August, like July, was very warm (mean temperatures: 18 - 19 °C) but had less sunshine and clearly more rainfalls (80 - 134 mm precipitation) than the preceding month.
September in some areas had the highest precipitation (160 - 257 mm) and lowest number of sunshine hours (70 - 90 hours) in at least 40 years. The monthly mean temperatures ranged between 13 °C and 14.4 °C.
The surface water mean temperatures in August and September reached, 15.9 °C, and 14.5 °C, respectively, which was above the climatological values.
In August, potentially toxic or bloom forming algae occurred only in non-critical concentrations in the Wadden Sea of Lower Saxony.
Near Helgoland, the abundance of diatoms was low. The concentrations of small flagellates increased slightly; of the larger dinoflagellates, Ceratium fusus and few Dinophysis norvegica and Prorocentrum micans were found. At the end of the month, an increase in the abundance of flagellates was observed, with many small species like Prorocentrum minimum.
Phaeocystis was quite frequent in the North Sylt Wadden Sea in early August - which has become a typical summer phenomenon. As in July, no unusual blooms occurred in this area in August. Observations of toxic algae were rare.
An unseasonably low abundance of plankton was observed in the coastal waters of Schleswig-Holstein. Diatoms (Rhizosolenia imbricata) were more abundant than dinoflagellates. The potentially toxic flagellate Fibriocapsa japonica was found at all stations, but in uncritical concentrations.
In the Wadden Sea of Lower Saxony, an increasing abundance of diatoms of the Pseudonitzschia group was observed between Accumer Ee and the river Jade in September. In the eastern area between Harle and Voslapp, slightly higher cell concentrations of the potentially toxic armoured dinoflagellate Dinophysis acuminata were observed in the second half of the month.
Off Helgoland, the diversity of diatom species (Chaetoceros socialis, Lithodesmium undulatum, Melosira sulcata, Odontella sinensis) was still large but concentrations were low. There were many different species of small naked flagellates. The abundance of Dinophysis acuminata increased toward the end of the month.
In the coastal waters of Schleswig-Holstein, plankton growth had clearly decreased. Although more diatom species were observed than dinoflagellates (Gymnodinium sp., Prorocentrum triestinum), cell concentrations remained low in almost all areas.
In the North Sylt Wadden Sea, nutrients in 2001 showed the typical seasonal cycle: a pronounced seasonality with high levels in winter and autumn, and low levels in summer. A strong decrease in nutrient concentrations began early in the year, during the spring bloom. Phosphate showed a slightly different seasonal cycle: from May, phosphate levels increased from about 0.1 µmol to about 1 µmol in autumn. As the phytoplankton season this year ended early because of predominantly cloudy weather in September, a marked nutrient increase was observed as early as mid-September. NH4 and NO2 reached their maxima in mid-November (NH4: ~14 µmol, NO2: 4 - 4.5 µmol). NO3 and Si increased steadily and reached about 30 µmol in late December.
In the German Bight, nutrient concentrations in August were very low. Both nitrate and phosphate levels were almost depleted, causing a standstill of plankton growth. Low nutrient levels were still measured in the coastal waters and parts of the Wadden Sea.
The oxygen situation in the deeper water layers of the German Bight was good this summer. The lowest levels were measured at the bottom of the Elbe glacial river valley, southwest of the Dogger Bank, and off Limfjord, with 60 - 70 % oxygen saturation. This deficit was compensated during the first autumn storms.
August was warm with above-average summer temperatures (around 18 °C) but a shorter duration of sunshine (200 - 232 hours), and clearly too wet (70 - 115 mm precipitation).
September in many areas had the highest precipitation (130 - 160 mm) and the shortest duration of sunshine (70 - 95 hours) on record in at least 40 years. It also had some strong winds which often reached gale force.
Due to the exceptionally warm weather in July and August, water temperatures along the coast of Mecklenburg-Vorpommern were 2 K too high on average.
Also the bottom water in the shallow, unstratified water body between Warnemünde and Ahlbeck was clearly too warm, over 4 K above normal (August: 8.1 - 20.4 °C).
Due to a rapid temperature decline in September, the water temperatures were normal (surface layer: 14.5 - 16.8 °C; bottom water: 11.2 - 16.7 °C). The temperature stratification in the Mecklenburg Bight was weak.
In the Baltic Proper gale-force winds in early August caused the thermocline to drop to 20 m, with the surface temperature decreasing to 18 °C. The general trend of rising temperatures was also observed in the bottom water of the deep basins.
Surface water salinity (August: 5.8 - 9.6) was seasonably low along the entire coast of Mecklenburg-Vorpommern. Inflowing high-salinity water from the Belt Sea led to salinity increases in the bottom water (6.5 - 23.4).
In September, the surface salinity increased (6.8 - 14.7) despite exceptionally strong rainfalls. The salinity stratification was clearly weaker. Salinity values in the inner coastal waters were slightly below normal.
