The effect of increased nitrogen load on phytoplankton in a phosphorus‐limited lake

Abstract

Summary Widespread use of artificial fertilisers and the burning of fossil fuels and/or biomass release a large amount of reactive nitrogen into the atmosphere. So far, the effects of increasing nitrogen deposition from the atmosphere have mainly been studied in nitrogen‐limited limnic and marine systems. Interestingly, in phosphorus‐limited lakes, additional nitrogen input might not affect phytoplankton biomass, but rather increase mainly the degree of phosphorus limitation. The resulting effects on plankton communities are difficult to predict and quantify. To estimate the effects of increasing nitrogen load on a spring phytoplankton community in a primarily phosphorus‐limited system, a mesocosm experiment was performed in an oligotrophic lake, in which a gradient of six increasing nitrogen enrichment levels was applied. During the initial phytoplankton growth phase (spring bloom), molar, seston nitrogen:phosphorus ratios increased from 43 to 72 and carbon:phosphorus ratios from 328 to 542 with increasing nitrogen enrichment, indicating increased phosphorus limitation. Three commonly used phytoplankton biomass proxies (phytoplankton biovolume, chlorophyll a and particulate organic carbon) showed only minor responses to nitrogen enrichment. Different groups and species of phytoplankton varied in their responses to the nitrogen enrichment in both the growth phase (spring bloom) and the descending phase (clear water phase). Overall, we detected an effect of nitrogen enrichment on phytoplankton stoichiometry and community composition. The observed changes in the phytoplankton community combined with changes in abundances of heterotrophic nanoflagellates and ciliates indicate bottom‐up driven alterations of the basal food web due to increased nitrogen loads.

The bioelements, the elementome, and the biogeochemical niche

“…, Poxleitner et al. , Du ). Changes in N:P have been related to substantial changes in species composition of plant communities (Peñuelas et al.…”

“…This sequence of events has been observed in several field studies where shifts in the elemental composition of the media (water or soil) due to changing conditions (anthropogenic or natural) have been translated into changes in the species composition of species of the community (Sterner and Elser 2002). The species with more favorable elemental compositions gain then importance and/or there is species turnover (Novotny et al 2007, Yu et al 2011, Poxleitner et al 2016, Du 2017. Changes in N:P have been related to substantial changes in species composition of plant communities and changes in P loads to adjusted C:P ratios in zooplankton communities (Teurlincx et al, 2017).…”

Impacts of increasing nitrogen:phosphorus ratios on zooplankton community composition and whitefish (Coregonus macrophthalmus) growth in a pre‐alpine lake

“…This was measured in a previous mesocosm experiment conducted in the same lake, during the same season and with an identical mesocosm size (Poxleitner, Trommer, Lorenz, & Stibor, 2016). To compensate for natural nutrient loss through sedimentation, low background fertilisation was given twice a week.…”

“…The amount of nutrient input as background fertilisation was equivalent to a TP loss of 0.06 μg L −1 day −1 . This was measured in a previous mesocosm experiment conducted in the same lake, during the same season and with an identical mesocosm size (Poxleitner, Trommer, Lorenz, & Stibor, 2016).…”

Lake nutrient stoichiometry is less predictable than nutrient concentrations at regional and sub‐continental scales

“…Nutrient co-limitation by nitrogen (N) and phosphorus (P) is pervasive across a range of terrestrial, aquatic, and marine ecosystems (Elser et al 2007, Harpole et al 2011, Paerl et al 2016, suggesting that multiple elements should be considered in studies of ecosystem production. Elemental ratios can also influence a range of ecological processes , including community composition (Tilman et al 1982, Poxleitner et al 2016, likelihood of toxin-producing algal blooms (Davidson et al 2012, Michalak et al 2013, trophic interactions (Frost et al 2005), and interspecific competition (Hall 2004). The influence of N:P stoichiometry on a range of ecological phenomena, coupled with the reality that anthropogenic activities are shifting the balance of elements (Peñuelas et al , 2013, underscore the need to examine drivers of variability in both N and P over space.…”