In June/July 1994, July and September 1995 and in January 1996 several drift experiments and high resolution sampling transections in the salinity gradients were performed in the Pomeranian Bight to describe the mixing and biological and
Chemical and biological interactions in mixing gradients in the Pomeranian Bight 14
chemical transformation of introduced river water. However, the river Oder does not enter the bight directly. A shallow lagoon, the Szczecin Lagoon (Odra Haff), is situated between the river mouth and the open bight. The Szczecin Lagoon and the Pomeranian Bight are connected via the rivers Peene and Szwina and lead to mixing of river water with water from the bight. The lagoon water with a salinity range between 0.5 and 2 PSU (practical salinity units) enters the Pomer-anian Bight in a pulse like manner and in plumes of different size and is mixed with bight water within 2 or 3 days (v.Bodungen et al., 1995).
Water samples were taken at depths of 1–2, 5–6 and 8–10 m with a rosette sampler combined with sensors for conduc-tivity, temperature and density (CTD) as well as a sensor for fluorescence.
Inorganic nutrients were analysed using standard colorimetric methods according to Rohde & Nehring (1979) and Grasshoff et al. (1983).
For the determination of dissolved carbohydrates, samples were filtered through GF/F filters. Dissolved monosaccarides were estimated according to the 3-methyl-2-benothiazolon-hydrazon (MBTH) method (Johnson & Sieburth, 1977). For the determination of total dissolved carbohydrates (TCHO), filtered water samples were hydrolysed with 0.09N HCl at 100°C for 20h followed by the application of MBTH method.
For the characterisation of the degradation of carbohydrates by bacteria, glucosidase and glucosaminidase (chitinase) activity were investigated. Glucosidase degrades preliminary oligosaccharides. Glucosaminidase acts on aminopolysac-charides like chitin. The enzyme activities were determined according to Hoppe (1993) using the model substrates 4-methylumbelliferryl (MUF)-a-glucoside, -ß-glucoside and MUF-glucosaminide. For estimation of in situ hydrolysis of natural substrates, the hydrolysis rate (Hr (%h–1)) was measured at final concentrations of 100 nM MUF-a-or ß-glucoside and at in situ temperatures.
Turnover rates (To (%h–1)) of glucose were determined with D-[U-14C] glucose at final concentrations of 80nM at in situ temperatures (Jost & Pollehne, 1998).
Results
In winter, nitrogen and phosphorus from the Oder river were introduced into the Pomeranian Bight mainly in inorganic form. Reduced primary and bacterial production (<5% of summer values for primary production, <8% of summer values for bacterial production) were indications of low biological activities. In the growth season, these nutrients were trans-formed already in the lagoon by phytoplankton production and entered the bight as organic material as shown by the high particulate organic carbon and nitrogen concentrations of 229.4–510.3µmol l–1 and 29.4–36.4µmol l–1, respectively (Table 1). Phytoplankton biomass is the dominating component of introduced organic material. It accounted for up to 70%
of the total organic carbon. In winter, the decrease of inorganic nutrients in the salinity gradient in the Pomeranian Bight was caused by conservative dilution of lagoon water with water from the open bight. In that season, low biological activity left the inflowing nutrients without significant alteration. Figure 1 shows the behaviour of ammonia and nitrate in the salinity gradient. Phosphate and silicate had the same characteristics. In summer, inorganic nutrients were generally low.
Therefore, no or only small gradients were observed. The difference before and after dilution were about 0.3µmol l–1 for nitrate, about 0.1µmol l–1 for ammonia and 0,02µmol l–1 for phosphate.
Table 1 Concentration of nutrients, particulate organic matter and phytoplankton biomass in the river plume entering the Pomeranian Bight
Phytoplankton C(µmol l–1) 11.4–16.2 118.6–361.1
M. Nausch and E. Kerstan 15
Figure 1 Ammonia and nitrate concentrations in the salinity gradient in winter 1996
The highest concentrations of dissolved carbohydrates were measured near to the Swina mouth in the growing season.
