Sea-bed benthic-invertebrate assemblages of species within and proximal to an existing mussel farm off Taniwhanui Point, eastern Waiheke Island, are reported. Substratum type, whether predominantly mud, gravels or an admixture of the two, mud/gravels, is shown to influence infaunal species assemblage composition; the bivalve Theora lubrica, ostracods, amphipods and polychaetes characterise muddy substrata; polychaetes, particularly spionids and syllids, ostracods, amphipods, bivalves and ophiuroids characterise mud/gravel substrata; and diverse assemblages of polychaetes, bivalves, pagurid crabs, gastropods, ostracods, ophiuroids and nemertean worms characterise gravel substrata. Significant differences in sea-bed assemblages are reported along one transect inside and outside the farm over the three seasons during which surveys were conducted, summer, autumn, winter of 2008. Along the northern side of the mussel farm those sediments beneath the farm are characterised by greater abundances of polychaetes and crustaceans (Malacostraca), whereas sediments outside the farm are characterised by greater abundances of bivalves and ostracods. Sediments both inside and outside the north-eastern border of the farm during summer are characterised by similar abundances of polychaetes, bivalves and ostracods. Similarly, those sediments within and outside the farm along its southern border during summer are characterised by abundances of polychaetes, bivalves, crustaceans (Malacostraca) and gastropods. Measures of relative abundance, rarity and species richness are applied to sea-bed assemblages off eastern Waiheke Island to enable an appraisal of the spatial distribution of each within and outside the farm, and throughout the eastern Waiheke Island region. One of these measures, relative abundance, is then compared with other, albeit limited abundance data from previous soft-sediment surveys conducted throughout Hauraki Gulf. The most species rich and abundant sites off eastern Waiheke Island occur in gravelly substrata between Waiheke Island and Pakatoa Island, and between Rotoroa and Ponui Islands, in addition to beneath the southern portion of the existing mussel farm. Gravel-based substrata are recognised to be the most species rich and densely populated with invertebrates for this sediment type in Hauraki Gulf. Similarly, the muddy substrata off eastern Waiheke Island region appear to host more individuals and species than any other reported muddy substratum in Hauraki Gulf. The existing mussel farm is shown to significantly affect sea-bed communities, but in a manner that has not been previously reported in New Zealand. Species richness and abundance are greater beneath the farm, as are the proportions of very rare and uncommon taxa to more common and ubiquitous taxa. Sediments beneath the farm are not characterised by elevated abundances and richness of opportunistic species; and no obvious difference in sediment grain size is apparent along a transect extending from 20 m inside the farm to at least 110 m outside it. The biological footprint of the farm is limited, appearing to extend no further than 20 metres from the northern physical boundary of the farm; the gross sedimentary characteristics (grain size) do not differ significantly within and outside the farm. Within and immediately outside the farm species richness and abundance tend to increase during colder seasons; beneath the farm, species richness (d), abundance (N), Shannon index (H’) and Simpson index (1-λ’) were higher during May (autumn) and August (winter) than during February (summer); diversity values outside the farm were similar during summer and autumn, but species richness (d), evenness (J’), Shannon index (H’) and Simpson index (1-λ’) were all greater during winter. No opportunistic taxa are considered to be appropriate indicators of organically enriched environments, at least enrichment that can be intuitively linked to any direct effect of the existing mussel farm. One species, the heart urchin Echinocardium cordatum, only rarely occurs inside the physical farm boundary, so its relative abundance renders it an appropriate indicator species of mussel-farm impacts.

