Climate Change and Eutrophication Induced Shifts in Northern Summer Plankton Communities

Abstract

Marine ecosystems are undergoing substantial changes due to human-induced pressures. Analysis of long-term data series is a valuable tool for understanding naturally and anthropogenically induced changes in plankton communities. In the present study, seasonal monitoring data were collected in three sub-basins of the northern Baltic Sea between 1979 and 2011 and statistically analysed for trends and interactions between surface water hydrography, inorganic nutrient concentrations and phyto- and zooplankton community composition. The most conspicuous hydrographic change was a significant increase in late summer surface water temperatures over the study period. In addition, salinity decreased and dissolved inorganic nutrient concentrations increased in some basins. Based on redundancy analysis (RDA), warming was the key environmental factor explaining the observed changes in plankton communities: the general increase in total phytoplankton biomass, Cyanophyceae, Prymnesiophyceae and Chrysophyceae, and decrease in Cryptophyceae throughout the study area, as well as increase in rotifers and decrease in total zooplankton, cladoceran and copepod abundances in some basins. We conclude that the plankton communities in the Baltic Sea have shifted towards a food web structure with smaller sized organisms, leading to decreased energy available for grazing zooplankton and planktivorous fish. The shift is most probably due to complex interactions between warming, eutrophication and increased top-down pressure due to overexploitation of resources, and the resulting trophic cascades.

Warming, but not enhanced CO2 concentration, quantitatively and qualitatively affects phytoplankton biomass

“…In contrast, increasing biomass with warming was found for 2 experimental early summer blooms (Taucher et al 2012, Lewandowska et al 2014, as well as for a Baltic Sea long-term field study (1979− 2011) by Suikkanen et al (2013). In summer conditions, nutrient concentrations are naturally low.…”

“…Warming under such conditions has a positive effect on phytoplankton biomass. Suikkanen et al (2013) also suggested that warming was the key environmental factor explaining the general increase in total phytoplankton biomass in northern summer Baltic Sea communities in recent decades.…”

Effect of elevated CO<sub>2</sub> on organic matter pools and fluxes in a summer Baltic Sea plankton community

“…Regular monitoring of the Baltic Sea over the past 100 years has indicated higher rates of temperature increase (0.08 to 0.11 • C per decade) than the global average, along with a 20 % decrease in annual maximum ice extent (HELCOM, 2013). Observed shifts in the spring and summer phytoplankton community dynamics have been primarily associated with warming in northern Baltic Sea regions over the past three decades (Suikkanen et al, 2013).…”

Shift in diet composition of Indian mackerel Rastrelliger kanagurta – an analysis in relation to climate change

“…The significant increase in SST observed off Karnataka waters would have increased the productivity of phytoplankton which is reflected in the diet composition of mackerel. At a regional scale, increase in SST has shown to increase the phytoplankton biomass in the Baltic Sea (Suikkanen et al, 2013) and in the North Sea (McQuatters-Gallop et al, 2007).…”

“…A shift in the food web structure was observed in the Baltic Sea towards more microbial, less energy-efficient food webs consisting of lower food quality and smaller sized organisms, which in combination with warming may lead to decreased availability of energy for grazing zooplankton and fish (Suikkanen et al, 2013). Studies conducted by Klauschies et al (2012) on effects of warming on phytoplankton communities with different light intensities in the Baltic Sea projected that under high temperature and lower light levels a higher share of smaller phytoplankton is expected for the next decades.…”