The effect of salinity on the filtration rate of blue mussels, Mytilus edulis, from the brackish Great Belt (Denmark) and the low-saline Central Baltic Sea, respectively, was studied. First, we measured the effect of longterm (weeks) constant ambient salinities between 5 and 30 psu on the filtration rate of M. edulis collected in the Great Belt where the mean salinity is 17 psu. At salinities between 10 and 30 psu, the filtration rates did not vary much, but at 5 psu the filtration rates were significantly lower. Next, we studied dwarfed M. edulis (\25 mm shell length) from Central Baltic Sea (Askö, Sweden) where the mean salinity is 6.5 psu. The maximum filtration rate (F, ml min -1 ind.-1 ) as a function of shell length (L, mm) and dry weight of soft parts (W, mg) were found to be: F = 0.003L 2.71 and F = 0.478W 0.92

No abstract

The exhalant jet flow of mussels in conjunction with currents and/or other mussels may strongly influence the mussels' grazing impact. Literature values of mussel exhalant jet velocity vary considerably and the detailed fluid mechanics of the near-mussel flow generated by the exhalant jet has hitherto been uncertain. Computational modelling of this phenomenon depends on knowledge of the velocity distribution near the exhalant siphon aperture of mussels to provide appropriate boundary conditions for numerical flow models. To be useful such information should be available for a range of mussel shell lengths. Here, we present results of a detailed study of fully open mussels Mytilus edulis in terms of filtration rate, exhalant siphon aperture area, jet velocity, gill area and body dry weight, all as a function of shell length (mean ± SD) over the range 16.0 ± 0.4 to 82.6 ± 2.9 mm, with the corresponding scaling laws also presented. The exhalant jet velocity was determined by 3 methods: (1) measured clearance rate divided by exhalant aperture area, (2) manual particle tracking velocimetry (PTV) using video-microscope recordings, and (3) particle image velocimetry (PIV). The latter provides detailed 2-component velocity distributions near the exhalant siphon in 5 planes parallel to the axis of the jet and the major axis of the oval aperture, and hence estimates of momentum and kinetic energy flows in addition to mean velocity. Data obtained on particles inside the exhalant jet of filtered water was verified by the use of titanium dioxide seeding particles which were de-agglomerated by ultrasound to a size range of 0.7 to 2 µm prior to addition, to avoid retention by the gill filter of the mussels. We found that exhalant jet velocity was essentially constant at ~8 cm s −1 , and independent of shell length. Based on geometric similarity and scaling of mussel pump-system characteristics we found that these characteristics coincide approximately for all sizes when expressed as pressure head versus volume flow divided by shell length squared. KEY WORDS: Filtration rate · Exhalant siphon area · Particle image velocimetry · Particle tracking velocimetry · Exhalant jet velocity · Velocity field · Allometric equation · Scaling law Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 437: [147][148][149][150][151][152][153][154][155][156][157][158][159][160][161][162][163][164] 2011 will influence the grazing impact and thus the concentration distribution of food particles reaching the inhalant aperture; hence affecting the feeding conditions of the mussel. Of particular interest is the undesirable phenomenon of re-filtration where part of the once filtered and exhaled water reaches the inhalant aperture of either the same or other mussels. This has been studied to some extent for mussel beds where a number of attempts have been made to model phytoplankton concentration gradients caused by dense beds of filter-feeding bivalves in relatively strong currents with a high ...

In 2007, the invasive ctenophore Mnemiopsis leidyi was observed for the first time in Limfjorden (Denmark) where it exhibited mass occurrence in late summer while the indigenous and usually dominating common jellyfish, Aurelia aurita, was nearly absent. Both species were further studied in 2008 and 2009 and it was found that the additional predation pressure by M. leidyi caused the zooplankton stocks to be severely depressed. Here, we report on the population dynamics and predation impact of M. leidyi and A. aurita in Limfjorden in 2010 and 2011. In 2010, M. leidyi was observed in Limfjorden for the first time in August with the highest density and largest size in the central parts (Skive Fjord). The estimated half-life of zooplankton (copepods) was only important in Skive Fjord in mid August 2010 when the joint-predation impact of A. aurita and M. leidyi was 2.3 d. In 2011, no M. leidyi were observed on the first cruise (3 August), while during the second cruise (17 November) it was observed in large numbers. The western most location (Venø Bugt) was dominated by large sized (≤ 60 mm) M. leidyi, while the average size decreased towards the central parts of the fjord-system. The proportion of cydippid larvae increased from west to the central parts thus suggesting rapid reproduction and population-size expansion. The bio-volumes of ctenophores were highest in the central part with 85 ml m-3 in Løgstør Bredning, which may be compared to the greatest mean bio-volume of about 184 ml m-3 observed in the Black Sea in 1989 when the zooplankton and fish stocks collapsed. Analysis of available hydrographic data and model calculations indicates that re-invasion of M. leidyi from the North Sea seeded the autumn population in Limfjorden in mid-September.

No abstract