Supply and dispersal of mussel farm debris and its impacts on benthic habitats in contrasting hydrodynamic regimes

Abstract

This thesis is a study of the distribution, accumulation and subsequent effects of mussel debris, (faeces, pseudofaeces and mussel shells) on the benthic environment in and around mussel farms experiencing varying hydrodynamic energy. Side-scan sonagraphs at two sheltered farm sites reveal high acoustic backscatter beneath farm sites and low backscatter in the immediate surrounds. This suggests a sharp contrast in seabed material around the edge of the farm. Photography and sediment samples reveal the farms are underlain by mounds of mussel shells with faecal material infilling inter-shell voids. In contrast the surrounding seabed is naturally sedimented soft mud. Sediment from beneath the farms had total organic contents of 8-19 % decreasing sharply to natural levels 4-7 % 30m from the farm boundaries. A third site exposed to energetic waves and currents exhibits low acoustic backscatter zones beneath the farm. Photographs show the seabed is predominantly clean sand with only scattered clumps of shell and no visible faecal material. Sediments are typical of the natural setting. To estimate the initial accumulation of the falling mussel debris, current meter data, sediment flux rates, biodeposit settling velocity and water depth were combined to model the depositional distribution at the three mussel farm cultures. Model results were compared to seabed sediment samples collected at each site. At the sheltered sites, comparison of the modelled dispersal pattern and seabed identification of mussel biodeposits using sedimentological data indicated a very close association. Both were found to reduce to natural levels approximately 30-40 m from the farm site. Such similarity between the model and observed distribution of the mussel biodeposits indicated that once deposited on the seabed the biodeposits remain immobile due to the low hydrodynamic energy of the two sites. In contrast, the exposed site had only poor correlation between the dispersal model and identification of mussel biodeposits. The dispersal model indicates that the majority of mussel biodeposits initially accumulate beneath the farm. However, observations found no sign of any mussel biodeposits in sediment up to 200 m from the farm boundary. This lack of biodeposits was attributed to strong currents re-suspending and dispersing these biodeposits over a wide area thereby leaving little impact on the natural sediment. A separate study was also undertaken to examine the re-mobilisation of shell and biodeposits on the seabed. The entrainment speeds of mussel shells and biodeposits were examined using a combination of flume experiments coupled with theoretical considerations. Wave and current dissipation through a mussel farm structure was also examined with reference to impacts on the distribution of mussel debris and effects on the coastal zone downstream. Average entrainment speeds of between 25-30 cm/s were observed for mussels of market size (70-85 mm) and slightly lower speed for freshly seeded spat. Estimations of biodeposit re-suspension indicate near bed currents of at least 9.5-12 cm/s are required to re-suspend mussel biodeposits. Numerical modelling of current dissipation found some retardation of current speed resulting in possible sediment accumulation depending on the farm size and hydrodynamic regime. Finally, examination of the macroinvertebrate assemblage composition from samples taken inside and outside of the two low energy farm sites revealed significant differences, whilst no difference was observed for the relatively high-energy site. Taxa that best discriminated between the dissimilarities observed in macroinvertebrate assemblage composition inside and outside farms were principally polychaetes (more abundant inside) and ophiuroids (more abundant outside). Sediment total organic matter and number of mussel shells were the parameters that best explained the difference observed at the two relatively sheltered farm sites. No taxa or environmental variables were particularly good at discriminating or explaining differences observed inside and outside the relatively high-energy farm site.