PERIODS RELEVANT TO THE DEVELOPMENT OF BENTHIC COMMUNITIES
Seasonal variations in the food supply for the benthos as well as changes in the physical
characteristics of the sediment gave rise to the definition of certain periods which turned
out to be of high relevance for the development of the benthic communities (legends in fig.
1 and 2).
Physical characteristics of the sediment
Typical for a boreal region, temperature dropped gradually in the near bottom water
from autumn to winter ( 14°C to 2°C) and increased slowly towards spring (4°C in April).
The first period of observation coincided with the termination of “summer stagnation”, a
period in which anoxic conditions (hydrogen sulfide) were prevailing in the sediment
(Fig. 1 ). At that time, the stratification in the water column limited the access of oxygen to
the sediment. A storm in the beginning of October temporarily allowed the introduction
of oxygen into the sediment surface (“break up” of summer stagnation). During November, suboxic conditions were observed in the sediment. These conditions may have been
caused by both a chemical oxygen demand and increased biological activity. Stable oxic
conditions, however, were achieved in December and maintained through winter and
spring with the exception of a temporary reduction in redox potential in February and
April, which again may have been caused by increased biological activity ( Hargrave
1980)
Input of organic material into the sediment
Three periods of accumulation of organic material in the sediment surface could be
distinguished : in autumn, winter and spring (fig. 1). During November protein, carbohydrate and total organic matter accumulated with distinct separate peaks. This input
(“autumn input”) could be traced back to the breakdown and sedimentation of the
autumn phytoplankton bloom composed of dinoflagellates in the beginning and diatoms
at the end of the bloom (Graf et al. 1983). During winter, we observed a continuous slow
increase of organic material in the sediment surface (“winter input”). Resuspended
sediment, material from terrestrial origin as well as macrophyte debris eroded by winter
storms represented sources of the organic material. The breakdown of the spring phytoplankton bloom (mainly diatoms ; Peinert et al. 1982) led to an enrichment of organic
material during late March to mid April (“spring input”). In contrast to autumn, the first
peak in organic material was made up by carbohydrates.
RESPONSE OF THE BENTHIC COMMUNITY
In the following, the seasonal development of the benthic community will be discussed as
a reflection upon the different ecological situations and events as described above (cf.
headlines in fig. 1,2). During the successive periods, quite different types of benthic
metabolisms are involved, some of which are microbial-dominated (Fig. 2).
Break up of summer stagnation
During the anoxic period of summer stagnation, protein accumulated in the sediment
surface. The concentration of stored protein was even comparable to that measured after
the sedimentation of the phytoplankton blooms in autumn and spring, respectively.
Following the introduction of oxygen into the sediment (“break up”of summer stagnation ), protein concentrations significantly decreased. Parallel peaks in heat production
and ATP imply that the stored protein was rapidly consumed and incorporated into
57
Seasonal variations in the food supply for the benthos as well as changes in the physical
characteristics of the sediment gave rise to the definition of certain periods which turned
out to be of high relevance for the development of the benthic communities (legends in fig.
1 and 2).
Physical characteristics of the sediment
Typical for a boreal region, temperature dropped gradually in the near bottom water
from autumn to winter ( 14°C to 2°C) and increased slowly towards spring (4°C in April).
The first period of observation coincided with the termination of “summer stagnation”, a
period in which anoxic conditions (hydrogen sulfide) were prevailing in the sediment
(Fig. 1 ). At that time, the stratification in the water column limited the access of oxygen to
the sediment. A storm in the beginning of October temporarily allowed the introduction
of oxygen into the sediment surface (“break up” of summer stagnation). During November, suboxic conditions were observed in the sediment. These conditions may have been
caused by both a chemical oxygen demand and increased biological activity. Stable oxic
conditions, however, were achieved in December and maintained through winter and
spring with the exception of a temporary reduction in redox potential in February and
April, which again may have been caused by increased biological activity ( Hargrave
1980)
Input of organic material into the sediment
Three periods of accumulation of organic material in the sediment surface could be
distinguished : in autumn, winter and spring (fig. 1). During November protein, carbohydrate and total organic matter accumulated with distinct separate peaks. This input
(“autumn input”) could be traced back to the breakdown and sedimentation of the
autumn phytoplankton bloom composed of dinoflagellates in the beginning and diatoms
at the end of the bloom (Graf et al. 1983). During winter, we observed a continuous slow
increase of organic material in the sediment surface (“winter input”). Resuspended
sediment, material from terrestrial origin as well as macrophyte debris eroded by winter
storms represented sources of the organic material. The breakdown of the spring phytoplankton bloom (mainly diatoms ; Peinert et al. 1982) led to an enrichment of organic
material during late March to mid April (“spring input”). In contrast to autumn, the first
peak in organic material was made up by carbohydrates.
RESPONSE OF THE BENTHIC COMMUNITY
In the following, the seasonal development of the benthic community will be discussed as
a reflection upon the different ecological situations and events as described above (cf.
headlines in fig. 1,2). During the successive periods, quite different types of benthic
metabolisms are involved, some of which are microbial-dominated (Fig. 2).
Break up of summer stagnation
During the anoxic period of summer stagnation, protein accumulated in the sediment
surface. The concentration of stored protein was even comparable to that measured after
the sedimentation of the phytoplankton blooms in autumn and spring, respectively.
Following the introduction of oxygen into the sediment (“break up”of summer stagnation ), protein concentrations significantly decreased. Parallel peaks in heat production
and ATP imply that the stored protein was rapidly consumed and incorporated into
57