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assemblage  diatom  effects  genera  l−  nitrogen source  nitrogen  pelagic rotifers  pelagic  pseudo nitzschia  rotifers  μg l−  μg 
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Preview: Journal of Plankton Research - Advance Access

Journal of Plankton Research Advance Access

Published: Wed, 31 Jan 2018 00:00:00 GMT

Last Build Date: Wed, 31 Jan 2018 01:46:08 GMT


Dynamics of pelagic rotifers subject to trophic fluctuations in Upper Lake Constance (1963–2012)

Wed, 31 Jan 2018 00:00:00 GMT

This study evaluates a long-term monitoring dataset recording the pelagic rotifers of Upper Lake Constance (ULC) over almost 50 years. The same period has seen profound changes in trophic conditions in the lake, with total phosphorus concentrations rising from 18 μg L−1 in 1963 to 84 μg L−1 in 1984, and then declining back to 6.3 μg L−1 by 2012. A severe bias identified in calculations for density and abundance was successfully controlled using a density function for vertical rotifer distribution developed from a supplementary dataset. Six out of 21 genera were lost from the rotifer assemblage during eutrophication phase and failed to re-appear when oligotrophic conditions were restored. From 2007 onwards, individuals of the remaining rare genera remained scarce, while densities of the dominant genera increased significantly when phosphorus levels declined. Unlike other water bodies, and in contrast with the situation prior to eutrophication, the diversity of ULC rotifers today correlates negatively with trophic state. Grazing pressure exerted by both rotifer and crustacean communities decreased with increasing P concentrations indicating increasing top down control. Surprisingly, however, since 2008, the influence of dominant rotifers has even exceeded that of crustacean grazers with likely implications for the entire pelagic food web.

Interactive effects of temperature, CO2 and nitrogen source on a coastal California diatom assemblage

Wed, 17 Jan 2018 00:00:00 GMT

Diatoms are often considered to be a single functional group, yet there is a great deal of morphological, genetic and ecological diversity within the class. How these differences will translate into species-specific responses to rapid changes in the ocean environment resulting from climate change and eutrophication is currently poorly understood. We investigated the response of a natural diatom-dominated assemblage in coastal California waters to interactions between the variables nitrogen source (nitrate and urea), temperature (19 and 23°C) and CO2 (380 and 800 ppm) in a factorial experimental matrix using continuous culture (ecostat) methods. The community included diatoms of the cosmopolitan genera Pseudo-nitzschia and Chaetoceros, as well as Leptocylindrus and Cylindrotheca. Our results demonstrate strong interactive effects of these variables on community composition; notably, nitrogen source alone and nitrogen and CO2 together had a much greater influence on diatom community structure at 23°C compared with 19°C. In addition, warming and acidification interactions significantly increased cellular quotas of the neurotoxin domoic acid produced by Pseudo-nitzschia multiseries. In general, the effects observed for the factors tested differed significantly between the various diatom genera in this assemblage, suggesting potentially divergent responses of some of these ecologically and biogeochemically important phytoplankton taxa to interactions between global-scale and local-scale anthropogenic stressors in a changing ocean.