Limnology and Oceanography最新文献

The latitudinal diversity gradient predicts that tropical regions should have higher alpha, beta, and gamma diversity than temperate areas. However, only a few studies have assessed the temporal variability of the different components of diversity across climatic regions. In this study, we compare, using a spatial and temporal approach, the diversity of multiple taxa inhabiting tropical and Mediterranean temporary ponds. We sampled the biological communities of each set of ponds on three occasions during the same hydrological year. Under a spatial framework, we analyzed, alpha, beta, and gamma diversities. With a temporal approach, we compared the coefficients of variation in alpha diversity for each local community, and temporal beta diversity. Differences between regions and sampling periods were tested using generalized linear mixed models. We found higher gamma and alpha diversity in the tropical ponds, as expected given the latitudinal differences between them. However, phytoplankton and microinvertebrates from the Mediterranean region, matched or even exceeded tropical alpha diversity on some occasions. Spatial beta diversity did not differ between regions, and it showed lower values at the middle or the end of the hydroperiod in bacteria, micro- and macroinvertebrates and amphibians. Thus, processes homogenizing and heterogenising pond metacommunities must be balanced in both studied regions. Temporal variation in alpha and beta diversity was similar for ponds in both regions, except for macroinvertebrates and amphibians, suggesting differential effects on community variation observable only in animals with longer life-spans, at our temporal scale of analysis.

The Laurentian Great Lakes provide economic support to millions of people, drive biogeochemical cycling, and are an important natural laboratory for characterizing the fundamental components of aquatic ecosystems. Small phytoplankton are important contributors to the food web in much of the Laurentian Great Lakes. Here, for the first time, we reveal and quantify eight phenotypically distinct picophytoplankton populations across the Lakes using a multilaser flow cytometry approach, which distinguishes cells based on their pigment phenotype. The distributions and diversity of picophytoplankton flow populations varied across lakes and depths, with Lake Erie standing out with the highest diversity. By sequencing sorted cells, we identified several distinct lineages of Synechococcales spanning Subclusters 5.2 and 5.3. Distinct genotypic clusters mapped to phenotypically similar flow populations, suggesting that there may not be a clear one-to-one mapping between genotypes and phenotypes. This suggests genome-level differentiation between lakes but some degree of phenotypic convergence in pigment characteristics. Our results demonstrate that ecological selection for locally adapted populations may outpace homogenization by physical transport in this interconnected system. Given the reliance of the Lakes on in situ primary production as a source for organic carbon, this work sets the foundation to test how the community structure of small primary producers corresponds to biogeochemical and food web functions of the Great Lakes and other freshwater systems.

Nutrient loading of freshwater and marine habitats has increased during the last century as a result of anthropogenic activities. From the 1980s onwards, following implementation of new policy targeting eutrophication, total phosphorus (TP) and total nitrogen (TN) loads were reduced in many European waters. Often, however, decreases in TP were stronger as compared to TN, leading to increased TN : TP ratios. Our analysis shows that the large and shallow lake IJsselmeer (the Netherlands) experienced a similar trend, whereas TN was reduced by 50%, TP was reduced by 89% between 1975 and 2018. Most of this nutrient load reduction was achieved before the year 2000, changes in nutrient concentrations in the lake became smaller afterwards, especially for TN, leading to a further increase in stoichiometric imbalance up to a yearly averaged TN : TP (molar) of 296 in 2018. The observed changes in nutrients were accompanied by a decline in total phytoplankton biomass, and slight declines in phytoplankton genus evenness and diversity. Although biomass decreases likely resulted from the overall decrease in nutrient availabilities, the reduced diversity may have resulted from the shift toward very high TN : TP ratios that indicate relatively low TP levels and enhanced competition for phosphorus. Overall, our findings demonstrate long-term trends with decreased phytoplankton biomass and diversity following reduced nutrient concentrations and enhanced stoichiometric imbalance. Ultimately, such changes at the food web base may alter the structure and functioning of the entire aquatic food- web in lake IJsselmeer.

