Limnology and Oceanography最新文献

In aquatic ecosystems, allochthonous nutrient transport to the euphotic zone is an important process that fuels new production. Here, we use high-resolution physical and biogeochemical observations from five summers to estimate the mean vertical nitrate flux, and thus new production over the Northeast U.S. Shelf (NES). We find that the summertime nitrate field is primarily controlled by biological uptake and physical advection–diffusion processes, above and below the 1% light level depth, respectively. We estimate the vertical nitrate flux to be 8.2 ± 5.3 × 10⁻⁶ mmol N m⁻² s⁻¹ for the mid-shelf and 12.6 ± 8.6 × 10⁻⁶ mmol N m⁻² s⁻¹ for the outer shelf. Furthermore, we show that the new production to total primary production ratio (i.e., the f-ratio), consistently ranges between 10% and 15% under summer conditions on the NES. Two independent approaches—nitrate flux-based new production and O₂/Ar-based net community production—corroborate the robustness of the f-ratio estimation. Since ~ 85% of the total primary production is fueled by recycled nutrients over sufficiently broad spatial and temporal scales, less than 15% of the organic matter produced in summer is available for export from the NES euphotic zone. Our direct quantification of new production not only provides more precise details about key processes for NES food webs and ecosystem function, but also demonstrates the potential of this approach to be applied to other similar datasets to understand nutrient and carbon cycling in the global ocean.

The capacity of aquatic systems to buffer acidification depends on the sum contributions of various chemical species to total alkalinity (TA). Major TA contributors are inorganic, with carbonate and bicarbonate considered the most important. However, growing evidence shows that many rivers, estuaries, and coastal waters contain dissolved organic molecules with charge sites that create organic alkalinity (OrgAlk). This study describes the first comparison of (1) OrgAlk distributions and (2) acid–base properties in contrasting estuary-plume systems: the Pleasant (Maine, USA) and the St. John (New Brunswick, CA). The substantial concentrations of OrgAlk in each estuary were sometimes not conservative with salinity and typically associated with very low pH. Two approaches to OrgAlk measurement showed consistent differences, indicating acid–base characteristics inconsistent with the TA definition. The OrgAlk fraction of TA ranged from 78% at low salinity to less than 0.4% in the coastal ocean endmember. Modeling of titration data identified three groups of organic charge sites, with mean acid–base dissociation constants (pK_a) of 4.2 (± 0.5), 5.9 (± 0.7) and 8.5 (± 0.2). These represented 21% (± 9%), 8% (± 5%), and 71% (± 11%) of titrated organic charge groups. Including OrgAlk, pK_a, and titrated organic charge groups in carbonate system calculations improved estimates of pH. However, low and medium salinity, organic-rich samples demonstrated persistent offsets in calculated pH, even using dissolved inorganic carbon and CO₂ partial pressure as inputs. These offsets show the ongoing challenge of carbonate system intercomparisons in organic rich systems whereby new techniques and further investigations are needed to fully account for OrgAlk in TA titrations.

Saline wetlands play a crucial role in climate regulation through their robust cooling effect, attributed to rapid carbon sequestration and minimal methane production. However, a comprehensive understanding of the mechanisms controlling their greenhouse gas (GHG) balance is lacking, particularly in salt marshes that are fully or partially submerged due to rising sea levels. We conducted a controlled manipulative experiment to test the effect of water levels on GHG emissions, including four water table levels: -10, 0, +5 cm and a fluctuating water table. We used soil cores from a Spartina anglica-dominated salt marsh and examined the CO₂ and CH₄ fluxes over a growing season. Daylight CO₂ uptake and dark CO₂ emission were highest at the -10cm water table, while CH₄ emissions were lowest at this water table. CO₂ and CH₄ fluxes were primarily driven by air and water temperature and solar irradiance. Our results indicate that salt marshes with near-surface water levels (-10 to 5 cm) function as potent CO₂ sinks and minor sources of CH₄ during the growing season. The high photosynthetic carbon assimilation combined with low CH₄ fluxes resulted in a Global Warming Potential value of -326 g CO₂eq m⁻² on a 100-year scale. Our study accounted for CH₄ fluxes, CO₂ uptake and emission together, and identified the mechanisms controlling CO₂ and CH₄ exchange. This approach is crucial for evaluating the potential of saline tidal wetlands as net carbon sinks and for developing scientifically sound climate mitigation policies.

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.

The recovery of isolated reef systems is a complex process that is usually associated with the supply of coral larvae from distant reefs (or large-scale connectivity). However, a frequently neglected process is the potential for supply within the reef itself (or local connectivity). In this study, we quantify and characterize the role of local connectivity over 21 yr of simulated annual coral spawning on an isolated coral reef atoll using outputs from a high-resolution biophysical model (< 150 m horizontal resolution) along with network analysis. We find that approximatively half of the coral reef larvae dispersal remains local (within 100 s m to 10 s km of release location), while the remaining half contributes to long-distance dispersal (> 100 s km) and is exported away from the system. Local dispersal plays a pivotal role in creating a highly-connected network across the reef, enhancing exchanges of larvae within the same reef patches (local retention), across reef zones (e.g., lagoon, reef flat), and across the larger reef system. Finally, we show that this highly-connected network exhibits a certain level of robustness, even when exposed to environmental stressors such as thermal-induced mortality. Our findings highlight the previously overlooked role of local scale dispersal in driving recovery of isolated reef systems and emphasize the importance of targeted local management actions, indicating that efforts directed at enhancing and preserving local connectivity can have a substantial impact on the overall health and resilience of isolated reef 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.