Monika Gerth

Dynamic features in the western Baltic Sea investigated using NOAA-AVHRR data

The western part of the Baltic Sea is a shallow transition area which is characterized by the water exchange between the Baltic and the North Sea. The dynamic processes in this area are forced by sea-level differences between the North Sea and Baltic Sea as well as by local wind and freshwater input. The response patterns are modified by Earths rotation, bathymetric structures and the alignment of the coast. Series of satellite images permit synoptical views of these pattern and their temporal and spatial development.

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.

Variation of optical properties in the Baltic Sea and algorithms for the application of remote sensing data

The variation of optical properties of water was studied in the open Baltic Sea and in coastal waters of the Pomeranian Bight for more than ten years. The optical properties in the open Baltic Sea are strongly influenced by plankton blooms with typical phytoplankton compositions. This led to significant seasonal variations in spectral reflectances. In coastal waters of the Pomeranian Bight, the origin of the water masses and the dwell period of the river water in different lagoon-like areas and bays determine the composition of the water constituents and their optical properties. Strong temporal and spatial variations in spectral reflectance, the spectral absorption of phytoplankton pigments, detritus and yellow substances were found within an area of 50 km2. The data set was used to develop ground truth algorithms to determine water constituents of Baltic Sea water from satellite data using CZCS, SeaWiFS, and MERIS.

Seasonal and interannual variations in satellite derived sea surface temperature of the Baltic Sea in the 1990s

Monthly mean Sea Surface Temperature (SST) maps derived from approximately 14.200 NOAA-AVHRR scenes for the period 1990–1998 were used to discuss seasonal and interannual climatic variations in the Baltic Sea in relation to air temperature and to solar radiation. In a validation analysis the comparison between SST data and time series as well as climatological in-situ temperatures showed very good correlations. The applicability of the meteorological parameters measured at the German coast is limited to the western and central Baltic Sea due to meteorological scales.

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.

Variation of specific optical properties and their influence on measured and modeled spectral reflectances in the Baltic Sea

Spatial and temporal variations due to seasonal and regional development of phytoplankton and the freshwater inflow in river mouth areas influencing the optical properties in the Baltic Sea. The phytoplankton blooms in different seasons are dominated by special taxonomical groups. The variation intervals for the concentration of different optical active water constituents are presented. Regional and seasonal variations in the absorption of phytoplankton, detritus and gelbstoff are discussed in relation to the sources. The specific absorptions of chlorophyll show more regional differences than temporal. Measured volume scattering functions from the open Baltic and Pomeranian Bight are compared with published data. The bb/b- ratio deliver the known values for different dominating water constituents, which influences also the spectral shape the backscattering coefficient. Derived spectral specific absorption and backscattering coefficients were used to control the reflectance model by comparison to measurements of the total internal reflectance. Selected inherent optical properties for the spring bloom were included in the inverse procedure by Krawczyk et al. for the derivation of water constituents from simultaneous satellite data of the Modulator Optical Scanner at the Indian Satellite IRS-P3.

Remote Sensing Applications in Coastal Zone Management

Satellite data of both sea surface temperature and ocean colour provide the opportunity to study dynamic features and the distribution of water constituents synoptically. A number of available and future satellite sensors with different spatial, temporal and spectral resolutions are presented. They allow to monitor both water temperature and quality in special areas influenced by algal blooms (including cyanobacteria) or in river water containing high concentrations of chlorophyll, suspended matter or yellow substances that change the turbidity. The optical properties and water constituents are also related to associated variables such as dissolved and particle-bound nutrients as well as inorganic and organic pollutants, especially in river mouth areas.

Chapter 13 Remote-sensing studies of the exceptional summer of 1997 in the Baltic Sea: The warmest August of the century, the Oder flood, and phytoplankton blooms

Abstract The summer of 1997 was exceptional in the Baltic Sea. Stable high pressure over Scandinavia in the first 10 days of August, associated with high solar radiation, resulted in the highest air and water temperatures of this century. These warm and calm conditions were favorable for a strong bloom of cyanobacteria in the southern Gotland Sea. In addition, two strong rainfall periods in the drainage area of the Oder River caused the centurys most extensive regional flooding, with floodwaters reaching the Pomeranian Bight in August. Satellite data from different sensors and spectral ranges were used to study these specialevents. The monthly mean summer sea surface temperature (SST) distribution for the years 1990-1998, as derived from the National Oceanic and Atmospheric Administration (NOAA) Advanced Very-High Resolution Radiometer (AVHRR) data, showed a large interannual variation. The mean SST in the Baltic Sea exceeded 22°C in August 1997, the warmest month of the 1990s. In addition to SST, visible data from NOAA AVHRR and Indian Remote-Sensing Satellites (IRS-P3 and IRS-1C) Multispectral Optical Sensor (MOS) and Wide Field Sensor (WiFS) were used to investigate the blooms of cyanobacteria and coccolithophores and the distribution of river discharge during the Oder flood.

The Upwelling Area Off Namibia, the Northern Part of the Benguela Current System

In the area off Namibia satellite remote sensing data of the visible and infrared spectral range were used to investigate the upwelling processes and the biological response. Satellite derived Sea Surface Temperature was applied to study upwelling processes in relation to the driving trade winds. The investigations were focussed on the intensity and horizontal extent, the temporal and spatial variability including inter-annual and climate scales. Ocean colour satellite data allow investigation of the response of surface water to the nutrient input into the euphotic zone by upwelling processes. Observations in the area revealed, that the phytoplankton development starts with species absorbing light in the visible spectral range (diatoms and dinoflagellates) and may end in persistent shallow surface filaments with light scattering algae blooms. These blooms of coccolithophores identified by different in situ methods changed the water colour by strong particle scattering to milky turquoise discolorations. Further milky turquoise discolorations were identified as sulphur plumes. Low wind periods support the degradation of organic matter and the development of hydrogen sulphide in the bottom layer. After the onset of the trade winds and the offshore transport of surface water hydrogen sulphide enriched waters reaches with the near-bottom counter current the coast. Upwelling transports the water to the surface where the hydrogen sulphide will be oxidised to elemental sulphur. An algorithm was developed to identify and separate the sulphur plumes from algae blooms and other features on the basis of highly spectrally resolved satellite data from the MERIS sensor. The algorithm identified only coastal plumes as sulphur and that offshore plumes are formed by coccolithophores. The sulphur season is the boreal spring starting in February and reaching the maximum in April.