Sea-bed benthic-invertebrate assemblages of species within and proximal to an existing mussel farm off Taniwhanui Point, eastern Waiheke Island, are reported. Substratum type, whether predominantly mud, gravels or an admixture of the two, mud/gravels, is shown to influence infaunal species assemblage composition; the bivalve Theora lubrica, ostracods, amphipods and polychaetes characterise muddy substrata; polychaetes, particularly spionids and syllids, ostracods, amphipods, bivalves and ophiuroids characterise mud/gravel substrata; and diverse assemblages of polychaetes, bivalves, pagurid crabs, gastropods, ostracods, ophiuroids and nemertean worms characterise gravel substrata. Significant differences in sea-bed assemblages are reported along one transect inside and outside the farm over the three seasons during which surveys were conducted, summer, autumn, winter of 2008. Along the northern side of the mussel farm those sediments beneath the farm are characterised by greater abundances of polychaetes and crustaceans (Malacostraca), whereas sediments outside the farm are characterised by greater abundances of bivalves and ostracods. Sediments both inside and outside the north-eastern border of the farm during summer are characterised by similar abundances of polychaetes, bivalves and ostracods. Similarly, those sediments within and outside the farm along its southern border during summer are characterised by abundances of polychaetes, bivalves, crustaceans (Malacostraca) and gastropods. Measures of relative abundance, rarity and species richness are applied to sea-bed assemblages off eastern Waiheke Island to enable an appraisal of the spatial distribution of each within and outside the farm, and throughout the eastern Waiheke Island region. One of these measures, relative abundance, is then compared with other, albeit limited abundance data from previous soft-sediment surveys conducted throughout Hauraki Gulf. The most species rich and abundant sites off eastern Waiheke Island occur in gravelly substrata between Waiheke Island and Pakatoa Island, and between Rotoroa and Ponui Islands, in addition to beneath the southern portion of the existing mussel farm. Gravel-based substrata are recognised to be the most species rich and densely populated with invertebrates for this sediment type in Hauraki Gulf. Similarly, the muddy substrata off eastern Waiheke Island region appear to host more individuals and species than any other reported muddy substratum in Hauraki Gulf. The existing mussel farm is shown to significantly affect sea-bed communities, but in a manner that has not been previously reported in New Zealand. Species richness and abundance are greater beneath the farm, as are the proportions of very rare and uncommon taxa to more common and ubiquitous taxa. Sediments beneath the farm are not characterised by elevated abundances and richness of opportunistic species; and no obvious difference in sediment grain size is apparent along a transect extending from 20 m inside the farm to at least 110 m outside it. The biological footprint of the farm is limited, appearing to extend no further than 20 metres from the northern physical boundary of the farm; the gross sedimentary characteristics (grain size) do not differ significantly within and outside the farm. Within and immediately outside the farm species richness and abundance tend to increase during colder seasons; beneath the farm, species richness (d), abundance (N), Shannon index (H’) and Simpson index (1-λ’) were higher during May (autumn) and August (winter) than during February (summer); diversity values outside the farm were similar during summer and autumn, but species richness (d), evenness (J’), Shannon index (H’) and Simpson index (1-λ’) were all greater during winter. No opportunistic taxa are considered to be appropriate indicators of organically enriched environments, at least enrichment that can be intuitively linked to any direct effect of the existing mussel farm. One species, the heart urchin Echinocardium cordatum, only rarely occurs inside the physical farm boundary, so its relative abundance renders it an appropriate indicator species of mussel-farm impacts.

The Baltic Sea is the most polluted sea in the world. Its hydrological conditions and ongoing eutrophication are a high threat for marine biodiversity and ecosystems. Additionally, eutrophication has negative effects on the wellbeing of countries and their societies in the Baltic Sea Region (BSR). Actions to mitigate eutrophication in the Baltic Sea have been implemented through on-land measures in the last 40 years. Although the improvement in the marine environment is notable, it happens very slowly. In order to combat eutrophication, there is a need for a combination of on-land and in-situ measures. In this study, blue mussel farming practices are presented as one of the in-situ measures to combat eutrophication in the Baltic Sea. Blue mussel farming has been implemented in Sweden since the 1980s and has potential to not only mitigate the amounts of nutrients that accumulate in the sea but also brings a circular approach to resource use. In this study, stakeholders from four different sectors that are closely related to blue mussel farming practices and Baltic Sea issues have been interviewed with the aim of making a comprehensive analysis of stakeholder perceptions of blue mussel farming practices in the BSR. Interviewed stakeholders represent four different sectors - academia, entrepreneurs, municipalities and NGOs. A comprehensive analysis of stakeholders’ perceptions on blue mussel farming practises from environmental, social and economic perspective is presented. All interviewed stakeholders are actors in Sweden and represent Swedish perspective on blue mussel farming activities. Potential causes for different perceptions across sectors are discussed.

