Limnology and Oceanography最新文献

Doliolids have a unique ability to impact the marine microbial community through bloom events and filter feeding. Their predation on large eukaryotic microorganisms is established and evidence of predation on smaller prokaryotic microorganisms is beginning to emerge. We studied the association between microorganisms and wild-caught doliolids in the Northern California Current system. Doliolids were collected during bloom events identified at three different shelf locations with variable upwelling intensity. We discovered doliolids were associated with a range of prokaryotic microbial functional groups, which included free-living pelagic Archaea, SAR11, and picocyanobacteria. The results suggest the possibility that doliolids could feed on the smallest members of the microbial community, expanding our understanding of doliolid feeding and microbial mortality. Given the ability of doliolids to clear large portions of seawater by filtration and their high abundance in this system, we suggest that doliolids could be an important player in shaping the microbial community structure of the Northern California Current system.

The production of the secondary metabolite dimethylsulfoniopropionate (DMSP) by marine microalgae has a strong impact on the global sulfur cycle, as DMSP is the precursor of the climate active gas dimethylsulfide. Quantifying the impact of abiotic parameters on DMSP production is needed to accurately depict DMSP production in ecosystem models. In this study, we investigated if de novo production of DMSP was upregulated under short-term elevated irradiance and ultraviolet A radiation (UVA-R). We exposed high-light and low-light acclimated cultures of Emiliania huxleyi, Tetraselmis sp., Thalassiosira oceanica, and Phaeodactylum tricornutum to high irradiance and UVA-R treatments and followed de novo DMSP production and carbon fixation. We show that combined photosynthetically active radiation and UVA-R resulted in increased net photoinhibition rates, but decreased specific DMSP production and growth compared to non-UVA-R treatments for all species. Photoacclimation to high photosynthetically active radiation resulted in a decreased UVA-R sensitivity and positively affected the DMSP-to-carbon concentration ratios within the cultures. We conclude that there is no active short-term upregulation of DMSP production under elevated photosynthetically active radiation and UVA-R. Instead, the production of DMSP in response to light-stress is closely coupled to particulate organic carbon production in all cases. While the relatively high cellular concentrations of DMSP do not exclude a de facto antioxidant function, its production is likely regulated by other cellular processes, for example, an overflow mechanism. The data of this study aim to improve the mechanistic understanding of DMSP synthesis, as well as to quantify DMSP production rates in different marine phytoplankton species.

Water browning, induced by allochthonous dissolved organic carbon (DOC) input, has become a widespread phenomenon in boreal lakes over the past decades. Directly quantifying aquatic organisms' responses to increased DOC concentrations is essential for projecting carbon cycle processes in freshwater ecosystems. In this study, we assessed the impacts of DOC addition on the growth of three freshwater planktonic groups: phytoplankton, zooplankton, and bacteria, and explored potential drivers behind variations in effect size. Background DOC concentrations vary between 0.5 and 25 mg L⁻¹, while total phosphorus concentrations span from 0.0003 to 1.55 mg L⁻¹. Based on a meta-analysis of 804 observations from 47 publications, we found that DOC addition had a significant positive effect on bacteria, while it had a small but negative impact on both phytoplankton and zooplankton. In different climate zones, DOC addition often stimulated bacterial growth, but it exerted either positive or negative effects on phytoplankton and zooplankton. Additionally, the effect sizes of both phytoplankton and zooplankton showed a significant negative relationship with the magnitude of DOC enrichment, while bacteria exhibited positive responses. Furthermore, the effect sizes of these three taxa correlated negatively with background total phosphorus concentrations and positively with the DOC : total phosphorus ratio. A significant negative correlation between effect size and experimental duration was observed for bacteria. In summary, this synthesis indicates that excessive DOC loading can inevitably inhibit phytoplankton and zooplankton growth. Future studies should focus on the interactions between DOC addition and global change factors to improve forecasts of carbon-climate feedback in aquatic ecosystems.

Phytoplankton community size structure influences the production and fate of organic carbon in marine food webs and can undergo strong seasonal shifts in temperate regions. As part of the Northeast US Shelf (NES) Long-Term Ecological Research program, we measured net primary production (NPP) rates and chlorophyll a (Chl a) concentrations in three phytoplankton size classes (< 5, 5–20, and > 20 μm) during winter and summer for 3 yr along a coastal-to-offshore transect. Mean depth-integrated NPP was 37% higher in summer than winter, with limited cross-shelf differences because of significant interannual variability. When averaged across the shelf, depth-integrated NPP was dominated by the > 20 μm size class in winter and generated equally by the three size fractions in summer because of substantial contributions from cells > 20 μm at the Chl a maximum depth. Furthermore, the relationship between Chl a and NPP, in terms of relative contributions, varied by size class. Variations in this relationship have implications for models of primary productivity on the NES and beyond. In comparison to historical NPP data, we identified equivalent levels of winter NPP but observed a 25% decrease in summer NPP, suggesting a potential reduction in the seasonality of NPP on the NES. Together, our results highlight seasonal shifts in NPP rates of different phytoplankton size classes, with implications for food web structure and export production. These data emphasize the importance of quantifying size-fractionated NPP over time to constrain its variability and better predict the fate of organic carbon in coastal systems under environmental change.