![Temporal development of shell length (mm) frequency distribution of Mytilus edulis in April and July 2004 and 2005 on the offshore research platform FINO 1 in 1 m depth. [n, total number of individuals per sampling date (i.e. all replicate samples pooled) used for shell length determination; numbers of individuals per length class were standardized to 1 m 2 .]](https://www.researchgate.net/profile/Bela-Buck/publication/229020221/figure/fig3/AS:667855066521602@1536240464305/Temporal-development-of-shell-length-mm-frequency-distribution-of-Mytilus-edulis-in_Q320.jpg)
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Colonization of an artificial hard substrate by Mytilus edulis in the German Bight
Abstract and Figures
The colonization of the underwater construction of an offshore research platform in the German Bight by Mytilus edulis was investigated. Mussel abundance, biomass and percentage coverage of the construction were determined from summer 2003 to summer 2005 from different water depths using digital underwater images and scrape samples of the hard substrate fauna. Growth was estimated from shell length distributions. In 2003 settlement of M. edulis was low at the platform mainly due to a temporal mismatch between platform construction and occurrence of competent larvae. In summer 2004 mussel abundance increased remarkably in the intertidal and upper subtidal. Abundance and biomass increased up to 30,000 individuals m (2 and approximately 40 kg m (2 in summer 2005. At the end of the investigation period, the upper part of the platform foundation was completely covered by M. edulis. Lower parts remained sparsely colonized. Mussel growth rates were high under offshore conditions because of favourable environmental conditions and reduced biological constraints. Cumulative effects from wind farm entities are estimated. Mussel accumulations will be an important component in the estimation of ecological implications of offshore wind farming at least at the local scale.
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... De même, ces habitats offrent de nouvelles surfaces de fixation pour les espèces sessiles telles que les mollusques bivalves (p.ex. moule bleue, huîtres), dont la présence a déjà été reportée sur les bouées houlomotrices et les fondations des infrastructures offshores (Joschko et al., 2008). L'un des rôles écologiques principaux du benthos, est de contribuer aux cycles biogéochimiques par la régénération des nutriments (p.ex. ...
... M. edulis is a gregarious intertidal to shallow subtidal organism that occurs in high population densities, forms large aggregated structures (Christensen et al., 2015) and dominates the French mussel farm production (Agreste 2019). In particular, it is a major colonizer of anthropogenic hard substrates of offshore buildings (Krone et al., 2013), such as wave power buoys and the foundations of offshore infrastructure (Joschko et al., 2008). Very few studies have evaluated the in situ biological colonization of submarine power cables (Carlier et al., 2019). ...
Ces dernières années, la volonté d’exploiter les énergies marines renouvelables s’est renforcée et les projets de construction en haute mer se multiplient. L’électricité ainsi produite est acheminée jusqu’à la côte par un réseau de câbles sous-marins généralement enfouis dans le sédiment. Or, ces derniers émettent des champs magnétiques alternatifs AC ou continus DC d’intensité élevée (jusqu’à 30 fois supérieure au champ géomagnétique), dont les effets potentiels sur la faune marine sont encore mal connus. De nombreux organismes marins utilisent en effet le champ magnétique terrestre pour orienter leur déplacement à petite et large-échelle. Dans ce contexte, cette thèse avait pour objectif d’explorer les réponses comportementales d’organismes benthiques, lors d’exposition à des champs magnétiques d’intensités similaires à celles théoriquement émises par les câbles sous-marins. Selon une approche multi-modèles ciblant des groupes taxonomiques variés, les expérimentations ont été menées en milieu contrôlé, sur la raie bouclée Raja clavata, l’étrille Necora puber, la moule bleue Mytilus edulis et le couteau arqué Ensis magnus. Les champs magnétiques artificiels ont été émis grâce à un dispositif, surnommé le Magnotron, basé sur le principe des bobines de Helmholtz et couplé à une interface numérique permettant le contrôle des intensités générées. Des comportements à forte valeur écologique ont été étudiés : chez la raie le comportement de camouflage, chez l’étrille les comportements de mise à l’abri, d’alimentation et de déplacements et chez la moule et le couteau, les activités de filtration et de bioturbation, respectivement. De manière générale, les expositions aux champs magnétiques artificiels n’ont pas causé de changements comportementaux significatifs chez aucune des quatre espèces. Ces travaux de thèse sont les premiers à évaluer la magnéto-sensibilité des mollusques bivalves et fournissent des données précieuses pour de futures recherches. Il est maintenant nécessaire d’évaluer les effets d’expositions de moyenne et longue-durée et d’explorer la sensibilité des jeunes stades de vie.
