Emma Wiik

Chemical and Biological Responses of Marl Lakes to Eutrophication

Abstract Eutrophication remains one of the foremost environmental issues threatening the quality of surface waters yet comparatively little is known of the timing, magnitude and characteristics of nutrient-related changes in highly calcareous (marl) lakes. This review focuses on marl lake ecology and chemistry, their known responses to eutrophication, and also highlights questions that remain unanswered. In good condition, marl lakes support a diversity of macrophytes, especially Characeae and Potamogetonaceae, which can grow to considerable depth. High water transparency and low phosphorus and phytoplankton concentrations are facilitated by the coprecipitation of marl and phosphorus. Although large amounts of phosphorus can be thus removed, buffering against eutrophication, macrophyte communities can undergo significant change under rather low nutrient concentrations. Maximum colonisation depth declines and tolerant species replace sensitive species, with losses particularly among charophytes. Marl lakes are therefore ecologically highly sensitive. The effects of coprecipitation on long-term burial of phosphorus are contested. Several palaeolimnological studies have identified iron complexes as more important than calcite, as chemical conditions in the sediment may promote either calcite dissolution or calcite-bound phosphorus exchange, or possibly both. Some marl lakes have been shown to have phosphorus concentrations which, compared with other lake types, are higher than expected in winter and lower in summer. The phosphorus binding capacity of marl sediment has not to our knowledge been adequately researched. Marl precipitation may be inhibited by high phosphate or organic matter concentrations in the water, or when biological communities effecting precipitation (picoplankton, charophytes, epiphytes) are disturbed. Highly impacted marl lakes having low species diversity and lacking precipitation may be misidentified as eutrophic, high-alkalinity lakes. More studies addressing the interaction between external loading, phosphorus cycling and marl precipitation in relation to biological communities are required to assess to what extent marl lakes can buffer eutrophication, and what factors contribute to disturbed marl precipitation.

The coming and going of a marl lake: multi-indicator palaeolimnology reveals abrupt ecological change and alternative views of reference conditions

Eutrophication is the most pressing threat to highly calcareous (marl) lakes in Europe. Despite their unique chemistry and biology, comprehensive studies into their unimpacted conditions and eutrophication responses are underrepresented in conservation literature. A multi-indicator palaeolimnological study spanning ca 1260 to 2009 was undertaken at Cunswick Tarn (UK), a small, presently eutrophic marl lake, in order to capture centennial timescales of impact. Specific aims were to 1) establish temporal patterns of change (gradual/abrupt) across biological groups, thereby testing theories of resistance of marl lake benthic communities to enrichment, and 2) compare the core record of reference condition with prevailing descriptions of high ecological status. Analyses of sediment calcium (Ca), phosphorus (P), pigments, diatoms, testate amoebae, cladocerans, and macrofossils, revealed three abrupt changes in ecosystem structure. The first (1900s), with biomass increases in charophytes and other benthic nutrient-poor indicators, supported ideas of resistance to eutrophication in Chara lakes. The second transition (1930s), from charophyte to angiosperm dominance, occurred alongside reductions in macrophyte cover, increases in eutrophic indicators, and a breakdown in marling, in support of ideas of threshold responses to enrichment. Core P increased consistently into the 1990s when rapid transitions into pelagic shallow lake ecology occurred and Cunswick Tarn became biologically unidentifiable as a marl lake. The moderate total P at which these changes occurred suggests high sensitivity of marl lakes to eutrophication. Further, the early record challenges ideas of correlation between ecological condition, charophyte biomass and sediment Ca. Instead, low benthic production, macrophyte cover, and Ca sedimentation, was inferred. Management measures must focus on reducing external nutrient and sediment loads at early stages of impact in order to preserve marl lakes.

