Effect of Salinity-Changing Rates on Filtration Activity of Mussels from Two Sites within the Baltic Mytilus Hybrid Zone:The Brackish Great Belt(Denmark)and the Low Saline Central Baltic Sea

Mussels from two sites within the Baltic mussel (Mytilus edulis × M. trossulus) hybrid zone were used in a comparative study on the effects of salinity-changing rates on filtration activity. The acute effect of varying salinity-changing rates was found to be similar in M. edulis from the brackish Great Belt and in M. trossulus from the low saline Central Baltic Sea, and the relationships could be described by linear regression lines through 0.0 indicating that the acute effect of deteriorating conditions at decreasing salinities is the opposite as for improving conditions when the salinity is subsequently increased. Further, both M. edulis and M. trossulus acclimatized to 20 psu reacted to an acute salinity change to 6.5 psu by immediately closing their valves whereupon the filtration rate gradually increased during the following days, but only M. trossulus had completely acclimatized to 6.5 psu within 5 days which may be explained by different genotypes of M. edulis and M. trossulus which probably reflected an evolutionary adaptation of the latter to survive in the stable low-salinity Baltic Sea.

Effect of Salinity on Growth of Mussels,Mytilus edulis,with Special Reference to Great Belt(Denmark)

The effects of salinities between 10 and 30 psu on the growth of blue mussels, Mytilus edulis, were studied in laboratory feeding experiments and compared to the growth of mussels suspended in net-bags in the brackish water Great Belt, Denmark. In the laboratory, 3 series of growth experiments were conducted: in Series #1, groups of mussels were exposed to 10, 15, 25 and 30 psu, in Series #2, two groups of mussels were exposed to 10 and 30 psu, respectively, for 15 days (first period) where upon the mussels were exposed to the reversed salinities for another 15 days (second period). In Series #3, two groups of mussels were initially exposed to 15 and 25 psu for 22 days whereupon the mussel groups were exposed to the reversed salinities for another 17 days. In the laboratory experiments there was a tendency towards reduced growth with decreasing salinity, reflected as reduced shell growth rate and decreasing weight specific growth rate with falling salinity. The shell growth rate was relatively low in the first feeding period compared to the second period, and mussels that were initially exposed to 10 psu, where the growth was low, exhibited fast growth when subsequently exposed to 30 psu, and reversed when 30 psu mussels were exposed to 10 psu. The study showed that mussels are able to adjust growth at changing salinities, and the observed effect of salinity could partly be explained by a temporary shell valve closure after a sudden change in salinity. The specific growth rate of mussels measured in laboratory experiments at salinities between 15 to 25 psu (4.2% to 4.8% d–1) were comparable to the growth of mussels in the field experiment (3.2% to 4.0% d–1) where the salinity varied between 24 and 13 psu during the growth period.

Invasion of Danish and Adjacent Waters by the Comb Jelly Mnemiopsis leidyi—10 Years After

The invasive ctenophore Mnemiopsis leidyi, which comes from North America’s east coast, was observed in Danish waters for the first time in 2007. Since then, the new invader has every summer spread in Danish and adjacent waters (i.e. North Sea, Limfjorden, Skagerrak, Kattegat, Belt Sea, Baltic Sea). The invasive comb jelly has apparently come to stay, as it has no effective enemies. Possible harmful effects of M. leidyi which feeds voraciously on zooplankton, fish eggs and larvae, have so far not been thoroughly studied in Danish waters, although dedicated attempts have been made in Limfjorden and in the central Baltic Sea. Over the last 10 years, the Danish national environmental monitoring program did not include gelatinous zooplankton, but new initiatives have been recently taken. A brief overview of our current knowledge on the impact of M. leidyi in Danish waters is given here.

In Situ Filtration Rates of Blue Mussels(Mytilus edulis)Measured by an Open-Top Chamber Method

Blue mussels, Mytilus edulis , form dense beds of both commercial and ecological importance, and many attempts have been made to determine their filtration rate. The total time in which mussels actually utilise their filtration capacity in nature varies greatly, making in situ methods for filtration rate measurements relevant. Further, it is being debated to what extend filtration rates measured in the laboratory using cultivated algal cells may apply for mussels in nature. In the present study, we have used an open-top chamber setup in order to allow repeated in situ filtration rate measurements of M. edulis using ambient natural phytoplankton and free-living bacteria. We found that the in situ measured filtration rates are comparable to filtration rates obtained in laboratory studies using different methods and controlled diets of cultivated algal cells. Further, we found that the retention efficiency of free-living bacteria was between 22.2% and 29.9%, in good agreement with values from laboratory studies. Our findings support the assumption that mussels in nature tend to use their filtration capacity when the phytoplankton concentration is above a certain lower trigger level.

Environmental Factors and Seasonal Variation in Density of Mussel Larvae(Mytilus edulis) in Danish Waters

Mussel larval densities may fluctuate considerably on both small spatial and short temporal scales. So far, only few and scattered data on the occurrence of mussel larvae have been reported from Danish waters. However, seasonal variation in density of blue mussel (Mytilus edulis) larvae as related to environmental factors (temperature, salinity, phytoplankton biomass) is basic information of relevance for future line-mussel farming in Danish waters. Here we report on the density of mussel larvae in a number of potential farming sites in the inner Danish waters. The mussel larval density measured in Skive Fjord, an eutrophicated inner branch of Limfjorden, during a period of 20 years, from 1989 to 2009, along with corresponding temperatures and chlorophylla, makes up the most important series of data reported here. In most years, a pronounced spring density peak and a subsequently lower autumn peak could be seen in Skive Fjord, but most conspicuous in the period 1993 to 2002 where the mean maximum spring larval density was 319 ± 260 ind·l-1. Further, data on mussel larval densities have been recorded on 4 locations in the Great Belt region: Kerteminde Bay in 2008 to 2011, and in 2008 at 3 other locations: Musholm Bay, Svendborg Sund, and Horsens Fjord. The maximum spring densities in the studied waters were observed in Skive Fjord, typically in May, whereas 10 to 100 times lower peak densities were found at the other locations studied. The reported observations show that mussel larvae are omnipresent in the studied areas and it is suggested that the larval density is sufficient forrecruitment to future line-mussel farms.