... This can be explained by the combination of the relatively high carbon assimilation rate of this species (although significantly lower in CC + AQ), the feeding type ('direct' filter feeder) and the high total biomass in all three environmental scenarios. This also reflects the natural in situ situation in which M. edulis is capable of significantly reducing the primary producer standing stock (Slavik et al., 2019), because of its generally high energy demands (Joschko et al., 2008) and subsequent high clearance rates (Prins et al., 1991;Voet et al., 2021). ...
The rapid development of blue economy and human use of offshore space triggered the concept of co-location of marine activities and is causing diverse local pressures on the environment. These pressures add to, and interact with, global challenges such as ocean acidification and warming. This study investigates the combined pressures of climate change and the planned co-location of offshore wind farm (OWF) and aquaculture zones on the carbon flow through epifaunal communities inhabiting wind turbines in the North Sea. A ¹³C-labelled phytoplankton pulse-chase experiment was performed in mesocosms (4 m³) holding undisturbed hard-substrate (HS) communities, natural sediment with infauna, and mobile invertebrate predators. Carbon assimilation was quantified under current and predicted future-climate conditions (+3 °C and −0.3 pH units), as well as a future-climate co-use scenario with blue mussel (Mytilus edulis) aquaculture. Climate change induced an increase in macrofaunal carbon assimilation as well as an organic enrichment of underlying sediments. Dynamic (non-)trophic links between M. edulis and other HS epifauna resulted in shifts among the species contributing most to the phytoplankton-derived carbon flow across climate scenarios. Increased inter- and intraspecific resource competition in the presence of M. edulis aquaculture prevented a large increase in the total assimilation of phytoplankton by HS fauna. Lower individual carbon assimilation rates by both mussels and other epifauna suggest that if filter capacity by HS epifauna would approach renewal by advection/mixing, M. edulis individuals would likely grow to a smaller-than-desired commercial size. In the same scenario, benthic organic carbon mineralisation was significantly boosted due to increased organic matter deposition by the aquaculture set-up. Combining these results with in situ OWF abundance data confirmed M. edulis as the most impactful OWF AHS species in terms of (total) carbon assimilation as well as the described stress responses due to climate change and the addition of bivalve aquaculture.
... M. edulis is a gregarious intertidal to shallow subtidal organism that occurs in high population densities, forms large aggregated structures (Christensen et al. 2015) and dominates the French mussel farm production (Agreste 2019). In particular, it is a major colonizer of anthropogenic hard substrates of offshore buildings (Krone et al. 2013), such as wave power buoys and the foundations of offshore infrastructures (Joschko et al. 2008). Very few studies had evaluated the in situ biological colonization of submarine power cables (Carlier et al. 2019). ...
Along European coasts, the rapid expansion of marine renewable energy devices and their buried power cables, raises major societal concerns regarding the potential effects of their magnetic field emissions (MFs) on marine species and ecosystem functioning. MFs occur at a local spatial scale, which makes sessile species the primary target of chronic and high-intensity exposures. Some of them, as ecosystem engineers, have critical functions in coastal habitats whose behavioral alteration may drive profound consequences at the ecosystem level. In this context, the present experimental study explored the effects of short exposure to direct current MFs, on the feeding behavior of a widespread ecosystem engineer, the blue mussel (Mytilus edulis). A repeated measure design was carried out with adult mussels successively exposed to control treatment (ambient magnetic field of 47 µT) and artificial MF treatment (direct current of 300 µT produced by Helmholtz coils), as measured around power cables. The filtration activity was assessed through valve gap monitoring using an automated image analysis system. The clearance rate was estimated simultaneously by measuring the decrease in algal concentration using flow cytometry. Our findings revealed that mussels placed in MF treatment did not exhibit observable differences in valve activity and filtration rate, thus suggesting that, at such an intensity, artificial MFs do not significantly impair their feeding behavior. However, additional research is required to investigate the sensitivity of other life stages, the effects of mid to long-term exposure to alternative and direct current fields and to test various MF intensities.