Light availability for phytoplankton in shallow lakes is closely related to non-algal light attenuation (Kd_NALA, the fractional light absorption by non-algal substances, such as suspended sediments); thus, significant changes in global wind speed in recent decades may have a profound effect on light availability and algal blooms in shallow lakes. Herein, the eutrophic shallow Lake Taihu was selected to investigate the long-term dynamics of light availability and its effect on algal blooms. The results showed that Kd_NALA decreased from approximately 3.5 to 2.5 m⁻¹ with decreasing wind speed from 2005 to 2021, indicating the significantly increased light availability for phytoplankton despite surface photosynthetically active radiation exhibiting limited variability during the study period. In addition, both experimental (i.e., in situ nutrient enrichment experiments) and statistical approaches (i.e., deviations of Trophic State Index subindices) indicated that phytoplankton growth was primarily light-limited during the study period in Lake Taihu. Consequently, considerable increases in fraction of observed and maximal chlorophyll a yield at given nitrogen or phosphorus concentration were observed and were mostly related to decreasing Kd_NALA, which implied that decreasing Kd_NALA allowed phytoplankton utilize “unused nutrient-capacity” until the additional algal-turbidity induce further light limitation or nutrient limitation. As the effect of changes in global wind speed on Kd_NALA and algal growth received limited attention in the existing research, we revealed an underappreciated mechanism by which global changes in wind speed significantly affects algal biomass by influencing light availability, which may have profound effects on future algal bloom mitigation efforts in shallow lakes.

Global warming causes dramatic environmental change to Arctic ecosystems. While pelagic primary production is initiated earlier and its intensity can be increased due to earlier ice melt and extended open-water periods, sea-ice primary production is progressively confined on a spatio-temporal scale, leading to unknown consequences for the ice-associated (sympagic) food web. Understanding ecological responses to changes in the availability and composition of pelagic and sympagic food sources is crucial to determine potential changes of food-web structure and functioning in Arctic marine communities under increasingly ice-free conditions. Focus was placed on the importance of suspended particulate organic matter vs. sympagic organic matter for 12 zooplankton species with different feeding modes covering five taxonomic groups (copepods, krill, amphipods, chaetognaths, and appendicularians) at two ice-covered, but environmentally different, stations in the north-western Barents Sea in August 2019. Contributions of diatom- and flagellate-associated fatty acids (FAs) to total lipid content and carbon stable isotopic compositions of these FAs were used to discriminate food sources and trace flows of organic matter in marine food webs. Combination of proportional contributions of FA markers with FA isotopic composition indicated that consumers mostly relied, directly (herbivorous species), or indirectly (omnivorous and carnivorous species), on pelagic diatoms and flagellates, independently of environmental conditions at the sampling locations, trophic position, and feeding mode. Differences were nevertheless observed between species. Contrary to other studies demonstrating a high importance of sympagic organic matter for food-web processes, our results highlight the complexity and variability of trophic structures and dependencies in different Arctic food webs.

In estuaries, local processes such as changing material loads from the watershed and complex circulation create dynamic environments with respect to ecosystem metabolism and carbonate chemistry that can strongly modulate impacts of global atmospheric CO₂ increases on estuarine pH. Long-term (> 20 yr) surface water pH records from the USA's two largest estuaries, Chesapeake Bay (CB) and Neuse River Estuary-Pamlico Sound (NRE-PS) were examined to understand the relative importance of atmospheric forcing vs. local processes in controlling pH. At the estuaries’ heads, pH increases in CB and decreases in NRE-PS were driven primarily by changing ratios of river alkalinity to dissolved inorganic carbon concentrations. In upper reaches of CB and middle reaches of the NRE-PS, pH increases were associated with increases in phytoplankton biomass. There was no significant pH change in the lower NRE-PS and only the polyhaline CB showed a pH decline consistent with ocean acidification. In both estuaries, interannual pH variability showed robust, positive correlations with chlorophyll a (Chl a) during the spring in mid to lower estuarine regions indicative of strong control by net phytoplankton production. During summer and fall, Chl a and pH negatively correlated in lower regions of both estuaries, given a shift toward heterotrophy driven by changes in phytoplankton community structure and increases in the load ratio of dissolved inorganic nitrogen to organic carbon. Tropical cyclones episodically depressed pH due to vertical mixing of CO₂ rich bottom waters and post-storm terrestrial organic matter loading. Local processes we highlight represent a significant challenge for predicting future estuarine pH.