Eutrophication of coastal waters is a serious environmental problem with high costs for society globally. In eastern Skagerrak, reductions in eutrophication are planned through reduction of nitrogen inputs, but it is unclear how this can be achieved. One possible method is the cultivation of filter-feeding organisms, such as blue mussels, which remove nitrogen while generating seafood, fodder and agricultural fertilizer, thus recycling nutrients from sea to land. The expected effect of mussel farming on nitrogen cycling was modeled for the Gullmar Fjord on the Swedish west coast and it is shown that the net transport of nitrogen (sum of dissolved and particulate) at the fjord mouth was reduced by 20%. Existing commercial mussel farms already perform this service for free, but the benefits to society could be far greater. We suggest that rather than paying mussel farmers for their work that nutrient trading systems are introduced to improve coastal waters. In this context an alternative to nitrogen reduction in the sewage treatment plant in Lysekil community through mussel farming is presented. Accumulation of bio-toxins has been identified as the largest impediment to further expansion of commercial mussel farming in Sweden, but the problem seems to be manageable through new techniques and management strategies. On the basis of existing and potential regulations and payments, possible win-win solutions are suggested.

The post-larval growth pattern and dynamics of Amphiura filiformis (Echinodermata: Ophiuroidea) were investigated in 1991 at the sill of the Gullmarsfjord. A total of 201 specimens were examined in detail by video-techniques to analyse the growth patterns of disks, arms and arm segments. Maximum settling was recorded in October, with 7,100±7,400 (mean±SD) ind. m–2, after which settlement rapidly declined. The initial size of the settlers was around 300 µm in disk diameter with one arm segment developed. Specific growth rates during August–November were 0.42% day–1 for disk diameter and 1.76% day–1 for mean arm length. Growth of the oral width between 1.5 and 3 mm (3.6–9.7 mm disk diameter) shows a von Bertalanffy growth pattern, and the best fit for early growth was a model in which disk size increased exponentially with time. When combining the juvenile growth curve with adult growth curves from the literature, an asymptotic sigmoidal pattern appeared, which was best described by a Gompertz curve, suggesting that adult size and sexual maturity (ca. 4 mm disk diameter) are attained around 2 years after settlement. The inflexion point in the sigmoidal growth curve occurs ca. 1 year after settlement, at an approximate disk diameter of 2 mm. The higher growth rate after the early phase of life is suggested to be attributable to the attained capability of suspension feeding at this size, which in this species may be a more efficient means of acquiring energy than other modes of feeding. Predictions of disk diameter (up to 6 mm) from the reconstructed growth curve correspond well with measurements from the North Sea and elsewhere.

The presence of diarrheic shellfish toxins (DST) has been the main obstacle to mussel cultivation in Sweden. Monitoring of DST concentrations in mussels by HPLC for 12 years has shown great geographical and seasonal differences. Furthermore, individual mussels on the same farming strip may differ by a factor as great as 16. DST levels are usually low in summer, rise in September, remain high in the winter and vanish after the start of the spring bloom. A part of the Bohuslan fjord system north of the island Orust, which is secluded from the open sea by shallow sills, regularly shows low DST concentrations. This basin is among the most eutrophicated parts of the Bohuslan coastline. Cultivation of mussels in this area has been calculated to reduce the eutrophication, and results of field-tests support this view. As a consequence, a strategy for sea-gardening is being developed in which the presence of nutrients with primary production in the sea and cultivation of mussels are planned to balance one another.

The pelagic primary production and the secondary production of four species of benthic suspension feeders were frequently measured over a period of approximately one year in four different habitats. Simplified energy budgets over one-year periods are presented including ingestion, absorption, faeces production, respiration and secondary production. The production of a Mytilus edulis culture exceeded primary production by about 6.25 times, whereas semi-exposed shallow-water populations of Cerastoderma edule and Mya arenaria had a production roughly equal to primary production. In an exposed area, the latter species had a secondary production:primary production ratio of 0.15:1, whereas for a deeper-living (40 m) Amphiura filiformis population this relation was 0.0034:1. M. edulis followed by C. edule and M. arenaria in the semi-exposed habitat had the comparatively lowest Respiration:Absorption ratios and the highest Production:Absorption ratios. This study emphasizes the ecological importance of horizontal advective processes for energy transfer from the pelagic to the benthic system.

Experimental studies in a laboratory flume show that the sediment-living brittle-star Amphiura filiformis captures suspended particles. Feeding activity is a function of now velocity with few animals extending feeding arms in still water. Flow velocity also affects the orientation of feeding arms, and we suggest that this orientation is partly controlled by A. filiformis. By combining field measurements of current velocity and seston concentration with morphometrics and filtration models, a theoretical encounter rate of suspended particles was calculated for A. filiformis. In terms of organic content, A. filiformis can potentially balance growth and respiration with ingested seston although balance will strongly depend on retention efficiency and particle quality. Detailed measurements of flow around feeding arms revealed complex now patterns that will limit the applicability of available models of food encounter for passive suspension feeders, but our sensitivity analysis indicates that suspended aggregates may be especially important in the nutrition of this species.