... and mussels all require similar substrate to settle on, C. intestinalis and C. robusta are likely able to coexist with mussels because of partial habitat segmentation based on depth (Rius et al., 2011;Holthuis et al., 2015). In natural populations, mussels were more common nearer the surface, decreasing substantially below 3 m (Joschko et al., 2008), whereas both C. intestinalis (Laupsa, 2015) and C. robusta (Casso et al., 2018) colonized more abundantly with depth. The same partial separation of mussels and Ciona spp. ...
The set of four closely related solitary ascidians Ciona spp. were once considered a single cosmopolitan species, Ciona intestinalis, but are now recognized as genetically and morphologically distinct species. The possibility of ecological differences between the species was not widely considered in studies preceding the schism of Ciona spp. Consequently, there may be an over-generalization of the ecology of Ciona spp., with potential implications for the broad range of studies targeting these species, encompassing the evolution, development, genomics, and invasion biology of Ciona spp. We completed a comprehensive review of the ecology of Ciona spp. to establish the similarities and differences between the widely distributed Ciona robusta and C. intestinalis (and what little is known of the two other species, Ciona sp. C and Ciona sp. D). When necessary, we used study locations and the species' geographic ranges to infer the species in each study in the review. As expected, ecological similarities are the norm between the two species, spanning both abiotic and biotic interactions. However, there are also important differences that have potential implications for other aspects of the biology of Ciona spp. For example, differences in temperature and salinity tolerances likely correspond with the disparities in the geographic distribution of the species. Asymmetries in topics studied in each species diminish our ability to fully compare several aspects of the ecology of Ciona spp. and are priority areas for future research. We anticipate that our clarification of common and unique aspects of each species' ecology will help to provide context for future research in many aspects of the biology of Ciona spp.
... At this offshore wind farm, initial settling of blue mussels was observed in 2010 (i.e., ~6 months after installation), predominantly in the intertidal zone where the mussel population developed into a typical intertidal mussel belt similar to those described in Joschko et al. (2008) for the German Bight. As a mussel population further develops, the mussels grow and start occupying more space, but do not necessarily increase in numbers (Suchanek 1985). ...
The European Directive 2009/28/EC on the promotion of electricity produced from renewable energy sources in the internal electricity market, imposes a target figure for the contribution of the production of electricity from renewable energy sources upon each Member State. On 31 December 2019, Belgium submitted a National Energy and Climate Plan to the European Commission which envisions a target figure of 17.5% for the contribution of the production of electricity from renewable energy sources by 2030. Offshore wind farms in the Belgian part of the North Sea are expected to make an important contribution to achieve that goal. Within the Belgian part of the North Sea, a zone of 238 km² is reserved for the production of electricity from water, currents or wind. In that zone, eight wind farms are operational with a combined installed capacity of 2.26 MW. A second area for renewable energy of 285 km² is foreseen by the new Belgian marine spatial plan (2020-2026) with the government aiming for an installed capacity of 3.1 tot 3.5 GW in this zone. Prior to installing a wind farm, a developer must obtain a domain concession and an environmental permit. The environmental permit includes a number of terms and conditions intended to minimise and/or mitigate the impact of the project on the marine ecosystem. Furthermore, as required by law, the permit imposes a monitoring programme to assess the effects of the project onto the marine environment. Within the monitoring programme, the Royal Belgian Institute of Natural Sciences and its partners assess the extent of the anticipated impacts onto the marine ecosystem and aim at revealing the processes behind these impacts. The first objective is tackled through basic monitoring, focusing on the a posteriori, resultant impact quantification, while the second monitoring objective is covered by the targeted or process monitoring, focusing on the cause-effect relationships of a priori selected impacts.
This report, targeting marine scientists, marine managers and policy makers, and offshore wind farm developers, presents an overview of the scientific findings of the Belgian offshore wind farm environmental monitoring programme (WinMon.BE), based on data collected up to and including
2020.
... Regarding biological changes, these systems, considered as stepping stones and/or corridors, favour recruitment, dispersion and connectivity amongst species . Different studies were focused on the macrofauna community inhabiting the artificial hard substrata, investigating the rate of colonization (Joschko et al., 2008) and substrata shaping by autogenic ecosystem engineers (Mendez et al., 2015;Deidun et al., 2016). Moreover, the installation of barriers could favour the expansion of opportunistic and non-indigenous species (NIS) (Bulleri & Airoldi, 2005;Perkol-Finkel et al., 2012;Airoldi et al., 2015;Spagnolo et al., 2019). ...
