Klaus Nagel

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.

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.

The exceptional oder flood in summer 1997 — riverine mass and nutrient transport into the Pomeranian Bight

The discharge of river water and the amount of nutrients released into the Pomeranian Bight by the Oder river were investigated during the flood event in summer 1997. A barotropic box model of the lower Oder estuary was used to calculate the transport of riverine water through the channels connecting the Szczecin Lagoon and the Pomeranian Bight. The nutrient transport into the Baltic Sea was estimated, based on daily measurements of nutrient concentrations (NO3, NO2, NH4, Ntotal, PO4, Ptotal and SiO4) in the Swina and the Szczecin Lagoon and on the modelled water transport.

Nutrient Dynamics in the Pomeranian Bay (Southern Baltic): Impact of the Oder River Outflow

The Pomeranian Bay is a coastal region fed by the Oder River, one of the seven largest Baltic rivers, whose waters flow through a large and complex estuarine system before entering the bay. Nutrients (NO3−, NO2−, NH4+, Ntot, PO43−, Ptot, DSi), chlorophylla concentrations, oxygen content, salinity, and temperature were measured in the Pomeranian Bay in nine seasonally distributed cruises during 1993–1997. Strong spatial and temporal patterns were observed and they were governed by: the seasonally variable riverine water-nutrient discharges, the seasonally variable uptake of nutrients and their cycling in the river estuary and the Bay, the character of water exchange between the Pomeranian Bay and the Szczecin Lagoon, and the water flow patterns in the Bay that are dominated by wind-driven circulation. Easterly winds resulted in water and nutrient transport along the German coastline, while westerly winds confined the nutrient rich riverine waters to the Polish coast and transported them eastward beyond the study area. Two water masses, coastal and open, characterized by different chemical and physical parameters and chla content were found in the Bay independently of the season. The role of the Oder estuary in nutrient transformation, as well as the role of temperature in transformation processes is stressed in the paper. The DIN:DIP:DSi ratio indicated that phosphorus most probably played a limiting role in phytoplankton production in the Bay in spring, while nitrogen did the same in summer. During the spring bloom, predominated by diatoms, the DSi:DIN ratio dropped to 0.1 in the coastal waters and to 0.6 in the open bay waters, pointing to silicon limitation of diatom growth, similar to what is being observed in other Baltic regions.

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.

Impact of water temperature on nutrient concentrations in the Oder estuary in 1996–1998

The impact of extreme weather, in terms of temperature, that is the seventh coldest winter 1995/1996 and the warmest summer 1997 in this century, on some peculiarities in nutrient concentrations in the Świna Strait is discussed on the basis of monthly mean sea surface temperature maps. These maps were derived from synoptic remote sensing data of the Advanced Very High Resolution Radiometer (AVHRR) operating at the NOAA (National Oceanic and Atmospheric Administration) weather satellites. The Swina Strait drains ca. 75 % of the water from the Szczecin Lagoon which is a recipient of the Oder waters. Severe winter 1995/1996 with low water temperature and ice sheet in the Oder estuary were found to be responsible for record low oxygen and high ammonium concentrations (90 μmol dm-3) in the Swina Strait. Moreover, harsh winter conditions resulted in passive transport of ammonium through the estuary in 1996; in contrast, mild winter’97 caused significant reduction of ammonium loads exported to the Pomeranian Bay. Delayed spring’96 caused later than in 1997 freshet water outflow and it suppressed/delayed biological uptake of nutrients (spring contribution of inorganic to total nitrogen discharged was by 19 % higher in 1996 than in 1997). Delayed sedimentation of freshly produced organic matter, together with low water temperature must have reduced PO4 fluxes from the sediments and therefore strengthened and prolonged phosphorus limitation of primary production in the Oder estuary on the turn of spring and summer 1996. Extremely hot summer’97 must have intensified phosphate fluxes at water column/sediments interface as their concentrations before the flood’97 were considerably higher than comparable ones in 1996 and 1998. High temperature and low oxygen content in July/August 1996 and 1997 were responsible for ammonium peaks, reaching ca. 20 μmol dm-3 in the Świna bottom waters in summer 1996 and during the Oder flood 1997. Both, high and low water temperatures play extremely important role in nutrient transformation in the Oder estuary.