitation) and by decreases in dissolved inorganic nitrogen (indicative of successful eutrophication mitigation) and in biomass of a mixotrophic ciliate species. The trophic niche (assessed from isotope niche) was included as the best predictor for both mussel body condition and the observed decline in their total biomass. This study reveals that altered trophic relationships from climate-induced changes in the productivity base may strongly impact keystone species, with potential knock-on effects on ecosystem functions.