Peter Martens

The fate of faecal pellets in the North Sea

During an intensive survey in May/June 1986, 129 stations in the North Sea were examined in different water depths with respect to several hydrographical parameters. The relations of planktonic faecal pellets to food (chl-a) and producers (zooplankton-wet weight) as well as to temperature, salinity and water depth were investigated. The amount of faecal pellets was positively correlated with chl-a and zooplankton. The length of the faecal pellets was negatively correlated with water depth indicating a degradation during sinking. Hardly any formed faecal material was detectable under 100 m water depth. The ratio of pellets to zooplankton was positively correlated to chlorophyll and influenced by daytime, indicating diel variations in pellet production. It is assumed that in summer most of the planktonic faecal material is disintegrated in the upper water layer and the role of faeces in sedimentation is minor.

Effects of the severe winter 1995/96 on the biological oceanography of the Sylt-Rømø tidal basin

Abstract. Water temperature, salinity and precipitation, micronutrients (N, P, Si) and chlorophyll a concentrations in the Sylt-Rømø tidal basin (German Bight) deviated between the early 1990s, with mild winters, and the years 1996–1997 with a severe winter and a moderate winter. As a consequence of low temperature, offshore winds and low precipitation in the severe winter 1995/96, salinity peaked in February 1996 and nitrate concentrations were low. The latter further decreased in March as chlorophyll a peaked with an exceptional bloom of the diatom Odontella aurita, probably triggered by low water temperature. Winter temperatures and spring chlorophyll a in the Sylt-Rømø tidal basin correlate well with the climatic North Atlantic Oscillation index.

On trends in nutrient concentration in the northern Wadden Sea of Sylt

The mean values on hydrographical parameters in the Wadden Sea of Sylt taken weekly over a period of seven years (1975, 1976 and 1984 to 1988) were compared in order to detect trends in concentrations of nutrients or plankton. An increase in NO2, NO3, PO4 and Si could be found in summer. NH4 showed a negative trend. Corresponding with the rising nutrient-level, Chlorophyll-a and pH show a positive trend. Salinity decreased significantly. Changes in NH4 and NO2 were correlated with rainfall.

Zooplankton community respiration during the JGOFS pilot study

From March to July 1989 (JGOFS pilot study), measurements were carried out on the oxygen uptake of natural zooplankton communities on five drifting stations (about three weeks each) in the Atlantic from 18°N to 72°N after a method of Martens (1986). The weight specific respiration rate decreased from south to north parallel to the water temperature, whereas the amount of mesozooplankton increased. No significant differences in the oxygen uptake of the zooplankton community between the five stations could be found. A rough estimate showed that less than 2% of the phytoplankton carbon was assimilated by the mesozooplankton per day. This is thought to reflect a typical spring situation. The weight specific respiration rate is influenced significantly by the water temperature and the mean length of the zooplankter.

Long-Term Ecological Change in the Northern Wadden Sea

The Wadden Sea is a shallow coastal region in the south eastern North Sea. Karl Mobius started ecological research in the northern Wadden Sea about 150 years ago studying the extensive oyster beds. With the foundation of a field station of the Biologische Anstalt Helgoland in List/Sylt in 1924 biological research in the Wadden Sea was continued to date. Several time series were initiated between the 1970s and the 1990s including a bi-weekly phytoplankton and zooplankton program and an observation program on macrobenthos. Three factors dominating the changes observed during the past decades are a rise in temperature, decreasing nutrients, and increasing invasions of non-native species. Phytoplankton blooms gradually decrease due to the combined effect of decreasing nutrient loads and increasing winter temperatures. Mean annual zooplankton abundance is stimulated by higher winter temperatures. Recently, invading species are increasingly dominating native mussel beds. For several invaders, a positive effect of temperature was shown. We expect that major pressures of change during the next years will be further species introductions, temperature increase, and reduced nutrient loads. On the long run (21st century), we expect sea level rise to be the key factor of coastal change through a loss of habitats with fine-grained sediments and intertidal sediments in general. A major challenge for coastal research will be to disentangle the interactive effects of these pressures on the long-term development of the Wadden Sea.

Inorganic phytoplankton nutrients in the Wadden Sea areas off Schleswig-Holstein. II. Dissolved ortho-phosphate and reactive silicate with comments on the zooplankton

In the summer months of 1984, 1986, 1987 and 1988, several stations off the west coast of Schleswig-Holstein were investigated with regard to the situation of ortho-phosphate, reactive silicate and mesozooplankton. An increase in phosphate was observed, whereas an increase in silicate could only be found in the areas influenced by the River Eider. Polychaete larvae show a decreasing tendency in the summer months; this is thought to be influenced by water temperature.