Secondary production and population dynamics of the brittle star Amphiura filiformis (O. F. Muller) were studied between June 1990 and November 1991 at 40 m depth in the Skagerrak, west Sweden. Mean abundance of individuals > 1. mm in disc diameter was stable around 280 ind. m(-2) After a spawning maximum in June and July, settling post-larvae (ca 7000 ind. m(-2)) occurred in autumn 1991. Disc growth and gonad production accounted for ca 68.9 % (1.8 g AFDW m(-2) yr(-1)) of the total annual production in the population. About 13.3 % (0.34 g AFDW m(-2) yr(-1)) of the total production was allocated to regeneration of arms, probably a result of cropping by predators. Mean regenerated biomass in percent of total biomass for adult A. filiformis was between 12 and 30 % (mean 22 %). Annual production/biomass ratio was 0.46 yr(-1). The input of energy to arm regeneration indicates the importance of A. filiformis as an important food source.

Suspension feeding in adults of the Norway lobster Nephrops norvegicus (40–74 g) and the European lobster Homarus gammarus (280–350 g) was tested in experiments offering planktonic food items of different sizes from 200 to 600 μm and measuring the clearing capacity. Both lobster species were found to effectively clear water of food particles comprising nauplii of the brine shrimp Artemia salina of about 600 μm in size. These were reduced to 50% of the initial concentration within 5 h and to 90% within 12 h. When N. norvegicus was offered food particles averaging 200 μm, a significant reduction in average size occurred, indicating that the minimum retention size is around 200 μm. Fluorescently dyed Artemia salina were recovered in the stomach and intestine of lobsters proving that the filtered particles are passed to the digestive tract. Results from other experiments, using the blood pigment (haemocyanin) concentration as an index of nutritional state, indicated that the lobsters can get some nutritional advantage from suspension feeding. Suspension feeding in larger decapods has not been described previously, so the significance of this finding is discussed with respect to changes in behavioural and ecological role.

Studies of absorption, respiration and growth of Mytilus edulis L. were conducted at low temperatures, -1 to +4.8-degrees-C, with a natural seston composition. The bivalves actively ingested seston at -1-degrees-C and showed an absorption efficiency between 53 and 81% during the experimental period. The respiration rates ranged between 0.12 and 0.28 ml O2 h-1ind.-1 and the specific growth rate was 0.7% shell free dry weight per day. The results suggest that M. edulis can utilize the phytoplankton spring-bloom in boreal waters even at low temperatures.

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.

The open exposed Laholm Bay in the Kattegat is eutrophicated through riverine input, mainly of N. The benthic macrofauna down to 10 m depth (60 km2) is dominated by the suspension-feeding bivalves Cardium edule and Mya arenaria. To estimate the seasonal and annual consumption of seston by the suspension-feeders in Laholm Bay, we carried out three sets of observations. (1) The abundance and biomass of the macrofauna in this depth interval were assessed along eight transects. (2) The secondary production of the two bivalves was estimated over a 10-month period in two sampling squares. (3) The filtration rate of C. edule was determined in natural seawater in laboratory experiments during different seasons. The bivalves can in theory filter all of the water volume down to 10 m in 3 days and, thus, make a significant impact on the phytoplankton concentration. In our study, however, they filtered only approximately half of their potential feeding capacity, perhaps because food availability was low due to low turnover close to the bottom or due to physical disturbance. The majority of the phytoplankton is exported from the bay. Bivalve abundance, biomass, production and growth rate were moderate and generally lower than in adjacent areas to the north. In autumn, the bivalves consumed >90% of the seston in comparison with net-zooplankton consumption. An energy-flow diagram for the bivalves is presented including estimates of bivalve N excretion and biodeposition.

Marine eutrophication is studied in a multidisciplinary project in the Kattegat, western Sweden. Input of nutrients has increased several fold during this century; nitrogen ≈ 6 times, phosphrorus ≈ 10 times. This has caused elevated concentrations of nutrients in the water and increased primary production. Some adverse effects at the bottom have been noted annually from about August through October in the 1980's, e.g. fish catches have dropped and mortalities have been reported for fish and benthic animals including Nephrops noruegicus. In this paper we evaluate the effects on benthic macrofauna which are suggested to be caused mainly by oxygen deficiency occurring especially around the depth ofa strong halocline (≈ 15m), but also along a bottom transect down to 57 m. At some sites the benthic communities were impoverished, and at others only some species seemed to be especially sensitive, e.g. the bivalve Abra alba and the brittle star Amphiura filijormis. In shallow < 10 m) exp.sed waters an annually occurring mortality of molluscs was observed over large areas from 1980 onwards. Affected species were mainly the bivalves Cardium edule and Mya armaria. The causes behind this mortality may also be oxygen deficiency occurring during calm weather periods.

A long-line study of mussel culture occupying an area of 25 × 180 m in a sheltered bay in western Sweden commenced in June 1978 when spat first settled. Settlement on 50 mm × 6 m polypropylene bands hung from the long-lines reached more than 1000 Mytilus spat per meter of band and stabilized to about 400–500 per meter of band one year after settlement. The best results were obtained 0–2 m below the surface after the second summer where the highest density, greatest length and biomass, and most prolific production were obtained. The highest increase in biomass and the heaviest standing crop occurred during the second autumn when average mussel lengths of 60 mm were recorded at the 0–2 m depth 17–18 months after settlement. This was considered the optimum time for harvesting; the actual harvest took place in March 1980.

Energy-flow is described for a blue mussel long-line culture of 4 500 m2 which settled in June 1978 and was harvested in March 1980. Food quality varied with season. Phytoplankton blooms contained >95% of the seston protein and carbohydrate. However, in winter, >90% of these substances were bound in detritus. Consequently mussel energy content declined in both winters. Current speed had a theoretical linear relationship with long-line numbers during non-winter months. Particulate retention in the culture centre was 15–50%, suggesting culture numbers could be nearly doubled without significant growth depression.