SE Kattegat Research Articles

Manganese accumulation by the antennule of the Norway lobster Nephrops norvegicus (L.) as a biomarker of hypoxic events.

In laboratory tests, manganese accumulation by the appendages of the sediment burrowing Norway lobster, Nephrops norvegicus (L.) (including the lateral antennules) was approximately three times greater [600 μg Mn g −1 (dry weight) after 5 days in 20 mg Mn l −1] than that by the carapace. The accumulation was linearly dose-dependent (10–40 mg Mn l −1) and duration-dependent (2–30 days), and showed no decrease after 3 weeks in undosed seawater. A high manganese uptake to the lateral antennules during hypoxia in the field was verified from the SE Kattegat, Sweden. These results indicate that accumulation of Mn on the mobile appendages of the Norway lobster fulfils most of the criteria for a biomarker of exposure to hypoxia. Using these measurements in conjunction with Mn concentrations in the internal tissues, it may be possible to resolve both the timing and the extent of the Mn exposure and the underlying hypoxic event.

Responses to hypoxia of plaice, Pleuronectes platessa, and dab, Limanda limanda, in the south-east Kattegat: distribution and growth

Plaice, Pleuronectes platessa, and dab, Limanda limanda, were sampled with a Glommen lobster trawl at 25 to 40 m depth in the SE Kattegat during spring and autumn of 1984 to 1990. During autumn, hypoxia (O2-concentration < 3 mg I−1) occurred in the bottom water below the halocline for four to ten weeks every year, except in 1984 when moderate hypoxia (O2-concentration 3–5 mg 1−1) occurred. Biomass of both species was shown to be negatively correlated with oxygen concentration during autumn. Further, a decrease in population mean total length was observed during the study period in both spring and autumn samples. Laboratory studies of growth of juvenile plaice and dab, at 15° C and 30–34%, showed that growth is reduced at 50 and 30% O2-saturation for both species during a 20 d period. There was some adaptation to hypoxia resulting in less reduction of growth during the second half of the experiment. The frequency of fish eating was reduced in plaice at 30% O2-saturation. Reduced mean total length of the plaice and dab population of the SE Kattegat is discussed in view of sublethal effects of oxygen deficiency.

Nutrient recycling by coastal sediments: effects of added algal material

The deposition of a Skeletonerna bloom onto sediment collected in a coastal bay was simulated and the effects on 02, N and P fluxes were recorded. A comparison of calculated net production and observed net efflux of inorganic nitrogen (IN = ammonium + nitrate) for the period prior to the addition of algae, indicated that two-thirds of the produced ammonium was nitrified and subsequently denitrified. Algal additions induced a dramatic and persistent rise in oxygen consumption (1.6 X ) and release of IN (4.5 X) . DON (dissolved organic nitrogen, 2 X), and Pod3(3 X). Of the added algal N (76 mm01 N m-2) , 33 % was recycled as NH,' and DON during the 19 d of incubation and, according to Pod3measurements, the same was true for the algal P, giving a half-life of the algal biornass of ca 4 wk. Comparison of calculated net production and observed net efflux of IN indicated a considerably decreased denitrification/mineralisation ratio due to algal additions. Extrapolating to the size of Laholm Bay, SE Kattegat (sampling site), suggests that the settlement of algae (realistic amount used here) would make the sediment the most important nutrient source of this bay for a few weeks period. DON was a significant part of the released, non-gaseous nitrogen, both before (46 %) and after (27 %) the addition of algae. In cores subjected to anaerobic conditions, the concentrations of DON decreased considerably, suggesting that the production of DON was inhibited during such conditions and that much of the DON in this sediment was easily utilized.