Concentrations of TCHO up to 15.1µmol l–1 (Figure 2) and MCHO up to 4.8µmol l–1 (Figure 3) were estimated. In winter, the values were significantly lower. TCHO and MCHO concentrations of about 4µmol l–1 and 1µmol l–1 were determined. Dissolved carbohydrates had the same concentrations in summer and in autumn. The pattern of dissolved carbohydrates in the salinity gradient varied from year to year and from season to season. TCHO showed a significant decrease with increasing salinities in July 1994 and September 1995. During the other investigations, the TCHO concen-trations remained on the same level: 3–4µmol l–1 in January 1996 and about 7.3µmol l–1 in July 1995. MCHO showed a clear relationship to salinity only in autumn 1995 (Figure 3). Its concentrations scattered from 1.1 to 3.4µmol l–1.
Figure 2 TCHO concentrations in the surface layer in winter and in the growth season
Chemical and biological interactions in mixing gradients in the Pomeranian Bight 16
Figure 3 MCHO concentrations in salinity gradients
The hydrolysis rate of the carbohydrate degrading enzymes a-glucosidase, ß-glucosidase and glucosaminidase showed the same pattern in the salinity gradient and between the seasons. The values of these enzyme activities were in the same range and decreased linearly in the salinity gradient. In summer, glucosidase activity reduced from 13.9% h–1 to 0.3%
h–1 and the glucosaminidase activity from 9.9% h–1 to 0.2% h–1 during mixing processes. Bacteria are the carrier of these enzymes. The decrease of glucosidase activities is higher (by factor 46) than the decrease of bacterial counts (by factor 2.5). From that it can be deduced that the activity per bacterial cell decreased and that the specific activity is the dominant factor influencing the pattern of glucosidase activities. In winter, glucosidase- and glucosaminidase activities amounted to only 8% of summer values (Figure 4).
Figure 4 Glucosidase activities in winter and in the growth season
Glucosidase- and glucosaminidase activity correlated with the MCHO and TCHO (Table 2). These relationships were especially clear at low glucosidase activities up to 4.5% h–1 and glucosaminidase activities up to 2% h–1 (Figure 5). At higher glucosidase activities in the outflowing lagoon water where the highest bacterial production (Jost & Pollehne 1998) and enzyme activities were measured this relationship was not evident.
Table 2 Correlation coefficients between dissolved carbohydrate concentrations and glucosidase activities up to 4.5%h–1 and glucosaminidase activities up to 2% h–1
MCHO µmol l–1 TCHO µmol l–1 glucosidase activity (% h–1) 0,38
n=36, p=0.05
0,22 n.s.
glucosaminidase activity (% h–1) 0,81 n=36, p=0.01
0,67 n=33, p=0.01
M. Nausch and E. Kerstan 17
Figure 5 Relation between MCHO and TCHO concentrations and glucosidase activities
The uptake of low molecular weight substances by bacteria, determined as glucose turnover (To), was highest in summer. At this time, glucose turnover rates up to 30% h–1 were measured in the outflowing lagoon water. After dilution in the salinity gradient the values were reduced to 1–2% h–1. In winter, turnover rates ranged between 0.2 and 0.5% h–1. The quotient between To and Hr (To/Hr) can be used as an index for coupling of glucose uptake and release via enzymatic degradation by bacteria. In the outflowing lagoon water, a median To/Hr-ratio of 1.6 was determined in winter and a ratio of 4.2 in summer. In the growing season, the To/Hr-ratio had a relatively constant level in a salinity range between 1.9 and 7 PSU. Between 7 and 7.8 PSU, the quotient rose up to 21.4 (Figure 6). The increase is due to the fact that Hr was more (factor 4.8) reduced than To (factor 2.1). At salinities >5 PSU, the relationship between uptake of glucose and the hydrolysis rate of carbohydrates and the concentration of MCHO could be observed. In this range, the distribution of dissolved monosaccharides and the To/Hr-quotient were independent from the salinity. There was a negative correlation between the To/Hr-quotient and MCHO concentrations (Figure 7).
