Impact of change in climate and policy from 1988 to 2007 on environmental and microbial variables at the time series station Boknis Eck, Baltic Sea

Abstract

Phytoplankton and bacteria are sensitive indicators of environmental change. The temporal development of these key organisms was monitored from 1988 to the end of 2007 at the time series station Boknis Eck in the western Baltic Sea. This period was characterized by the adaption of the Baltic Sea ecosystem to changes in the environmental conditions caused by the conversion of the political system in the southern and eastern border states, accompanied by the general effects of global climate change. Measured variables were chlorophyll, primary production, bacteria number, -biomass and -production, glucose turnover rate, macro-nutrients, pH, temperature and salinity. Negative trends with time were recorded for chlorophyll, bacteria number, bacterial biomass and bacterial production, nitrate, ammonia, phosphate, silicate, oxygen and salinity while temperature, pH, and the ratio between bacteria numbers and chlorophyll increased. Strongest reductions with time occurred for the annual maximum values, e.g. for chlorophyll during the spring bloom or for nitrate during winter, while the annual minimum values remained more stable. In deep water above sediment the negative trends of oxygen, nitrate, phosphate and bacterial variables as well as the positive trend of temperature were similar to those in the surface while the trends of salinity, ammonia and silicate were opposite to those in the surface. Decreasing oxygen, even in the surface layer, was of particular interest because it suggested enhanced recycling of nutrients from the deep hypoxic zones to the surface by vertical mixing. The long-term seasonal patterns of all variables correlated positively with temperature, except chlorophyll and salinity. Salinity correlated negatively with all bacterial variables (as well as precipitation) and positively with chlorophyll. Surprisingly, bacterial variables did not correlate with chlorophyll, which may be inherent with the time lag between the peaks of phytoplankton and bacteria during spring. Compared to the 20-yr averages of the environmental and microbial variables, the strongest negative deviations of corresponding annual averages were measured about ten years after political change for nitrate and bacterial secondary production (~ −60%), followed by chlorophyll (−50%) and bacterial biomass (−40%). Considering the circulation of surface currents in the Baltic Sea we interpret the observed patterns of the microbial variables at the Boknis Eck time series station as a consequence of the improved management of water resources after 1989 and – to a minor extent – the trends of the climate variables salinity and temperature

Long-term trends at the Boknis Eck time series station (Baltic Sea), 1957–2013: does climate change counteract the decline in eutrophication?

“…Rising temperatures further enhance remineralisation of organic matter that is deposited on the bottom, resulting in increased oxygen consumption (Hoppe et al, 2013). This hypothesis is further supported by the lacking trend of bottom water ammonium concentration.…”

Summer upwelling at the Boknis Eck time-series station (1982 to 2012) – a combined glider and wind data analysis

“…Variability from seasonal to long term have been described for a number of parameters sampled at BE (e.g. Lennartz et al, 2014;Hoppe et al, 2013) while episodic, event-type variability has not systematically being addressed. Here we investigate one prominent process that triggers drastic changes in the interior parameter fields, namely summer upwelling events.…”

“…It is a ship survey-based time series with approximately monthly resolution, and was first surveyed in April 1957. Details on data collection and parameters have been described elsewhere Dale et al, 2011;Bertics et al, 2013;Hoppe et al, 2013;Lennartz et al, 2014). Here we will use bottle data acquired at standard depth (1, 5, 10, 15, 20, 26/25 m).…”

Implementation of European marine policy: New water quality targets for German Baltic waters

“…However, early chl.a concentrations can be calculated from historical phytoplankton data, which are available from Kiel Fjord for 1905 and 1906 [54]. Using a conversion factor of 50, as applied by Hoppe et al [29], the average phytoplankton carbon biomass of 55 mg/m 3 corresponds to a chl.a concentration of 1.1 mg/m 3 . This concentration meets the suggested target of 1.3 mg/m³ chl.a very well.…”