Hypoxia, salinity and temperature as structuring factors for marine benthic communities in a eutrophic area

During 1983 to 1990 the benthic macrofauna was studied at eight stations along a depth gradient from 5 to 53 m in the SE Kattegat on the Swedish west coast. The area is almost non-tidal and characterized by a strong halocline at about 15 m with brackish water (S=12 to 25) at the surface and oceanic water (S=32 to 34) below the halocline. Increasing eutrophication of the area has caused annual periodic below-halocline oxygen deficiency in the 1980s. In this paper, we present spatial and temporal benthic-community structural differences between the fauna above, around and below the halocline. As a consequence of the extensive and severe hypoxia below the halocline, especially in autumn 1988, significant reductions of the benthic fauna were recorded. We conclude that the salinity variation around the halocline in combination with oxygen stress at these depths and just below the halocline have great impact on the benthic faunal structure and biomass. Our long-term field observations suggest that benthic sub-littoral communities can survive lower oxygen concentrations than previously presented.

Energy flow through the SE Kattegat: A comparative examination of the eutrophication of a coastal marine ecosystem

Energy flow through the major compartments of the ecosystem of the south-east Kattegat has been assessed based on observations made between 1985–1989. In this increasingly eutrophic semi-estuarine coastal area, energy inputs are dominated by autochthonous production during the spring and autumn bloom periods (>90%), and by allochthonous fluvial sources during the winter (>50%). An unbalanced annual carbon budget is presented, which demonstrates the predominance of infaunal benthic suspension feeders in the shallow areas (above 13 m), where their demand alone is 2.4 times the calculated carbon supply to the sediment. Benthic demand does not diminish below 13 m, but the proportion utilized by macrofaunal deposit feeders and meiofauna increases with lower suspension feeder demand, emphasizing the close coupling between pelagos and benthos at all depths in this system. Excess benthic demand over planktonic production is assumed to be met by lateral imports across the system boundaries. Intermittent summer hypoxia below the halocline (mean depth 15 m) has a severe effect on the benthos of the area, leading to a reduction in epifaunal predation prior to the reduction of macrobenthic organisms. Such events lead to the temporary accumulation of sedimentary carbon before late autumnal mixing initiates the reoxygenation and eventual recolonization of the affected areas. Comparisons with published carbon budgets for the Chesapeake Bay and the Baltic Sea suggest major differences between the relative roles of pelagos, and benthos in these systems. In the Baltic, which is fuelled predominantly by autochthonous processes, pelagic carbon flows are about 4 times those of the benthos. Allochthonous inputs predominate throughout much of the year in the Chesapeake Bay, where pelagic flows are proportionately 1.5 × those of the benthos, whereas in the SE Kattegat benthic flows exceed those within the pelagos by about 25. The higher populations of epifaunal and nektonic predators found in the SE Kattegat system, as opposed to the other two systems, is a corollary of the greater energy flows through the benthic components. The demonstrable vulnerability to anoxia of these commercially important organisms emphasizes the sensitivity of such coastal systems to the consequences of increased nutrient inputs.

Influence of sublittoral microphytobenthos on the oxygen and nutrient flux between sediment and water. A laboratory continuous-flow study