The hydrodynamic effects of long-line mussel farms are studied through a two-pronged approach. Large-scale hydrodynamic effects are investigated through the use of field measurements, primarily at a large mussel farm in Golden Bay, New Zealand (230 long-lines, covering an area of 2.45 km by 0.65 km). The research focuses on three areas: the effect of the farm on currents, mixing and stratification, and the dissipation of wave energy. Measurements are also made of the forces on long-line anchor ropes, and a limited investigation is made of phytoplankton depletion. The second approach is the use of laboratory drag measurements and Particle Tracking Velocimetry (PTV) to study the effect of mussel dropper (vertical lengths of mussel-encrusted crop rope) roughness and spacing on flow at small scales. These experiments provide data on very rough cylinders, and on cylinder arrays. The field measurements show that the local effects of mussel farms on currents are significant, but that magnitudes of the effects depend on dropper density, mussel sizes, orientation of the long-lines to the flow, and other parameters that are necessary to characterise the complex interactions between a farm and the flow. The drag on the submerged structures reduces water velocities within the Golden Bay farm by between 47% and 67%. Mussel farms present a porous obstacle to the flow, and flow that does not pass through the farm must be directed around or beneath it. The field measurements indicate that at the study site, most of the flow is diverted around the farm despite its large horizontal dimensions. The droppers at the study site extend over most of the water column (average dropper length ~ 8 m, average water depth ~ 11 m), providing a restriction to the flow beneath the farm. The strength of the density stratification may also favour a horizontal diversion. The flow around the farm is essentially two-dimensional. This suggests that two-dimensional numerical models should be sufficient to obtain reasonable predictions of the velocity drop within, and the diversion around, mussel farms. A simple two-dimensional pipe-network model gives reasonable estimates of the velocity within the farm, demonstrating that the drag of the farm may be adequately parameterised through local increases of bed friction. A wake in the form of reduced velocities extends downstream of the farm, and a mixing layer analogy suggests that this wake spreads slowly. The downstream extent of the wake cannot be determined, although it is likely to be limited by the tidal excursion. The degree of vertical mixing caused by the flow through a mussel farm cannot be quantified, although there are clear interactions between the stratification and the farm. Two mixing mechanisms are considered. A shear layer is generated beneath the farm due to the difference in velocities between the retarded flow within the farm and the flow beneath. Shear layers beneath mussel farms are likely to be weak unless the ambient currents are strong. It will be necessary for stratification to be weak or non-existent for this mechanism to generate significant mixing. The second mechanism is smaller-scale turbulence generated by the mussel droppers. Although the efficiency of this form of mixing is likely to be low, the large number of mussel droppers suggests that there will be some enhancement of vertical mixing. Frequency-dependent wave attenuation is recorded, and is predicted with some success by an analytical model. Both the model and the field data show that wave dissipation increases as the wave period decreases. Wave energy dissipation at the study site averages approximately 10%, although the measurements are made during a period of low wave heights (Hs < 0.25 m). Measurements of long-line anchor rope tension at two study sites indicate that the loadings are induced by the tide, currents, and waves. Dynamic wave loadings may be significant, and higher wave forces are measured at the offshore end of a long-line. The issue of seston or phytoplankton depletion is considered briefly through the examination of fluorescence, turbidity, and acoustic backscatter data. Although the results are consistent with a reduction of seston within the farm, differences between the inside and outside of the farm are not statistically significant. Mussel droppers resemble extremely rough circular cylinders, with the mussel shells forming the surface roughness elements. Drag measurements and PTV flow visualisation are used to investigate the importance of the large surface roughness, and the influence of dropper spacing and long-line orientation on flow. Drag measurements conducted with smooth and rough cylinders show that high surface roughness (ks/D ~ 0.092) has little effect on the drag coefficient of single cylinders in the range 4,000 < Re < 13,000, yet increases the drag coefficient of a row of cylinders normal to the flow. High surface roughness on single cylinders has the effect of shortening the near-wake region, increasing the peak turbulent kinetic energy (TKE) behind the cylinder, and decreasing the Strouhal number (St = 0.21, 0.19, 0.17 for ks/D = 0, 0.048, and 0.094 respectively). Arrays of rough cylinders (ks/D = 0.094) demonstrate similar flow characteristics to those of smooth cylinders. At cylinder spacings of S/D < 2.2, the surface roughness acts to favour the formation of a particular metastable wake pattern, whereas different metastable wake patterns are formed each run behind the smooth cylinders. The experiments show that the drag on single row arrays of cylinders are related to the cylinder spacing (increasing drag with decreasing spacing), and the drag also varies with the sine of the angle to the flow, except where the array is at low angles to the flow. The PTV measurements provide new data regarding the two-dimensional distributions of velocity, TKE, and turbulence statistics behind the cylinder arrays.

