'i I Nutrient" Algae and Zooplanktlln
XS
The development of phytoplankton in 1996 was unusual (Fig. 5.1.3). Because of
the cold winter and the extraordinary favourable light conditions in the Wadden
Sea (Chaps. 3.2; 7) the large diatom Coscinodiscus concinnus bloomed. The bloom
began in March and lasted until June. According to cell numbers, the bloom was
inconspicuous (between 400 and 6400 cells I'), but its biomass can account for
over 90 % of the total biomass. Diatom species, which normally occurred in the
Wadden Sea at this time, were rarely found, except for small Chaetoceros species.
Simultaneously to the C. concinnus bloom, a bloom of the haptophyceae
Phaeocystis fJouchetii occurred. This bloom reached maximal cell numbers of
nearly 200 millions cells per litre. Usually this species is found in northern latitudes and has a higher tolerance towards low temperature than Phaeocystis globosa (Jahnke 1987). The simultaneous appearance of the two cold-adapted algae
C. concinnus and P. pouchetii are in line with the extraordinary low temperatures
in spring 1996,
5.1.4.3
Zooplankton
The development of the zooplankton in the backbarrier system of Spiekeroog
could be divided into four seasonal groups.
I. Winter plankton: It has few species and individuals and is dominated by copepods.
2. Spring plankton: Increasing amount of species and individuals with first meroplanktonic larvae and often hydromedusae in great numbers.
3, Early summer plankton: Maximal number of species with high individual numbers, leading to highest diversity in spite of high amounts of meroplanktonic
larvae and young copepods.
4. Late summer/autumn plankton: The decrease of individual numbers and the
increase of species numbers leads to a similar high diversity as in early summer.
In 1994, zooplankton reached maximum numbers in June, following the spring
phytoplankton bloom (Fig, 5.1.3). The zooplankton maximum was mainly composed of copepods and larvae of molluscs and polychaetes. Whereas copepods
occurred all over the year, larvae of molluscs were found only in May and June.
Larvae of polychaetes were found irregularly until the end of August. After the
bloom of the diatom Leptocylindrus ssp. in summer, an increase of the copepods
was observed, who possibly profited from this bloom, In winter, zooplankton was
dominated by copepods.
Compared with 1994, the total amount of zooplankton was much lower in 1995
and extended only 1/4 of the densities of the year before. The larvae of the polychaetes and molluscs reached low numbers. Only the numbers of copepods were
similar to the year before. The total amount of zooplankton reached maximum
values from June to September and then decreased to lower winter values.
In the year 1996, the zooplankton also showed differences with the years before
(Fig. 5.1.3) The individual numbers were unusually high and showed two maxima.
The first maximum appeared in the middle of May during the bloom of
C. ('oneil/nus. It consisted of copepods and polychaete larvae. A second maximum
XS
The development of phytoplankton in 1996 was unusual (Fig. 5.1.3). Because of
the cold winter and the extraordinary favourable light conditions in the Wadden
Sea (Chaps. 3.2; 7) the large diatom Coscinodiscus concinnus bloomed. The bloom
began in March and lasted until June. According to cell numbers, the bloom was
inconspicuous (between 400 and 6400 cells I'), but its biomass can account for
over 90 % of the total biomass. Diatom species, which normally occurred in the
Wadden Sea at this time, were rarely found, except for small Chaetoceros species.
Simultaneously to the C. concinnus bloom, a bloom of the haptophyceae
Phaeocystis fJouchetii occurred. This bloom reached maximal cell numbers of
nearly 200 millions cells per litre. Usually this species is found in northern latitudes and has a higher tolerance towards low temperature than Phaeocystis globosa (Jahnke 1987). The simultaneous appearance of the two cold-adapted algae
C. concinnus and P. pouchetii are in line with the extraordinary low temperatures
in spring 1996,
5.1.4.3
Zooplankton
The development of the zooplankton in the backbarrier system of Spiekeroog
could be divided into four seasonal groups.
I. Winter plankton: It has few species and individuals and is dominated by copepods.
2. Spring plankton: Increasing amount of species and individuals with first meroplanktonic larvae and often hydromedusae in great numbers.
3, Early summer plankton: Maximal number of species with high individual numbers, leading to highest diversity in spite of high amounts of meroplanktonic
larvae and young copepods.
4. Late summer/autumn plankton: The decrease of individual numbers and the
increase of species numbers leads to a similar high diversity as in early summer.
In 1994, zooplankton reached maximum numbers in June, following the spring
phytoplankton bloom (Fig, 5.1.3). The zooplankton maximum was mainly composed of copepods and larvae of molluscs and polychaetes. Whereas copepods
occurred all over the year, larvae of molluscs were found only in May and June.
Larvae of polychaetes were found irregularly until the end of August. After the
bloom of the diatom Leptocylindrus ssp. in summer, an increase of the copepods
was observed, who possibly profited from this bloom, In winter, zooplankton was
dominated by copepods.
Compared with 1994, the total amount of zooplankton was much lower in 1995
and extended only 1/4 of the densities of the year before. The larvae of the polychaetes and molluscs reached low numbers. Only the numbers of copepods were
similar to the year before. The total amount of zooplankton reached maximum
values from June to September and then decreased to lower winter values.
In the year 1996, the zooplankton also showed differences with the years before
(Fig. 5.1.3) The individual numbers were unusually high and showed two maxima.
The first maximum appeared in the middle of May during the bloom of
C. ('oneil/nus. It consisted of copepods and polychaete larvae. A second maximum