Increasing employment of artificial barriers to combat coastal erosion is causing alterations of coastal ecosystems. Understanding their ecological functioning is key to planning management activities and minimising their negative impacts on the ecosystem. In this paper, changes in biodiversity and in functional traits of macrofaunal shallow communities were investigated throughout 4 years of study in the central Mediterranean. Biodiversity indices (S, d, H 0) showed the highest mean values during in-operam phases (5.7, 1.99 and 2.07, respectively) whilst functional diversity (FDis) and functional redundancy (FDiv/H 0) showed the highest mean values in ante-operam (4.34 and 2.44, respectively). The PERMA-NOVA showed differences both in species composition and functional traits between sampling periods. b-diversity was driven by the replacement of species over years. The non-indigenous polychaete Lum-brinerides neogesae, recorded for the first time during in-operam, supplied the highest differences between years (14.5%, SIMPER). During in and post-operam phases an increase of sub-surface deposit feeder, tolerant, and pioneer categories was recorded. When the classic taxonomic approach is combined with novel functional biodiversity attributes, a wide range of biota responses to a disturbance can be better described and the efficient mitigation solutions to minimize negative impacts of human activities proposed.
... Regarding biological changes, these systems, considered as stepping stones and/or corridors, favour recruitment, dispersion and connectivity amongst species . Different studies were focused on the macrofauna community inhabiting the artificial hard substrata, investigating the rate of colonization (Joschko et al., 2008) and substrata shaping by autogenic ecosystem engineers (Mendez et al., 2015;Deidun et al., 2016). Moreover, the installation of barriers could favour the expansion of opportunistic and non-indigenous species (NIS) (Bulleri & Airoldi, 2005;Perkol-Finkel et al., 2012;Airoldi et al., 2015;Spagnolo et al., 2019). ...
Increasing employment of artificial barriers to combat coastal erosion is causing alterations of coastal ecosystems. Understanding their ecological functioning is key to planning management activities and minimising their negative impacts on the ecosystem. In this paper, changes in biodiversity and in functional traits of macrofaunal shallow communities were investigated throughout 4 years of study in the central Mediterranean. Biodiversity indices (S, d, H′) showed the highest mean values during in-operam phases (5.7, 1.99 and 2.07, respectively) whilst functional diversity (FDis) and functional redundancy (FDiv/H′) showed the highest mean values in ante-operam (4.34 and 2.44, respectively). The PERMANOVA showed differences both in species composition and functional traits between sampling periods. β-diversity was driven by the replacement of species over years. The non-indigenous polychaete Lumbrinerides neogesae, recorded for the first time during in-operam, supplied the highest differences between years (14.5%, SIMPER). During in and post-operam phases an increase of sub-surface deposit feeder, tolerant, and pioneer categories was recorded. When the classic taxonomic approach is combined with novel functional biodiversity attributes, a wide range of biota responses to a disturbance can be better described and the efficient mitigation solutions to minimize negative impacts of human activities proposed.
... In biomass, M. edulis is by far the dominant species present on turbine foundations (Joschko et al., 2008;Krone et al., 2013). The redistribution of OM deposition fluxes caused by OWFs under different scenarios of placement and turbine types for a new concession area are described in an accompanying paper (Ivanov et al. in review). ...
... various algae species (Atalah et al., 2016). More specifically, the soft growth comprises varying species of algae, anemones, and sponges and varies strongly depending on environmental conditions such as water depth, temperature, salinity, predation as well as competition and availability of food and space (Wolfram and Theophanatos, 1985;Joschko et al., 2008). This soft growth likely has a large influence on the boundary layer development and was not recreated during the modelling of the surrogates which were made of smooth 3D-printing material. ...
Against the background of a drastically increased demand of marine proteins, off-bottom, bivalve aquaculture, provides significant potential for production growth when moved into more energetic marine waters. Hence, research, industry and politics are currently proposing the development of new offshore sites. The highly energetic conditions at these sites present a challenging environment for bivalve aquaculture. In this work, physical experiments of suspended bivalves provide new knowledge on the commonly used design parameters: the drag and inertia coefficients. Live bivalves and manufactured surrogate models at a 1:1 scale were tested in a towing tank as well as under waves. The drag coefficient of live blue mussels was determined to be C d = 1.6 for Reynolds numbers between 2.3 × 10 4 and 1.4 × 10 5. The inertia coefficient obtained from the wave tests was C m = 2.1 for Keulegan Carpenter numbers KC < 10. In a pursuit to better understand the differences between live mussels and surrogates in laboratory conditions, the analysis revealed that appropriate surrogates can be identified. A method to determine the characteristic diameter of mussel dropper lines is suggested. The results facilitate the future design of aquaculture systems in high-energy environments and allow for an integration into numerical models.