Figure 6 Relation of glucose turnover and glucosidase activity in the salinity gradient
Chemical and biological interactions in mixing gradients in the Pomeranian Bight 18
Figure 7 Relation between To/Hr-quotients and concentrations of MCHO and TCHO at salinities >5 PSU
Discussion
Organic and inorganic material introduced into the Pomeranian Bight can be modified by physical dilution or by transfor-mation via biological processes. These processes cannot be distinguished clearly, biological processes are masked by physical dilution. However, we could show by the example of glucose turnover and enzymatic carbohydrate degradation that an interaction of different parts of planktonic community existed.
Carbohydrates are produced as a result of photosynthesis and are released into seawater by exudation from phytoplankton and cell lysis as well as sloppy feeding of zooplankton (Klok et al., 1984, Mopper et al., 1991, Münster &
Chrost, 1991). We assume that phytoplankton was the main source of dissolved carbohydrates in the Pomeranian Bight because the highest TCHO and MCHO concentrations were found near the Swina mouth where the phytoplankton biomass was also highest (Jost & Pollehne, 1998). Zooplankton biomass in the outflowing lagoon water was not higher than in the open bight. However, there was a shift from limnetic to more marine species (Postel & Mumm, 1995).
For stock parameters (POC and chlorophyll) as well as for activity parameters (primary production, bacterial production) a linear decrease was observed in all gradients (Jost & Pollehne, 1998). In contrast to that, dissolved carbohydrates had not such a strong relationship to salinity. Especially in summer, the MCHO concentrations were not correlated with the salinity. According to Jost & Pollehne (1998), the primary production near the Swina mouth is more light-limited than in the open bight. Respiratory processes exceeded the primary production and a negative carbon balance was calculated for the whole water column. Due to the deeper light penetration, the carbon balance in the open bight was positive. These relationships between autotrophic and heterotrophic processes could have an influence on the concentrations of TCHO and MCHO.
Bacteria are the main consumers of low molecular weight substances and they possess extracellular enzymes for carbo-hydrate degradation. For the characterisation of extracellular enzymes the maximum enzyme activity (Vmax) is used (Hoppe, 1993). In this context, the enzyme activities at low MUF-substrate concentrations (Hr) were used for a better description of the in situ substrate hydrolysis. The distribution of Vmax of glucosidase activities in the salinity gradient of the Pomeranian Bight is shown in Nausch et al. (1998). Vmax and Hr correlated.
TCHO are substrates for glucosidase- and glucosaminidase activity. The mechanism of substrate stimulation can be made responsible for the correlation of these parameters. The rapid degradation of TCHO and uptake of MCHO can lead to a constant or lower level of dissolved TCHO and MCHO in this area. The relationship between glucosidase activity and MCHO was not so tight because MCHO can be directly released by phytoplankton in addition to the release after degradation of polysaccharides.
Figure 7 demonstrates the connection between glucose turnover, the hydrolysis of carbohydrates and the concentrations of MCHO and TCHO at salinities >5PSU. The To/Hr quotient correlated with MCHO as well with TCHO. The negative correlation between the To/Hr quotient and TCHO can be explained by substrate stimulation as a regulatory mechanism of extracellular enzyme activities (Münster, 1991, Rath et al., 1993, Karner & Rassoulzadegan, 1995). The decrease of TCHO may cause a lower stimulation of glucosidase activity with the result that the importance of hydrolysis products for bacterial uptake is reduced. This assumption was supported by the decrease of the specific glucosidase activities in
M. Nausch and E. Kerstan 19
the salinity gradient. The negative correlation between To/Hr quotients and MCHO can be attributed to the turnover which exceeded the hydrolysis and caused the decrease of MCHO-concentrations coming from other sources.
Acknowledgements
This study was funded by the German Ministry for Education, Research and Technology (03F0105B). We are grateful to Dr. K. Nagel, Dr. F. Pollehne and Dr. G. Jost for values of POC, PON, primary production and bacterial glucose turnover.
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Vadims Jermakovs and Hans Cederwall 21 ICES Cooperative Research Report, No. 257 Baltic Marine Science Conference, Rønne, Denmark, 22–26 October 1996