5.2 Biogeochemical Processes and Interactions
115
both in the overall pattern as well as in the single depth profiles. The annual variations indicated an intensified anoxic mineralization activity during the summer
months, which could shift by one month or be prolonged until autumn. For example, the lowest downcore phosphate concentrations were recorded in spring 1994
and 1995, followed by an increase of the concentrations in summer and autumn,
which was delayed in 1995 by around one month.
Immediately after the ice winter 1995/96 high pore water concentrations of
ammonia, phosphate and sulphide were recorded up to mid February. Due to a
stable weather condition with easterly winds, the inundation periods of the tidal
flats on the Groninger Plate were very short during that time, which led to a reduced pore water exchange and thus to an enrichment of mineralization products in
the pore water. After a change in the weather conditions, the nutrient concentrations returned to a normal level. The development in 1996 was comparable to the
preceding years. The maximum of the phosphate concentrations occurred in January and February, followed by a minimum in spring. In May and June 1996, simultaneously with the "black spot event" (Chap. 7) it came to an unusually high
rise of the phosphate concentrations again, while the values measured in July and
September 1996 were comparable to those of the previous years. A rise of the
silicate concentrations in June 1996 could be an indication to the deposition and
dissolution of a diatom bloom (Coscinodiscus concinnus). Also the phosphate-,
sulphide and ammonia concentrations indicated an increased release of nutrients
caused by mineralization processes during this time.
Enrichment experiments
Enrichment experiments were carried out in the years 1994 and 1995. In the course
of these experiments, defined quantities of organic matter were buried in the sediment and the temporal development and change of the pore water chemistry was
followed (Langner 1997, Oelschlager unpubl. data). In both years, additional control and reference sites were established and sampled at the same time as the experimentally polluted sites. In the control sites the sediments were treated in the
same way as in the experimental sites, but no organic material was introduced into
the sediment. Reference surfaces remained untreated.
In 1994, the experimental addition of organic matter took place by burying macroalgae (Enteromorpha sp.) in 10-15 cm sediment depth. The introduced algal
material corresponded to a quantity of 0,83 kg C/m 2 • During the initial phase of the
experiment, daily samples were taken. Later on, sampling intervals were increased.
The input of organic matter into the sediment caused a quick reaction of the
pore water chemistry. This was initially indicated by high sulphide concentrations,
increased ammonia- and strongly decreased sulphate concentrations. In some profiles, the sulphide horizon reached the sediment surface and led to the formation of
black spots. The vertical position of the maximum concentrations of S2, P0 4 and
NH4+changed with time and moved towards the sediment surface. During the winter 1994/95, the sediment chemistry regenerated and the values of experiment and
reference sites were comparable afterwards. This regeneration could have been due
to an intensified exchange of pore water during storm events. However, higher
sulphide-concentrations measured in summer 1995 still indicated the consequences
of the enrichment.
115
both in the overall pattern as well as in the single depth profiles. The annual variations indicated an intensified anoxic mineralization activity during the summer
months, which could shift by one month or be prolonged until autumn. For example, the lowest downcore phosphate concentrations were recorded in spring 1994
and 1995, followed by an increase of the concentrations in summer and autumn,
which was delayed in 1995 by around one month.
Immediately after the ice winter 1995/96 high pore water concentrations of
ammonia, phosphate and sulphide were recorded up to mid February. Due to a
stable weather condition with easterly winds, the inundation periods of the tidal
flats on the Groninger Plate were very short during that time, which led to a reduced pore water exchange and thus to an enrichment of mineralization products in
the pore water. After a change in the weather conditions, the nutrient concentrations returned to a normal level. The development in 1996 was comparable to the
preceding years. The maximum of the phosphate concentrations occurred in January and February, followed by a minimum in spring. In May and June 1996, simultaneously with the "black spot event" (Chap. 7) it came to an unusually high
rise of the phosphate concentrations again, while the values measured in July and
September 1996 were comparable to those of the previous years. A rise of the
silicate concentrations in June 1996 could be an indication to the deposition and
dissolution of a diatom bloom (Coscinodiscus concinnus). Also the phosphate-,
sulphide and ammonia concentrations indicated an increased release of nutrients
caused by mineralization processes during this time.
Enrichment experiments
Enrichment experiments were carried out in the years 1994 and 1995. In the course
of these experiments, defined quantities of organic matter were buried in the sediment and the temporal development and change of the pore water chemistry was
followed (Langner 1997, Oelschlager unpubl. data). In both years, additional control and reference sites were established and sampled at the same time as the experimentally polluted sites. In the control sites the sediments were treated in the
same way as in the experimental sites, but no organic material was introduced into
the sediment. Reference surfaces remained untreated.
In 1994, the experimental addition of organic matter took place by burying macroalgae (Enteromorpha sp.) in 10-15 cm sediment depth. The introduced algal
material corresponded to a quantity of 0,83 kg C/m 2 • During the initial phase of the
experiment, daily samples were taken. Later on, sampling intervals were increased.
The input of organic matter into the sediment caused a quick reaction of the
pore water chemistry. This was initially indicated by high sulphide concentrations,
increased ammonia- and strongly decreased sulphate concentrations. In some profiles, the sulphide horizon reached the sediment surface and led to the formation of
black spots. The vertical position of the maximum concentrations of S2, P0 4 and
NH4+changed with time and moved towards the sediment surface. During the winter 1994/95, the sediment chemistry regenerated and the values of experiment and
reference sites were comparable afterwards. This regeneration could have been due
to an intensified exchange of pore water during storm events. However, higher
sulphide-concentrations measured in summer 1995 still indicated the consequences
of the enrichment.