... To do so, we used output generated by Ivanov et al. (2021) who implemented a model describing filtration by blue mussels on turbine substructures in a hydrodynamic model of the Southern North Sea, including the full BPNS and parts of the bordering French and Dutch coastal zones (Figure 1; Ivanov et al., 2020). In biomass, M. edulis is by far the dominant species present on turbine foundations (Joschko et al., 2008;Krone et al., 2013). Whereas alterations to sediment and OM transport to sediments from the water column under different scenarios of turbine placement in a new concession area are evaluated in an accompanying paper (Ivanov et al., 2021), we used the OM deposition fluxes as input for a model of sediment biogeochemistry (OMEXDIA, Soetaert et al., 1996a), to investigate potential changes to mineralization pathways in the sediment. ...
Offshore windfarms (OWFs) offer part of the solution for the energy transition which is urgently needed to mitigate effects of climate change. Marine life has rapidly exploited the new habitat offered by windfarm structures, resulting in increased opportunities for filter- and suspension feeding organisms. In this study, we investigated the effects of organic matter (OM) deposition in the form of fecal pellets expelled by filtering epifauna in OWFs, on mineralization processes in the sediment. OM deposition fluxes produced in a 3D hydrodynamic model of the Southern Bight of the North Sea were used as input in a model of early diagenesis. Two scenarios of OWF development in the Belgian Part of the North Sea (BPNS) and its surrounding waters were calculated and compared to a no-OWF baseline simulation. The first including constructed OWFs as of 2021, the second containing additional planned OWFs by 2026. Our results show increased total mineralization rates within OWFs (27–30%) in correspondence with increased deposition of reactive organic carbon (OC) encapsulated in the OM. This leads to a buildup of OC in the upper sediment layers (increase by ∼10%) and an increase of anoxic mineralization processes. Similarly, denitrification rates within the OWFs increased, depending on the scenario, by 2–3%. Effects were not limited to the OWF itself: clear changes were noticed in sediments outside of the OWFs, which were mostly opposite to the “within-OWF” effects. This contrast generated relatively small changes when averaging values over the full modeling domain, however, certain changes, such as for example the increased storage of OC in sediments, may be of significant value for national / regional carbon management inventories. Our results add to expectations of ecosystem-wide effects of windfarms in the marine environments, which need to be researched further given the rapid rate of expansion of OWFs.
The presented study focuses on the development of offshore wind farms in conjunction with open ocean aquaculture within the German Bight. For aquaculture enterprises in the open ocean an extensive cultivation of various species, blue mussels (Mytilus edulis), oysters (Ostrea edulis, Crassostrea gigas) and seaweed (Laminaria saccharina), is considered. However, without the solid foundations for wind turbines, such as monopiles and tripods as anchor or connection points, economic installations of equipment for mariculture would not be possible in view of the high-energy environment in this part of the North Sea. Thus, one of the most important questions pertains whether it is technically possible and economic feasible to use the offshore foundation structures as fixation device for aquaculture operations, such as a longline as one possible culture design. A longline culture is an open-water suspended technology in which cultured species are on-grown on ropes or diverse substrates, such as rope collectors for the catch of mussel spat, suspended from anchored and buoyed surface or subsurface horizontal ropes (longline).
For the calculation of additional foundation costs generated through the attached longline system knowledge on the supplement loads and stresses as well as on the required constructive modifications of the foundation must be obtained. The development and the conceptual design of offshore foundation structures are complex and require an interdisciplinary approach. In the presented project we aim to evaluate the induced loads between a foundation structure for offshore wind energy turbines and a longline system and to develop appropriate connection points. As monopile and tripod foundations are the most common foundation structures to date, these both will be considered within the modeling approach which calculates the respective loads from wind and waves. Both foundation structures are in the dimension of 4-5 MW wind turbines, while the monopile design will be calculated for a water depth of 10 m whereas the tripod for a water depth of 30 m.
