XX
5 Spatial and Temporal Distnhutlon Patterns
In 1996, most of the copepods (esp. Temora longicornis) could use Coseinodiscus cone inn us as a food source and their numbers increased already during the
bloom. The copepods already built fat droplets in spring, which was not observed
at this time in the two preceding years, indicating a good utilization of C. eoneinnus. Only the numbers of Centropages hamates did not increase during the bloom
of C. eoneinnus, as this species has a longer generation time and therefore occurs
usually later in the Wadden Sea.
5.1.7
Relations Between Phytoplankton and Meroplanktonic Larvae
The amount of meroplanktonic larvae in the water column can be influenced by
several factors, e.g. the intensity of spawning, abiotic conditions (e.g. water temperature), food supply and predation pressure. Spawning and predation were not
assessed. For the remaining factors an evaluation of their intluence of appearance
of larvae will be given.
The food of bivalve larvae consists mainly of microalgae and bacteria. The size
offood particles required by bivalve larvae is still under debate. J0rgensen (1981)
states that particles should not exceed 2-5 fJm in size, but Baldwin & Newell
(1991) observed larval uptake of particles smaller than 30 fJm. To assess the intluence of food on the larvae, the algal fraction smaller 30 fJm was considered here.
Larvae of polychaetes have a wide ranging food spectrum. They feed on small
algae as the larvae of bivalves do. Some larvae of polychaetes are known to colonize aggregates in the water and feed on bacteria and detritus (Shanks & del
Carmen 1997).
In order to estimate the food supply for meroplanktonic larvae the fraction of
the small algae « 30 fJm) was calculated. To this fraction belong, among others,
Asterionellopsis Rlacialis, Skeletonema eostatum, small species of Chaetoeeros
and Phaeoeystis spp. Including Phaeocystis spp. in this fraction « 30 fJm) can be
disputed, because these algae also occur in colonies, which are substantially larger
than 30 fltl1. However. in the backbarrier system of Spiekeroog these algae often
occurred as individual cells and were therefore counted in this fraction.
Except for algal blooms, the fraction of small algae accounted for 75 to 90 % of
total algal numbers. During the diatom bloom in spring 1994 the share was reduced
to 15 %. At the beginning of the bloom of Coscinodiscus concinnus in 1996, the
fraction of small algal cells was low, but reached nearly 100 % in April.
Cold winters with sea-ice formation in the Wadden Sea ("ice winters",
Chaps. 3.2, 7) indirectly intluence the numbers of larvae of the different species:
Cold sensitive species as Lanice conehilcga do not survive temperatures below
2 DC (Buhr 1991), so that their spawning can be decreased by the reduction of the
adult population. According to Honkoop & van der Meer (1997) ice winters can
also result in an increase of the amount of larvae of some species, if the egg development commences before the winter. Then, in a mild winter with poor nutrition,
the eggs can be resorbed by the adult. But in a cold winter, the metabolism is reduced and all eggs can mature. The generally strong recruitment of Macoma
balthiea after ice winters could be explained by this mechanism (loc. cit.).
5 Spatial and Temporal Distnhutlon Patterns
In 1996, most of the copepods (esp. Temora longicornis) could use Coseinodiscus cone inn us as a food source and their numbers increased already during the
bloom. The copepods already built fat droplets in spring, which was not observed
at this time in the two preceding years, indicating a good utilization of C. eoneinnus. Only the numbers of Centropages hamates did not increase during the bloom
of C. eoneinnus, as this species has a longer generation time and therefore occurs
usually later in the Wadden Sea.
5.1.7
Relations Between Phytoplankton and Meroplanktonic Larvae
The amount of meroplanktonic larvae in the water column can be influenced by
several factors, e.g. the intensity of spawning, abiotic conditions (e.g. water temperature), food supply and predation pressure. Spawning and predation were not
assessed. For the remaining factors an evaluation of their intluence of appearance
of larvae will be given.
The food of bivalve larvae consists mainly of microalgae and bacteria. The size
offood particles required by bivalve larvae is still under debate. J0rgensen (1981)
states that particles should not exceed 2-5 fJm in size, but Baldwin & Newell
(1991) observed larval uptake of particles smaller than 30 fJm. To assess the intluence of food on the larvae, the algal fraction smaller 30 fJm was considered here.
Larvae of polychaetes have a wide ranging food spectrum. They feed on small
algae as the larvae of bivalves do. Some larvae of polychaetes are known to colonize aggregates in the water and feed on bacteria and detritus (Shanks & del
Carmen 1997).
In order to estimate the food supply for meroplanktonic larvae the fraction of
the small algae « 30 fJm) was calculated. To this fraction belong, among others,
Asterionellopsis Rlacialis, Skeletonema eostatum, small species of Chaetoeeros
and Phaeoeystis spp. Including Phaeocystis spp. in this fraction « 30 fJm) can be
disputed, because these algae also occur in colonies, which are substantially larger
than 30 fltl1. However. in the backbarrier system of Spiekeroog these algae often
occurred as individual cells and were therefore counted in this fraction.
Except for algal blooms, the fraction of small algae accounted for 75 to 90 % of
total algal numbers. During the diatom bloom in spring 1994 the share was reduced
to 15 %. At the beginning of the bloom of Coscinodiscus concinnus in 1996, the
fraction of small algal cells was low, but reached nearly 100 % in April.
Cold winters with sea-ice formation in the Wadden Sea ("ice winters",
Chaps. 3.2, 7) indirectly intluence the numbers of larvae of the different species:
Cold sensitive species as Lanice conehilcga do not survive temperatures below
2 DC (Buhr 1991), so that their spawning can be decreased by the reduction of the
adult population. According to Honkoop & van der Meer (1997) ice winters can
also result in an increase of the amount of larvae of some species, if the egg development commences before the winter. Then, in a mild winter with poor nutrition,
the eggs can be resorbed by the adult. But in a cold winter, the metabolism is reduced and all eggs can mature. The generally strong recruitment of Macoma
balthiea after ice winters could be explained by this mechanism (loc. cit.).