Influence of sublittoral microphytobenthos on the flux of oxygen and inorganic nutrients ( N , P, Si) at the sediment-water interface was studied using undisturbed cores of sandy and muddy sediment incubated in a laboratory continuous-flow system, either in darkness or with a 16/8 h L/D cycle at in situ light level during summer. Sediment was collected In July at 15 m depth in a non-tidal, stratified bay in SE Kattegat. To test whether the higher content of inorganic nutrients below the halocline, compared with surface waters, could stimulate microphytobenthic growth, 2 levels of nutrient concentrations were used. Diel variations were found In L/D cores, but not in darkened cores, for oxygen, dissolved inorganic nitrogen and phosphorus content in the water overlying the sediment. The flux of NH4+, NO3and ~ 0 ~ ~ out of the sediment decreased during light periods and occasionally a net uptake was recorded. Light-induced O2 production, and correlations between A fluxes (differences between day and night fluxes of 0, and nutrients), chlorophyll a content and algal cell numbers in the sediment, indicate that the decreased outflux of IN and POA3was mediated by photosynthetic organisms. Diel variations were not studied for silicon, but a significantly lower outflux, or even an uptake, of Si(OH), from L/D cores supports this conclusion. This suggests that diatoms play a major role in the nutrient flux between sediment and water. Also, the differences in pore-water nutrient gradients between LID and dark cores point to the importance of sediment-associated organisms. Daily (24 h) net fluxes of nutrients were primarily out of the sediment, but the magnitude depended on both light conditions and sediment type. Daily net outflux was significantly lower in U D cores than in darkened cores for all nutrients except NO3in muddy cores and NO2in sandy cores. Net uptake in L/D cores was recorded for Si(OH)4 and NO3In sandy sediment. Outflux of nutrients was significantly higher from muddy sediments in comparison w ~ t h sandy sediments (except NO3-), especially in permanent darkness. No significant effect of nutrient enrichment on the abundance of sublittoral benthic microalgae could be shown. Results suggest that m~crophytobenthos can influence sediment-water exchange of inorganic nutrients even at sublittoral depths, and when measuring nutrient flux in permanently darkened cores from depths around 15 m in the Kattegat, summer flux rates will be overestimated by a factor varying between 2 and 6, depending on sediment type.

Effects of oxygen depletion on the ecology, blood physiology and fishery of the Norway lobster Nephrops norvegicus

Biomass, population structure, food selection and blood (haemolymph) physiology of the Norway lobster Nephrops norvegicus (L.) were investigated in SE Kattegat, an area where low oxygen concentrations (t 2 m1 1-', 30 % 02-saturation) have occurred in the bottom water for 1 to 3 mo periods in most years in the 1980's. During the study period (October 1984 to September 1989) lobster biomass decreased in the area from 10.8 kg h-' (catch per u n ~ t effort) to zero (estimated during the last 12 nlo of the investigation). Males contributed on average 78 % of the population density, except in September 1988 during severe hypoxia when a reversed sex ratio was found and females (even berried) dominated (75 % of density). The food of N. norvegicus belonged to 4 major groups; crustaceans, echinoderms, molluscs and polychaetes. The dominant species eaten within these groups were also found to be dominants in the benthic infauna. This suggests that N. norvegicus are not feeding selectively but taking available organisms indiscriminately. In the field and in laboratory experiments N. norvegicus increased blood pigment (haemocyanin, Hcy) concentration in moderate hypoxia (20 to 40 '10 0 2 saturation), and reduced it in severe hypoxia (10 to 20 % 02-saturation). At 02-saturations below 15 % N. norvegicus ceased feeding and had empty stomachs. Thus in low oxygen concentrations the lobsters suffer from hypoxia-induced starvation rather than lack of food. Survival of N. norvegicus exposed to 15 and 10 % 02-saturation was 4 wk and 2 to 4 d, respectively. After return to normoxia recovery of blood Hcy concentration was slow, probably due to lack of copper in the diet, which is essential for Hcy synthesis. We consider blood Hcy concentration to be a promising 'in situ' biomarker with ecological relevance.

Influence of microphytobenthos on the nutrient flux between sediment and water- a laboratory study

The importance of microphytobenthos as a regulator of the flux of inorganic phosphorus and nitrogen between sediment and water was investigated in a laboratory study. Sediment from the SE Kattegat was incubated in nutrient-enriched seawater during exposure to varying light quantities (0, 5, 10, and 30 pE m-' S' ) . Microphytobenthos growth was limited by both inorganic nutrients and light quantity. When exposed to no-light conditions, chlorophyll a content of the sediment decreased only slightly and remained at an almost constant level (50 to 55 mg m-2) for several weeks, and increased rapidly when exposed to light. A relationship was observed between light quantity and the rate and direction of the flux of NH: and PO:-, but not NO;, between the sediment and overlying water The indirect influence of the microphytobenthos on the nutrient flux, i.e. by changes in the oxygen concentration, seemed more important than the direct effect by uptake by the algae. The rate of release of phosphate from the sediment exposed to darkness was up to 0.2 mm01 PO4 m-2 d-l The maximum mean rate of ammonia release was 1.6 mm01 m-' d-' in the dark and ca 0.1 mm01 m-2 d-' at 5 yE S-'. When the amount of light was 3 10 pE m-' S-', the microbenthic activity (oxygen production and increased nutrient requirements) prevented the release of PO4 and NH4 from the sediment.