To validate the model outcomes, an entire monopile-longline-monopile construction is set up in the offshore waters 10 nautical miles off the Island of Sylt at a water depth of about 17 m. At this site, the deformation behavior of the entire system will be determined on the basis of analytically calculated loads supported by the measurement of local stresses at the junction of the longline with the foundation. An additional measurement of forces will be performed on selected mussel collectors. To evaluate the observed loads with respect to environmental conditions, namely currents and waves, accompanying sensors will be installed on site. These achieved datasets will be used for the verification of the computer model for mechanical loads at the longline. Finally it is intended to scale up the model to the proportions of a wind farm while considering the interactions in the wavefield in which the longline system will be located.
At a subtidal, soft-bottom site in the western Baltic Sea, mussel (Mytilus edulis) patches co-occur with high predator abundances. Sea star (Asterias rubens) biomasses, in particular, exceed reported values considered sufficient for restricting mussels to the intertidal zone. To determine how mussels can persist in the face of intense predation, we decomposed patch space occupancy into the relative contributions of newly arriving individuals (recruitment) and of increases in body size of the individuals already present in the patch over 13 mo. Sea stars, as major predators, were only able to control 77% of the potential per capita recruitment rate of 91 individuals/yr in 2 m depth. The remaining recruitment rate of 21 individuals/yr was sufficient to allow patches to occupy 1.6 times more space per year. Transplantation of patches to 6 m depth, where recruitment is negligible, revealed that Sea stars were also ineffective in controlling mussel coverage through consumption of larger mussels (>1 yr, >30 mm shell length). In deeper water, space occupancy of patches through increases in mussel body size was able to balance predation mortality, demonstrating that mussels attained a relative refuge in size at only 33 mm shell length. Based on the measured shell growth rates, mussels attain this size after ≃ 5 mo. In situ observations of Asterias feeding activity, the ratios between necessary predator sizes to attack prey of a given size, and predator size distributions suggest that sea stars were on average too small to feed effectively on adult (> 1 yr) mussels. Probably, Asterias cannot respond to abundant prey and increase its maximal body size at the site because salinities are at its lower tolerable limit (12-18 g/kg). Thus, bottom-up factors such as high prey productivity in concert with subtle size-based ineffectiveness of the predator population allow otherwise unstable predator-prey populations of a generalist predator and its preferred prey to coexist. AlthoUgh mussel predators were unable to decimate mussels to local extinction, a release of experimental mussel patches from predation with strong recruitment (2 m depth) resulted in an approximately sevenfold yearly areal increase in shallow treatments, which would lead to a 100% mussel cover at the site within 1 yr. Given that mussels can dominate both rocky substratum and soft sediment, we also studied the effect of substratum quality in factorial combination with presence/absence of predation and water depth on mussel abundance. Attachment to stable substratum did not affect recruitment to the patches or patch space occupancy, but it completely prevented patch dislodgment and subsequent drift. In contrast to rocky shores, mussel patch dislodgment may represent the major mode of patch dispersal and new patch formation in soft-bottom environments as demonstrated by a drift collector fence.
Techniques for the induction of spawning of Mytilus edulis, the delay of spawning and the rearing of the larvae are described.
Abundance of larvae and primary plantigrades in the blue mussel Mytilus edulis L. was studied quantitatively in the western part of the Dutch Wadden Sea. Observations were made in seven years between 1981 and 1996. In general, a first large peak of larvae appeared in May, followed by one or more much smaller peaks until September. For pelagic larvae an approximate life time of four to six weeks was calculated (on average about five weeks). Pelagic larvae appeared to have a relatively higher survival chance at low larval concentrations than at high concentrations. No relation could be demonstrated between time of larval peaks and spring tides. Year-to-year differences in abundance of plantigrades did not predict mussel recruitment success on nearby tidal flats.
Temporal and spatial (depth) patterns of shell growth were studied in the mussel Mytilus edulis in relation to water temperature and potential food availability, at an offshore oil platform, Holly (ARCO), in the Santa Barbara Channel, California. Length-specific growth rates were highest from late May to July and at a depth of 9 m. The time to achieve a length of 50 mm from recruitment was estimated at 6–8 months. Growth rates were not correlated with water temperature, using multiple regression and correlation analysis. Temporal variation in the growth of 20- and 35-mm mussels correlated with chlorophyll a concentration at time lags of 2 and 4 wk, respectively. Variation in growth of mussels with depth was more closely associated with the concentration of particulate organic carbon than with chlorophyll a. Our results indicate that water temperature can be disregarded as an important factor in regulating mussel growth in California waters, but that growth could vary in association with well-documented regional variation in phytoplankton biomass.