Günther Nausch

The surface water CO2 budget for the Baltic Proper: a new way to determine nitrogen fixation

Abstract Six cruises were performed in the eastern Gotland Sea in about monthly intervals between March and September 2001 in order to establish a surface water carbon/nitrogen budget and to derive N2 fixation rates. Balancing the concentration changes of total CO2 and dissolved organic carbon, the CO2 air/sea exchange and the effect of mixed layer deepening yielded the net production of particulate organic carbon (POC) for each time interval. A total net production of 3.5 mol m−2 was obtained, which is consistent with earlier estimates of the POC export into the deep water. Measured C/N ratios in the particulate organic matter (POM) were applied to calculate the net formation of particulate organic nitrogen (PON), which was used to balance the nitrogen pools and fluxes. Taking into account the changes in dissolved organic nitrogen, the atmospheric input of NO3−/NH4+, and mixed layer deepening, the N2 fixation was calculated for each period. A total N2 fixation of 318±53 mmol m−2 was obtained, which substantially exceeded any previous estimate for the Baltic Proper (14–260 mmol m−2) and which corresponded to almost twice the spring bloom nitrogen consumption. The temporal distribution was characterized by an unexpected peak (108 mmol m−2) in late spring that was attributed to the activity of Aphanizomenon. A second maximum (106 mmol m−2) was found for the June–July period when Nodularia dominated the planktonic community.

Anthropogenic nutrient load of the Baltic Sea

Abstract The discharge of nutrients is investigated in relation to their sources and effects in two case studies. The reduction of 47% in the phosphorus load from Denmark to marine areas between 1989 and 1993 has resulted in significantly lower phosphorus concentrations in most Danish coastal waters, and tendency to decrease can be seen in the Belt Sea and Kattegat as well. No general changes in nitrogen concentrations have been observed. This is due to the fact that more than 80% of the nitrogen load in Danish waters originate from diffuse agricultural sources. In the Pomeranian Bight strong nutrient gradients are generated by the mixing of Odra river water and coastal water. The spreading of the river plume could be exactly observed especially in winter, when biological activity is low. In general, different types of distribution, transport and modification patterns can be described. The annual input of nutrients from the catchment area to the Baltic Sea was estimated to be around 1000 kt N and 46 kt P. As a result, winter concentrations of phosphate and nitrate are characterized by positive overall trends in the surface layer in all subregions of the Baltic Proper for the period 1969 to 1993. These trends stem mainly from the strong increase in the 1970ies and early 1980ies. Thereafter, the concentrations of both nutrients fluctuate strongly around a high level. The drastic decrease in fertilizer consumption since the late 1980ies mainly caused by the great economic changes in the countries of the former East Bloc is not yet significantly reflected in decreasing winter concentrations, but first signs already have been found in the decrease in averaged phosphate concentrations in winter, especially in the Arkona and Bornholm Seas.

The Baltic Sea 1994—Consequences of the hot summer and inflow events

The temperatures in the western and central Baltic Sea were determined by the relatively mild winter 1993/94 causing positive anomalies of 0.5–1 K in both the surface layer and the intermediate water. The summer 1994 was among the warmest in the past 60 years and produced extremely high temperatures and anomalies of 4–6 K in the surface layer of the whole Baltic Sea.

The Baltic Sea in 1995— Beginning of a new stagnation period in its central deep waters and decreasing nutrient load in its surface layer

Positive anomalies of the surface heat flux caused by the mild winter and warm summer influenced temperatures in the surface layer and in the cold intermediate water of the Baltic Sea in 1995. The water exchange across the sills in the entrances to the Baltic Sea was relative small in comparison with the years before. That favours the beginning of a new stagnation period in central Baltic deep waters.

Die hydrographisch-chemischen Bedingungen in der westlichen und zentralen Ostsee im Jahre 1993

Der Salzwassereinbruch im Januar 1993 war von herausragender Bedeutung fur die hydrographischchemischen Bedingungen in den untersuchten Ostseeregionen. Er fuhrte vor allem im Tiefenwasser des Bornholmbeckens und des Danziger Beckens zu einer starken Zunahme der Salz- und Sauerstoffkonzentrationen. Seine Auswirkungen auf das ostliche Gotlandbecken waren schwacher und im Gotlandtief durch starke Fluktuationen der chemischen Parameter gepragt. Nur Anfang Juli war auf dieser Station die gesamte Wassersaule kurzzeitig frei von Schwefelwasserstoff.

The exceptional Oder flood in summer 1997 — distribution patterns of the oder discharge in the Pomeranian Bight

The exceptional Oder flood in summer 1997 was a unique event in order to investigate the impacts on and the consequences for the ecosystem of the Baltic Sea of about 6.5 km3 additional water loaded with nutrients and contaminants and discharged within only 5 weeks. About 15 institutions participated in this investigation in both the Szczecin Lagoon and the Pomeranian Bight. The Baltic Sea Research Institute Warnemunde studied the water and nutrient inflow, the spreading of the Oder discharge, and the impact of the discharge on the ecosystem. The main topic of the presented investigations is a detailed study of the spatial and temporal spreading of the extreme river discharge in the Pomeranian Bight and the southern Baltic Sea by satellite data, ship observations and continuous buoy measurements as well as numerical modelling.

The Baltic Sea in 1996 — continuation of stagnation and decreasing phosphate concentrations

The severe winter of 1995/96, which was among the coldest in the western and central Baltic Sea since 1946 (4th rank), caused negative temperature anomalies in the winter surface and summer intermediate waters. The surface temperatures remained below the long-term mean until July, but reached positive anomalies between 2 and 4 K in August due to sunny high pressure weather conditions and weak winds.

The Baltic Sea in 1998 — characteristic features of the current stagnation period, nutrient conditions in the surface layer and exceptionally high deep water temperatures

The winter of 1997/1998 was mild in the northern Baltic Sea and even very mild in the southern Baltic, among the warmest of this century. The summer and autumn were relatively cool in northern Europe.

The baltic sea in 1997-impacts of the extremely warm summer and of the exceptional oder flood

The winter of 1996/97 can be characterized as moderate in the western and central Baltic Sea but was only weak in the Gulfs of Bothnia, Finland and Riga. The temperatures in the winter surface and summer intermediate layers met the long-term mean. The summer of 1997 was the warmest since 1890. Water temperatures reached positive anomalies up to 6 K in the 10 m upper layer due to sunny weather conditions and weak winds during August.

The exceptional Oder Flood in summer 1997 — the fate of nutrients and particulate organic matter in the Baltic Sea

The distribution of dissolved inorganic nutrients, particulate organic matter and phytoplankton pigments in the Oder plume was investigated at peak discharge of the Oder River during the exceptional flood event in summer 1997. Mixing diagrams of dissolved inorganic nutrients (NO3-, PO43-, SiO44-) reveal a nearly complete removal of nitrate during the first steps of estuarine mixing, whereas phosphate and silicate were still available over the entire salinity range. In contrast to silicate and phosphate, the nitrate riverine end-member concentration was about 3–4 times lower than during peak discharge in spring. It appears that during the summer flooding event inorganic nitrogen was not as available as in spring due to a stronger dilution effect and the advanced retention of nitrogen by land vegetation within the drainage area in summer. Therefore, algal biomass production in the Pomeranian Bight was most certainly nitrogen limited and significant removal of other dissolved inorganic nutrients by phytoplankton blooming did not occur.