Changes in nutritional quality and nutrient limitation regimes of phytoplankton in response to declining N deposition in mountain lakes

Abstract

Phytoplankton play a key role in supporting aquatic food webs. However, the effects of ongoing large-scale changes in the concentrations and stoichiometry of important biological compounds [dissolved inorganic N (DIN), total phosphorus (TP), dissolved organic carbon (DOC) and DIN:TP] on the development and nutritional quality of phytoplankton for higher trophic levels are unclear. We conducted lake studies and in situ bioassay experiments in two Swedish mountain regions [Abisko (north) and Jämtland (south)] with different N deposition and where lakes in each region were distributed along a similar gradient in lake DOC (2-7 mg L −1) to assess whether differences in nutrients, DOC and DIN:TP induced differences in phytoplankton quantity [chlorophyll a (Chl-a) and seston carbon (C)] and quality [seston C:N:P stoichiometry and fatty acid (FA) composition]. Using long-term monitoring data from lakes in these two mountain regions, we found declining longterm trends in N deposition and lake DIN and total TP concentrations, but not in lake DIN:TP. Lakes in Abisko received lower N deposition and had lower DIN:TP than those in Jämtland. Phytoplankton was N-to NP-limited in Abisko lakes but NP dual-limited in Jämtland lakes. The N fertilization effects induced by higher DIN:TP were weak on phytoplankton quantity but strong on phytoplankton quality. The phytoplankton had lower eicosapentaenoic acid (EPA) content and higher P content (lower seston C:P) in Abisko compared to in Jämtland. In addition, the quality of the DOC (as indicated by its aromaticity and SUVA) influenced not only the light conditions and the seston C:P ratios, but also the FA composition. We found higher bacteria FA concentrations in seston in Abisko than in Jämtland, despite lower amounts of FA of terrestrial origin in Abisko. Our findings suggest that declining N deposition and enhanced colored terrestrial C loadings leads to lower nutritional quality of basal resources for higher consumers in mountain lakes.

Lowered nutritional quality of plankton caused by global environmental changes

“…Methods for FA analysis of seston and zooplankton (or invertebrates) are reported in Bergström et al (2020) and Grieve and Lau (2018 Detailed descriptions of the capillary column and the GC-MS program are given by Bergström et al (2020). Contents of individual FA in seston and zooplankton are reported as mg FA g −1 seston C and mg FA g −1 dry mass, respectively.…”

Nutritional quality of littoral macroinvertebrates and pelagic zooplankton in subarctic lakes

“…The relative importance of the two habitats (pelagic and littoral) for consumers is particularly interesting in northern ecosystems, which are experiencing rapid environmental changes, i.e., rising temperatures and precipitation, increasing terrestrial runoff and potential shifts in primary production toward pelagic dominance (Creed et al 2018; Hayden et al 2019). These may have profound effects on lake food webs, especially on the importance and quality of both pelagic and littoral primary producers as well as bacterial contribution to basal resources, available for invertebrate consumers and fish in the higher trophic levels (Ask et al 2009; Creed et al 2018; Hayden et al 2019; Bergström et al 2020).…”

What Colin Reynolds could tell us about nutrient limitation, N:P ratios and eutrophication control

“…The effect of P-limitation on phytoplankton species composition and food quality for higher trophic levels has been well studied and described (Lukas et al, 2011a, b;Sommer et al, 2012). Bergström et al (2020) analysed N-and P-limitation in two Swedish mountain regions in the context of declining loads of macro-nutrients and found that a proportionally stronger decrease of N (leading to more phases of N-limitation) has a pronounced impact on phytoplankton quality (including as nutrition for higher trophic levels). Elser et al (2009) pointed out that relatively few phytoplankton species have an enhanced ability to capture P, and where excess N shifts waterbodies to P-limitation, these will be favoured, outcompeting others.…”