Effects of experimental nitrogen fertilization on planktonic metabolism and CO2 flux in a hypereutrophic hardwater lake

Hardwater lakes are common in human-dominated regions of the world and often experience pollution due to agricultural and urban effluent inputs of inorganic and organic nitrogen (N). Although these lakes are landscape hotspots for CO2 exchange and food web carbon (C) cycling, the effect of N enrichment on hardwater lake food web functioning and C cycling patterns remains unclear. Specifically, it is unknown if different eutrophication scenarios (e.g., modest non point vs. extreme point sources) yield consistent effects on auto- and heterotrophic C cycling, or how biotic responses interact with the inorganic C system to shape responses of air-water CO2 exchange. To address this uncertainty, we induced large metabolic gradients in the plankton community of a hypereutrophic hardwater Canadian prairie lake by adding N as urea (the most widely applied agricultural fertilizer) at loading rates of 0, 1, 3, 8 or 18 mg N L-1 week-1 to 3240-L, in-situ mesocosms. Over three separate 21-day experiments, all treatments of N dramatically increased phytoplankton biomass and gross primary production (GPP) two- to six-fold, but the effects of N on autotrophs plateaued at ~3 mg N L-1. Conversely, heterotrophic metabolism increased linearly with N fertilization over the full treatment range. In nearly all cases, N enhanced net planktonic uptake of dissolved inorganic carbon (DIC), and increased the rate of CO2 influx, while planktonic heterotrophy and CO2 production only occurred in the highest N treatments late in each experiment, and even in these cases, enclosures continued to in-gas CO2. Chemical effects on CO2 through calcite precipitation were also observed, but similarly did not change the direction of net CO2 flux. Taken together, these results demonstrate that atmospheric exchange of CO2 in eutrophic hardwater lakes remains sensitive to increasing N loading and eutrophication, and that even modest levels of N pollution are capable of enhancing autotrophy and CO2 in-gassing in P-rich lake ecosystems.

Generalized Additive Models of Climatic and Metabolic Controls of Subannual Variation in pCO2 in Productive Hardwater Lakes

Spatio-temporal variation in climate and weather, allochthonous carbon loads, and autochthonous factors such as lake metabolism (photosynthesis and respiration) interact to regulate atmospheric CO2 exchange of lakes. Understanding this interplay in diverse basin types at different timescales is required to adequately place lakes into the global carbon cycle, and predict CO2 flux through space and time. We analyzed 18 years of data from seven moderately hard lakes in an agricultural prairie landscape in central Canada. We applied generalized additive models and sensitivity analyses to evaluate the roles of metabolic and climatic drivers in regulating CO2 flux at the intra-annual scale. In all basins, at mean conditions with respect to other predictors, metabolic controls resulted in uptake of atmospheric CO2 when surface waters exhibited moderate primary production, but released CO2 only when primary production was very low (5 − 13 μg L−1) or when dissolved nitrogen was elevated (>2000 μg L−1), implying that respiratory controls offset photosynthetic CO2 uptake under these conditions. Climatically, dry conditions increased the likelihood of ingassing, likely due to evaporative concentration of base cations and/or reduced allochthonous carbon loads. While more research is required to establish the relative importance Pavillon des sciences biologiques (SB), Université du Québec à Montréal, Montréal (Québec ), H2X 1Y4, Canada c ©2018 American Geophysical Union. All Rights Reserved. of climate and metabolism at other time scales (diel, autumn/winter), we conclude that these hard fresh waters characteristic of continental interiors are mainly affected by metabolic drivers of pCO2 at daily-monthly timescales, are climatically controlled at interannual intervals, and are more likely to ingas CO2 for a given level of algal abundance, than are softwater, boreal ecosystems. Keypoints: • In Canadian hardwater prairie lakes, calculated CO2 fluxes correlate mostly with pH, not DIC • Intra-annual CO2 correlates with algal abundance (-CO2) and prolonged clearwater phases (+CO2) • CO2 influx increases with drier weather conditions, and is reduced with extreme N loading c ©2018 American Geophysical Union. All Rights Reserved.