Effects of increased CO₂ concentration on nutrient limited coastal summer plankton depend on temperature

Abstract

Increasing seawater temperature and CO2 concentrations both are expected to increase coastal phytoplankton biomass and carbon to nutrient ratios in nutrient limited seasonally stratified summer conditions. This is because temperature enhances phytoplankton growth while grazing is suggested to be reduced during such bottom‐up controlled situations. In addition, enhanced CO2 concentrations potentially favor phytoplankton species, that otherwise depend on costly carbon concentrating mechanisms (CCM). The trophic consequences for consumers under such conditions, however, remain little understood. We set out to experimentally explore the combined effects of increasing temperature and CO2 concentration for phytoplankton biomass and stoichiometry and the consequences for trophic transfer (here for copepods) on a natural nutrient limited Baltic Sea summer plankton community. The results show, that warming effects were translated to the next trophic level by switching the system from a bottom‐up controlled to a mainly top‐down controlled one. This was reflected in significantly down‐grazed phytoplankton and increased zooplankton abundance in the warm temperature treatment (22.5°C). Additionally, at low temperature (16.5°C) rising CO2 concentrations significantly increased phytoplankton biomass. The latter effect however, was due to direct negative impact of CO2 on copepod nauplii which released phytoplankton from grazing in the cold but not in the warm treatments. Our results suggest that future seawater warming has the potential to switch trophic relations between phytoplankton and their grazers under nutrient limited conditions with the consequence of potentially disguising CO2 effects on coastal phytoplankton biomass.

Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions

“…Values for the following decreasing pH levels were 7.779 ± 0.012, 7.641 ± 0.007, 7.460 ± 0.001, 7.323 ± 0.030, and 7,159 ± 0.004. As expected, the pH T in all bags increased during the incubation period due to photosynthesis (Rost et al, 2008;Paul et al, 2016) (mean protons change per bag of −1.52 × 10 −8 ± 6.67 × 10 −9 mol L −1 over the 9-day experiment; Fig. 2).…”

“…Normality of the data was determined using a Shapiro-Wilk test at the 0.05 significance level and data were transformed (log or square root) when the normality was rejected (p < 0.05). To test for differences between pH and light treatments for the variables measured during the experiment (time series of nutrient and Chl a concentrations, phytoplankton, bacteria abundances, and dimethylated compound concentrations), we used a generalized least-square model that corrected for data autocorrelation due to time repetition (gls command in R studio, package nlme; also see the method described by Paul et al, 2016). In the model, time and pH were taken as two continuous factors; the light was included as a categorical factor (two levels: low light, high light).…”

“…A total of 12 incubation bags were separated into 2 groups of 6 bags to produce two similar sets of pH gradients (con- trol + 5 acidified bags; Table 1). The pH gradient method has been successfully applied by Schulz et al (2013) and Paul et al (2016) and is suitable when the possibility of replication is limited (see Cottingham et al, 2005, andHavenhand et al, 2010, for more details).…”

Acidification and warming affect prominent bacteria in two seasonal phytoplankton bloom mesocosms

“…However, in their autumn experiment Paul and colleagues (2015) found no effects of CO 2 on either blooming time or phytoplankton biomass whereas according to Sommer and colleagues (2015) changes in the composition of rare phytoplankton species were weak. In both studies, the only indications for CO 2 effects on phytoplankton were an interaction effect of CO 2 and temperature on phytoplankton biovolume and an increase in cell size with CO 2 enrichment (Sommer et al ., ) during the autumn experiment as well as an interaction effect of temperature and CO 2 on phytoplankton biomass because of reduced grazing in the summer experiment (Paul et al ., ).…”

“…() for the autumn experiment and Paul et al . () for the summer experiment]. Samples for amino acids analyses were taken between t 1 and t 4 , and those for the molecular characterisation of dissolved organic matter (DOM) on the day of filling, day 16 and day 21 ( t 4 ), both during the autumn experiment.…”