Eutrophication of seas, particularly near the coasts, is a topic of high current concern. Itleads to increased primary production and a higher frequency of algal blooms. The enhancedprimary production also increases the turbidity of the water, making it harder for ecologicallyimportant benthic macro algae, e.g. seaweed, to colonize bottoms on deeper waters. EU directives, as well as national law, demand decreases in nutrients, e.g. nitrogen, reachingthe sea. Another approach to decreasing the amount of nutrients in the sea could be musselfarming. Mussel farming is an established, albeit small, industry on the Swedish west coast. The useof mussels to remove nitrogen, however, is a more recent application. Lysekil municipalityhas been granted exemption from improving the nitrogen removal in Långevik waste watertreatment plant. Instead, mussel farms are used to remove nitrogen, which was estimated to bea less costly solution. This thesis investigates the advantages in money and energy in using mussel farming. Theuse of a mussel farm to remove nitrogen from sea water is compared with removing nitrogenin a WWTP. In addition, the production of mussel fertilizer, through a composting process, iscompared with producing a corresponding amount of artificial fertilizers. The results show that mussel farms are neither more economic nor energy efficient inremoving nitrogen and producing fertilizer than the conventional alternatives examined Mussel fertilizer, however, is not the most lucrative product that the mussels from the farmcan give. Alternatives, such as fodder or human consumption, are discussed in the report. Furthermore, the mussel fertilizer qualifies as organic fertilizer. A comparison with otherorganic fertilizers could prove mussels more economic or energy efficient, but such acomparison is out of the scope for this thesis. Finally, some data, e.g. the energy demand for nitrogen removal in the WWTP, were roughestimations. More research and investigations are needed in order to achieve more accurateconclusions. / Eutrofiering i haven, speciellt kustnära vatten, är ett högaktuellt problem. Det leder till ökadprimärproduktion och frekvens av algblomningar. Den förhöjda primärproduktionen lederäven till ökad turbiditet, något som försvårar för ekologiskt viktiga bentiska arter, t.ex. sjögräs(ålgräs), att kolonisera bottnar på djupare vatten. Så väl EU-direktiv som nationell lagstiftning kräver en minskning i näringsämnen, bl.a.kväve, som når havet. Ett verktyg för att klara av dessa minskningar kan vara musselodlingar. Musselodling är en etablerad, men liten, industri på den svenska västkusten. Musslor som enkväveavskiljningsmetod är däremot en ny tillämpning. Lysekil kommun har fått undantag frånatt bygga ut kvävereningen i Långevik vattenreningsverk. I stället används musselodlingar föratt ta bort kväve, vilket ansågs vara en billigare metod. Detta examensarbete undersöker fördelarna i pengar och energi i att användamusselodlingar för att få bort kväve från havsvatten, jämfört med att avskilja kväve ivattenreningsverk. Dessutom jämförs produktionen av musselgödning, genom att komposteramusslor från musselodlingen, med att producera motsvarande mängder med konstgödsel. Resultaten visar att musselodlingar vare sig är billigare eller mer energieffektiva än dekonventionella alternativen för att ta bort kväve och producera gödsel. Musselgödning är däremot inte den mest lönsamma produkt som musslorna frånmusselodlingen kan ge. Alternativ som foder och mänsklig konsumption diskuteras i rapporten. Vidare kan musselgödning godkännas som KRAV-gödsel. I en jämförelse mot andraKRAV-gödsel kan musselgödning visa sig mer ekonomisk eller energieffektiv, men en sådan jämförelse ligger utanför detta arbetes tidsram. Slutligen var vissa data grova uppskattningar, t.ex. energiåtgången för kväveavskiljning i vattenreningsverk. Mer forskning och undersökningar behövs för att nå mer tillförlitligaslutsatser. / www.ima.kth.se