114
:) SpatIal alld Temporal Diqnbutlllil Patterns
the increased POC-contents in the sediments of a mussel bed, the food quality of
the organic matter for further heterotrophic organisms was reduced by the nutritional uptake of the mussels.
5.2.1.3
Groninger Plate
The geochemical investigations on the Groninger Plate were carried out in order to
follow the reactions of the sediment- and pore water composition to an excessive
input of organic carbon to the sediments. The emphasis of the investigation was
on:
• monitoring the dynamics in the pore water chemistry of reference sites as background data for the experimentally enriched sediment sites
• field experiments on the enrichment of sediment with organic carbon
• improvement and use of a mathematical model on the development of black
spots.
The fieldwork started on the Groninger Plate in April 1994 and was continued until
September 1996. Except for the winter months, samples were taken at least once a
month mainly at the sandtlat sites GP I and GP3 (Chap. 3.1). During the entire
investigation, the macrobenthos of the study area was dominated by Arenicola
marina and Lanice conchilega (Chap. 3.6.2).
Reference dynamics
The field investigations concentrated mainly on the monitoring of the dynamics of
dissolved pore water nutrients. Sediment profiles of phosphate, ammonia, nitrite,
nitrate, silicate, sulphide, sulphate and DOC were measured to a depth of max.
50 cm. Samples were taken 2 to 8 times per month at station GP3 on the Groninger
Plate (Fig. 3.1.4), so that a relatively large number of sediment depth profiles was
available, which were analysed with a variety of statistical procedures (Langner
1997).
The distribution of nitrate and nitrite was very variable. However, while the
median values were close to the detection limit, enrichments occurred in spring
and autumn, which could be explained by local peaks in single depth profiles.
Compared to 1995, the values were generally lower in 1996 and showed little
variation over the year. A decline of the values which was recorded in June/July
1995 could not be observed in 1996. Nitrite and nitrate occurred in sediment
depths down to 25 cm although they are products of oxygen-dependent mineralization and consumed under anoxic conditions. Sporadic and small-scale maxima
pointed to locally increased bioturbation activity. which facilitated an entry of
oxygen down into deeper sediment layers.
With values below 300 !lmol dm", the sulphide concentrations of the pore water
remained at low levels during 1994 and 1995. Unusually high sulphide concentrations (up to I mmol dm\ were detected in the year 1996, but most of the values
were lower than 300 !lmol dm ' in that year as well.
The concentrations of ammonia. phosphate and sulphide as direct products of
mineralization usually showed similar dynamics below the 0, penetration depth,
:) SpatIal alld Temporal Diqnbutlllil Patterns
the increased POC-contents in the sediments of a mussel bed, the food quality of
the organic matter for further heterotrophic organisms was reduced by the nutritional uptake of the mussels.
5.2.1.3
Groninger Plate
The geochemical investigations on the Groninger Plate were carried out in order to
follow the reactions of the sediment- and pore water composition to an excessive
input of organic carbon to the sediments. The emphasis of the investigation was
on:
• monitoring the dynamics in the pore water chemistry of reference sites as background data for the experimentally enriched sediment sites
• field experiments on the enrichment of sediment with organic carbon
• improvement and use of a mathematical model on the development of black
spots.
The fieldwork started on the Groninger Plate in April 1994 and was continued until
September 1996. Except for the winter months, samples were taken at least once a
month mainly at the sandtlat sites GP I and GP3 (Chap. 3.1). During the entire
investigation, the macrobenthos of the study area was dominated by Arenicola
marina and Lanice conchilega (Chap. 3.6.2).
Reference dynamics
The field investigations concentrated mainly on the monitoring of the dynamics of
dissolved pore water nutrients. Sediment profiles of phosphate, ammonia, nitrite,
nitrate, silicate, sulphide, sulphate and DOC were measured to a depth of max.
50 cm. Samples were taken 2 to 8 times per month at station GP3 on the Groninger
Plate (Fig. 3.1.4), so that a relatively large number of sediment depth profiles was
available, which were analysed with a variety of statistical procedures (Langner
1997).
The distribution of nitrate and nitrite was very variable. However, while the
median values were close to the detection limit, enrichments occurred in spring
and autumn, which could be explained by local peaks in single depth profiles.
Compared to 1995, the values were generally lower in 1996 and showed little
variation over the year. A decline of the values which was recorded in June/July
1995 could not be observed in 1996. Nitrite and nitrate occurred in sediment
depths down to 25 cm although they are products of oxygen-dependent mineralization and consumed under anoxic conditions. Sporadic and small-scale maxima
pointed to locally increased bioturbation activity. which facilitated an entry of
oxygen down into deeper sediment layers.
With values below 300 !lmol dm", the sulphide concentrations of the pore water
remained at low levels during 1994 and 1995. Unusually high sulphide concentrations (up to I mmol dm\ were detected in the year 1996, but most of the values
were lower than 300 !lmol dm ' in that year as well.
The concentrations of ammonia. phosphate and sulphide as direct products of
mineralization usually showed similar dynamics below the 0, penetration depth,