Can microbenthic photosynthesis influence below-halocline oxygen conditions in the Kattegat?

Abstract Gross benthic oxygen uptake at 20 m depth in the Laholm Bay, SE Kattegat (measured in the laboratory on darkened cores) varied between 3.5 and 6.5 ml O2 · m−2 · h−1 for the period May through September. The integrated oxygen uptake for this period was calculated to 17.6 l O2 · m−2 or 8.0 g C · m−2 if RQ = 0.85. At 14–16 and 17–18 m depth a microbenthic primary production of 15 and 3.5 g C · m−2 · yr−1, respectively, has been measured with the 14C-technique. This corresponds to 28 and 6.5 l O2 · m−2 if PQ = 1.0. At 20 m production was 1 g C · m−2 · yr−1 (1.9 l O2). Microbenthic primary production is concentrated to the period May to September. Laboratory experiments show that a light intensity of approximately 10 µE · m−2 · s−1 (16L/8D) is sufficient to make the benthic community autotrophic (P/R> 1) when the heterotrophic activity corresponds to 10 ml O2 · m−2· h−1, i.e. above the maximal benthic respiration found during summer at 20 m depth in the Laholm Bay. Light intensity in summer often exceeds 10 µE · m−2 · s−1 at 15 m depth. We conclude that for coastal areas in SE Kattegat oxygen in below-halocline waters (>14 m depth) may be significantly influenced by photosynthesis of microbenthic algae. Eutrophication may initially improve benthic oxygen conditions through increased microbenthic photosynthesis. However, with more advanced stages of eutrophication plankton blooms are likely to cause a deterioration of the benthic light climate, diminishing benthic photosynthesis.

Connection between trace metals, chelators and red tide blooms in the Laholm Bay, SE kattegat—an experimental approach

The influence of the trace metals, copper, aluminum and iron, and of the strong complexing agents, EDTA and NTA, on phytoplankton growth in water from a brackish water bay was investigated through bioassay experiments. A diatom ( Skeletonema costatum (Grev.) Cleve) and a dinoflagellate ( Prorocentrum minimum (Pav.) J. Schiller) were used as test organisms. The growth of both phytoplankton species was strongly inhibited by copper. This inhibition was generally eliminated by EDTA and NTA. Both phytoplankton species were considerably less inhibited by aluminum than by copper at the same total metal concentration. While S. costatum responded to copper and chelator additions in the same way in sea water samples from different seasons, the growth of P. minimum exhibited pronounced seasonal variation. Other parameters than the values of pCu must be considered in order to account for the experimental results. This work supports the theory that alterations in contents of trace metals and natural chelators in sea water are important factors behind shifts in phytoplankton species composition.

Alkaline phosphatase activity in the Laholm Bay, south-eastern Kattegat

Measurements of alkaline phosphatase activity (APA) and experimental investigations of limiting nutrients for phytoplankton biomass formation were performed using water from Laholm Bay, SE Kattegat. Alkaline phosphatase activity was insignificant during the autumn and spring phytoplankton blooms which indicates that phosphorus is not a limiting nutrient for phytoplankton growth in the area. Nitrogen additions greatly increased chlorophyll production in the enrichment experiments during the spring bloom, and to a lesser extent the rest of the year. Water from rivers entering the Laholm Bay greatly stimulated phytoplankton growth when mixed with Laholm Bay water. APA increased in flasks with river water additions indicating phosphorus limitation of phytoplankton growth. The results are explained by the high N/P ratio in river water and low ratio in Bay water.