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.
Highlights
Environmental monitoring data are being collected worldwide to assess the current state of marine ecosystems, as well as to identify spatial and temporal changes associated with eutrophication, climate or other human induced pressures across the world’s oceans [1,2]
Based on statistical analysis of existing monitoring data collected in three sub-basins of the Baltic Sea for more than 30 years, we demonstrate strong effects by climate change and eutrophication on hydrography and plankton communities, causing a shift towards a food web structure composed of low-food-quality phytoplankton and small sized zooplankton species leading to decreased energy availability for predators
Dinophyceae and Prasinophyceae significantly increased in the northern Baltic proper (NBP) (Table 2; Fig. 2), and Bacillariophyceae decreased in Aland Sea (A S) (Table 2; Fig. 4)
Summary
Environmental monitoring data are being collected worldwide to assess the current state of marine ecosystems, as well as to identify spatial and temporal changes associated with eutrophication, climate or other human induced pressures across the world’s oceans [1,2]. The cumulative effects of global change, including climate warming and increased human population, followed by more intense industrialisation and agribusiness, will probably continue and intensify the course of eutrophication in estuarine waters. Global climate change will likely result in higher water temperatures, stronger stratification and increased inflows of freshwater and nutrients to coastal waters in many areas [3]. In nutrient-poor and high latitude waters, the spring bloom is the single seasonal peak of primary production, providing energy and matter base for zooplankton, benthic animals and fish [5]. Large changes in plankton abundances may have serious effects on ecosystem functioning via bottom-up and top-down effects cascading through the food webs [8,9]
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