Effects of nitrogen enrichment on zooplankton biomass and N:P recycling ratios across a DOC gradient in northern-latitude lakes

Abstract

We used data from whole-lake studies to assess how changes in food quantity (phytoplankton biomass) and quality (phytoplankton community composition, seston C:P and N:P) with N fertilization affect zooplankton biomass, community composition and C:N:P stoichiometry, and their N:P recycling ratio along a gradient in lake DOC concentrations. We found that despite major differences in phytoplankton biomass with DOC (unimodal distributions, especially with N fertilization), no major differences in zooplankton biomass were detectable. Instead, phytoplankton to zooplankton biomass ratios were high, especially at intermediate DOC and after N fertilization, implying low trophic transfer efficiencies. An explanation for the observed low phytoplankton resource use, and biomass responses in zooplankton, was dominance of colony forming chlorophytes of reduced edibility at intermediate lake DOC, combined with reduced phytoplankton mineral quality (enhanced seston N:P) with N fertilization. N fertilization, however, increased zooplankton N:P recycling ratios, with largest impact at low DOC where phytoplankton benefitted from light sufficiently to cause enhanced seston N:P. Our results suggest that although N enrichment and increased phytoplankton biomass do not necessarily increase zooplankton biomass, bottom-up effects may still impact zooplankton and their N:P recycling ratio through promotion of phytoplankton species of low edibility and altered mineral quality.

Increased Nitrogen Loading Boosts Summer Phytoplankton Growth by Alterations in Resource and Zooplankton Control: A Mesocosm Study

“…Combined with the low total and herbivorous zooplankton to phytoplankton biomass ratios in the treatments with N addition, the results indicate that phytoplankton was also favored by the reduced herbivory. Similarly, Bergström et al (2021) found much lower zooplankton to phytoplankton biomass ratios after wholelake fertilization with N along a gradient of lake DOC concentrations. It could be argued that the increased Chl-a to TP and TN ratios by N addition were not due to changes in zooplankton herbivory, but instead were a result of changes of cellular pigment contents between phytoplankton groups (Nicholls and Dillon, 1978).…”

Crustacean zooplankton densities in northern temperate lakes are related to habitat temperature across a wide gradient in lake dissolved organic carbon and nutrient content

“…However, the increases in DOC and resulting changes in resources were relatively subtle compared to the large gradient of DOC concentrations observed here. Additionally, other studies have demonstrated relationships between DOC and zooplankton, mediated by changes in phytoplankton taxonomic composition and digestibility (Bergström et al, 2021; Senar et al, 2019; Strandberg et al, 2020). Our results suggest that physiological limitations through reduced temperatures also may be an important consideration for diminished zooplankton production with increased lake DOC concentration.…”

“…However, the increases in DOC and resulting changes in resources were relatively subtle compared to the large gradient of DOC concentrations observed here. Additionally, other studies have demonstrated relationships between DOC and zooplankton, mediated by changes in phytoplankton taxonomic composition and digestibility (Bergström et al, 2021;Senar et al, 2019;Strandberg et al, 2020).…”

Biomass, community composition and N:P recycling ratios of zooplankton in northern high‐latitude lakes with contrasting levels of N deposition and dissolved organic carbon

“…Responses in phytoplankton mineral quality to changes in lake DIN:TP and DOC are therefore expected to be different from the responses in phytoplankton biomass (Deininger et al, 2017a; Bergström et al, 2020; Isles et al, 2021). Consequently, changes in phytoplankton biomass might not necessarily translate into changes in zooplankton biomass (McCauley & Kalff, 1981), due to concurrent changes in phytoplankton mineral quality (Bergström et al, 2021; Deininger et al, 2017b).…”

“…In combination, these processes lower DIN:TP and light availability in boreal lakes (Isles et al, 2020). Light availability will limit the extent to which phytoplankton can change their C:N:P stoichiometry following changes in lake DIN:TP (Bergström et al, 2021;Deininger et al, 2017a;Sterner et al, 1997). Responses in phytoplankton mineral quality to changes in lake DIN:TP and DOC are therefore expected to be different from the responses in phytoplankton biomass (Deininger et al, 2017a;Bergström et al, 2020;Isles et al, 2021).…”