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"Jeppesen, Erik"
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Low shifts in salinity determined assembly processes and network stability of microeukaryotic plankton communities in a subtropical urban reservoir
Background
Freshwater salinization may result in significant changes of microbial community composition and diversity, with implications for ecosystem processes and function. Earlier research has revealed the importance of large shifts in salinity on microbial physiology and ecology, whereas studies on the effects of smaller or narrower shifts in salinity on the microeukaryotic community in inland waters are scarce. Our aim was to unveil community assembly mechanisms and the stability of microeukaryotic plankton networks at low shifts in salinity.
Results
Here, we analyzed a high-resolution time series of plankton data from an urban reservoir in subtropical China over 13 consecutive months following one periodic salinity change ranging from 0 to 6.1‰. We found that (1) salinity increase altered the community composition and led to a significant decrease of plankton diversity, (2) salinity change influenced microeukaryotic plankton community assembly primarily by regulating the deterministic-stochastic balance, with deterministic processes becoming more important with increased salinity, and (3) core plankton subnetwork robustness was higher at low-salinity levels, while the satellite subnetworks had greater robustness at the medium-/high-salinity levels. Our results suggest that the influence of salinity, rather than successional time, is an important driving force for shaping microeukaryotic plankton community dynamics.
Conclusions
Our findings demonstrate that at low salinities, even small increases in salinity are sufficient to exert a selective pressure to reduce the microeukaryotic plankton diversity and alter community assembly mechanism and network stability. Our results provide new insights into plankton ecology of inland urban waters and the impacts of salinity change in the assembly of microbiotas and network architecture.
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Video abstract
Journal Article
Persistent internal phosphorus loading during summer in shallow eutrophic lakes
Nutrient availability, in particular of phosphorus (P), is a key factor for the structure and functioning of shallow lakes, and not least the sediment plays an important role by acting as both a nutrient source and sink. We used 21 years of monthly mass balance and lake water data from six shallow (mean depth = 1.2–2.7 m) and fast flushed (mean hydraulic retention time = 0.6–2.6 months) eutrophic Danish lakes (mean summer P concentrations ranging from 0.09 to 0.61 mg/l) to investigate long-term trends in yearly and seasonal patterns of P retention. To one of the lakes, the external P input was reduced by 70% in the early 1990s, whereas none of the other lakes have experienced major changes in external P loading for more than 20 years. All lakes showed a distinct seasonal pattern with high P concentrations and typically negative P retention during summer (up to −300% of the external loading from May to August). During winter, P retention was overall positive (up to 50% of the external loading from December to April). Internal P loading from the sediment delayed lake recovery by approximately 10 years in the lake with the most recently reduced external loading, but in all the lakes net release of P from the sediment occurred during summer. P release in the six lakes has not abated during the past decade, indicating that the sediment of eutrophic and turbid shallow lakes remains a net source of P during summer. The seasonal variations in P retention became more pronounced with increasing P levels, and retention decreased with increasing temperature, but increased if clear water conditions were established.
Journal Article
Consistent stoichiometric long-term relationships between nutrients and chlorophyll-a across shallow lakes
Aquatic ecosystems are threatened by eutrophication from nutrient pollution. In lakes, eutrophication causes a plethora of deleterious effects, such as harmful algal blooms, fish kills and increased methane emissions. However, lake-specific responses to nutrient changes are highly variable, complicating eutrophication management. These lake-specific responses could result from short-term stochastic drivers overshadowing lake-independent, long-term relationships between phytoplankton and nutrients. Here, we show that strong stoichiometric long-term relationships exist between nutrients and chlorophyll
a
(Chla) for 5-year simple moving averages (SMA, median
R²
= 0.87) along a gradient of total nitrogen to total phosphorus (TN:TP) ratios. These stoichiometric relationships are consistent across 159 shallow lakes (defined as average depth < 6 m) from a cross-continental, open-access database. We calculate 5-year SMA residuals to assess short-term variability and find substantial short-term Chla variation which is weakly related to nutrient concentrations (median
R²
= 0.12). With shallow lakes representing 89% of the world’s lakes, the identified stoichiometric long-term relationships can globally improve quantitative nutrient management in both lakes and their catchments through a nutrient-ratio-based strategy.
Nutrient limitation is a well-known control of phytoplankton growth, but predicting specific responses in individual lakes based on nutrient data alone has proven challenging. Here, the authors show that long-term signals of chlorophyll-a dynamics in shallow lakes can be captured based on stoichiometric effects of N and P concentrations along a continuum of total N:total P ratios.
Journal Article
Global lake phytoplankton proliferation intensifies climate warming
In lakes, phytoplankton sequester atmospheric carbon dioxide (CO
2
) and store it in the form of biomass organic carbon (OC); however, only a small fraction of the OC remains buried, while the remaining part is recycled to the atmosphere as CO
2
and methane (CH
4
). This has the potential effect of adding CO
2
-equivalents (CO
2
-eq) to the atmosphere and producing a warming effect due to the higher radiative forcing of CH
4
relative to CO
2
. Here we show a 3.1-fold increase in CO
2
-eq emissions over a 100-year horizon, with the effect increasing with global warming intensity. Climate warming has stimulated phytoplankton growth in many lakes worldwide, which, in turn, can feed back CO
2
-eq and create a positive feedback loop between them. In lakes where phytoplankton is negatively impacted by climate warming, the CO
2
-eq feedback capacity may diminish gradually with the ongoing climate warming.