In the surface water of the Baltic Proper salinities (6.0 - 7.5) were below the long-term mean values also in July/August. The halocline in the Arkona Basin was at 22 m, in the Bornholm Basin at 48 m, and in the Gotland Basin at 65 m.
The water levels in August, at 504 - 511 cm, were below the mean values.
A strong cyclone with gale-force winds in the western Baltic in early September caused elevated water levels of 560 - 580 cm. Typical of this month were water levels above or around generalized mean sea level.
In August a highly variable composition of microalgae populations was found in the Baltic coastal waters of Schleswig-Holstein. In the inner coastal inlets and bays, growths of non-toxic microalgae species were observed which changed from one week to the other. Large dinoflagellates of the genus Ceratium, which are typical of late summer, were abundant in all coastal waters. In mid-August, spreading of the diatom species Leptocylindrus danicus and Rhizosolenia pungens was observed. In late August, cell concentrations of the diatom Skeletonema costatum increased in the Flensburger Innenförde.
At the monitoring station Seebrücke Heiligendamm, mainly dinoflagellates Ceratium tripos and Cryptophyceae were found toward the end of the month. The abundance of cyanobacteria in this area was low.
In the Pomeranian Bight, chlorophyll concentrations increased considerably, and plankton was very rich in species and individuals. High concentrations of the blue-green alga Anabaena cf. spiroides were observed. Near Greifswalder Oie, the August mean value had tripled, indicating another conspicuously high chlorophyll-a concentration at this station in 2001. Small cryptoflagellates (Teleaulax and Plagioselmis) as well as undetermined Chrysophyceae and flagellates of the genus Pseudopedinella were dominant. Potentially toxic blue-green algae were recorded only at low densities.
Average chlorophyll-a concentrations were generally found in the inner coastal waters. Phytoplankton was dominated by small flagellates (undetermined Chrysophyceae) and by Pseudopedinella, Plagioselmis, Hemiselmis, Tetraselmis, and Teleaulax. In the waters of Greifswalder Bodden and Strelasund, various species of the coccoid blue-green alga Woronichinia were dominant. The typical summer mass development of Microcystis did not take place.
In September major variations in the species spectrum and cell concentrations were observed in the Baltic coastal waters of Schleswig-Holstein. The abundance of microalgae decreased from north to south. Filamentous blue-green algae were hardly observed any more.
Cell counts in the Flensburger Innenförde showed increased concentrations of diatoms (Rhizosolenia pungens, Cerataulina pelagica, Dactyliosolen fragilissima, Pseudo-nitzschia spp.). The dominant dinoflagellate species was Prorocentrum micans. In the inner parts of the inlet, the autotrophic ciliate Myrionecta rubra was observed at cell concentrations up to 62,000 cells/litre. In the Kiel Bight, cell numbers of the small dinoflagellate Prorocentrum minimum fluctuated substantially from one week to the other. The species spectrum off Heiligenhafen was relatively small. Average concentrations of the diatom Cerataulina pelagica were observed. Also the non-toxic blue-green alga Aphanizomenon was found. An interesting observation was a marked increase in several species (among others, Heterocapsa rotundata) in the Bay of Lübeck. The dinoflagellates Ceratium tripos, Ceratium fusus, Prorocentrum minimum, and Prorocentrum micans) had increased as well. At the monitoring station Seebrücke Heiligendamm, Dactyliosolen fragilissimus was the dominant species in early September. It once more reached the bloom concentration of August In mid-September.
Chlorophyll-a concentrations along the western outer coasts of Mecklenburg-Vorpommern exceeded the long-term monthly mean by over 200 % in some areas. The Pomeranian Bight was an exception, with cell concentrations at the preceding month's level (12.8 µg/L).
On the outer coast, phytoplankton was rich in species but relatively poor in individuals. Apart from the abundance of several Dinophyceae species of the genus Ceratium, which is typical of late summer, a minor bloom of Dactyliosolen fragilissimus was observed north of Boltenhagen. The potentially toxic plankton species Prorocentrum micans was found at concentrations of 12,000 cells/litre. In the eastern Baltic Sea areas between Hiddensee and Ahlbeck, small cryptoflagellates (Teleaulax, Plagioselmis, Hemiselmis) and the extremely small Dinophyceae species Heterocapsa rotundata continued to be dominant. In the Pomeranian Bight, various species of coccoid blue-green algae (Woronichinia) were found to have the highest partial biovolumes. In the total area of the eastern Baltic Sea, the number of µ-algae had decreased noticeably since August. Potentially toxic blue-green algae played an insignificant role.
In the inner coastal waters, concentrations of chlorophyll-a mostly showed the typical seasonal pattern. In the northern part of the Lower Warnow, the abundance of phytoplankton was considerably higher than the month before. Characteristic observations were an autumn bloom of Skeletonema costatum and extraordinarily large flagellates of the genus Pyramimonas as well as Cryptomonads. The abundance of phytoplankton in the Stettiner Haff showed a marked decrease from August to September. Compared to August, the species spectrum in the waters of Greifswalder Bodden and Strelasund was largely unchanged.