In lakes, phytoplankton sequester atmospheric carbon dioxide as organic carbon, but most of this organic carbon is recycled back to the atmosphere as carbon dioxide and methane, contributing to warming due to the higher radiative forcing of methane.
Journal Article
Nitrogen or phosphorus limitation in lakes and its impact on phytoplankton biomass and submerged macrophyte cover
We used data on nutrients, chlorophyll
a
(Chla) and submerged macrophyte cover from up to 817 Danish lakes to elucidate seasonal variations in nitrogen (N) and phosphorus (P) concentrations and to study the impact of N or its role in combination with P. In both deep and shallow lakes, we found marked seasonality in the ratio between total N and total P (TN:TP) and in the inorganic concentrations of nitrogen (DIN), indicating that N more easily becomes a limiting nutrient as summer proceeds. TN:TP reached its lowest values of <7 (by mass) in August in 25% of the shallow lakes. Chla generally related more strongly to TP than to TN, but at high TP concentrations TN explained more of the variability in Chla than TP. Macrophyte cover tended to decrease at increasing TN when TP was between 0.1 and 0.4 mg/l. At macrophyte cover above 20%, Chla was considerably lower compared with lakes with low macrophyte cover. We conclude that P is of key importance for the ecological quality of Danish lakes but that increased N concentrations, particularly in shallow lakes with moderate to high TP, may have significantly adverse effects on lake water quality and ecological status in summer.
Journal Article
Bimodality and alternative equilibria do not help explain long-term patterns in shallow lake chlorophyll-a
Since its inception, the theory of alternative equilibria in shallow lakes has evolved and been applied to an ever wider range of ecological and socioecological systems. The theory posits the existence of two alternative stable states or equilibria, which in shallow lakes are characterised by either clear water with abundant plants or turbid water where phytoplankton dominate. Here, we used data simulations and real-world data sets from Denmark and north-eastern USA (902 lakes in total) to examine the relationship between shallow lake phytoplankton biomass (chlorophyll-a) and nutrient concentrations across a range of timescales. The data simulations demonstrated that three diagnostic tests could reliably identify the presence or absence of alternative equilibria. The real-world data accorded with data simulations where alternative equilibria were absent. Crucially, it was only as the temporal scale of observation increased (>3 years) that a predictable linear relationship between nutrient concentration and chlorophyll-a was evident. Thus, when a longer term perspective is taken, the notion of alternative equilibria is not required to explain the response of chlorophyll-a to nutrient enrichment which questions the utility of the theory for explaining shallow lake response to, and recovery from, eutrophication.
Shallow lakes have long been considered an example of alternative equilibria in ecological systems. Here, the authors combine empirical data and simulations to show that the relationship of shallow lake chlorophyll-a with nutrient enrichment does not fit the theory of alternative stable states.
Journal Article
Significant fraction of CO2 emissions from boreal lakes derived from hydrologic inorganic carbon inputs
Lakes are a large source of CO
2
. An analysis of chemical and physical data from 5,118 boreal lakes reveals that a majority emit CO
2
originating primarily from terrestrial sources rather than CO
2
produced within the lakes.
Annual CO
2
emissions from lakes and other inland waters into the atmosphere are estimated to almost entirely compensate the total annual carbon uptake by oceans
1
,
2
,
3
. CO
2
supersaturation in lakes, which results in CO
2
emissions, is frequently attributed to CO
2
produced within the lake
4
,
5
,
6
,
7
,
8
. However, lateral inorganic carbon flux through watersheds can also be sizeable
9
,
10
,
11
. Here we calculated lake surface water CO
2
concentrations and emissions using lake pH, alkalinity and temperature from a compilation of data from 5,118 boreal lakes
12
. Autumn surface water CO
2
concentrations and CO
2
emissions from the 5,118 lakes co-varied with lake internal autumn CO
2
production. However, using a mass balance approach we found that CO
2
emission in the majority of lakes was sustained by inorganic carbon loading from the catchment rather than by internal CO
2
production. Small lakes with high dissolved organic carbon and phosphorus concentrations, shorter retention times and longer ice-free seasons had the highest CO
2
concentrations. CO
2
emissions from these small lakes was twice that of comparable lakes in colder regions, and similar to emissions from subtropical and tropical lakes. We conclude that changes in land use and climate that increase dissolved inorganic carbon may cause emission levels from boreal lakes to approach those of lakes in warmer regions.
Journal Article
Beyond the Plankton Ecology Group (PEG) Model: Mechanisms Driving Plankton Succession
The seasonal succession of plankton is an annually repeated process of community assembly during which all major external factors and internal interactions shaping communities can be studied. A quarter of a century ago, the state of this understanding was described by the verbal plankton ecology group (PEG) model. It emphasized the role of physical factors, grazing and nutrient limitation for phytoplankton, and the role of food limitation and fish predation for zooplankton. Although originally targeted at lake ecosystems, it was also adopted by marine plankton ecologists. Since then, a suite of ecological interactions previously underestimated in importance have become research foci: overwintering of key organisms, the microbial food web, parasitism, and food quality as a limiting factor and an extended role of higher order predators. A review of the impact of these novel interactions on plankton seasonal succession reveals limited effects on gross seasonal biomass patterns, but strong effects on species replacements.
Journal Article