Since the advent of nautical travel, humans have been transporting organisms into novel environments with little understand of the consequences.  The introduction of an invasive bivalve can have devastating consequences for an ecosystem.  The invasion of San Francisco Bay by the Asian clam, Corbula amurensis, led to the collapse of the fisheries industry and a dramatic change in the ecosystem functioning of the intertidal system there.  My thesis focuses on the consequences of the establishment of the invasive bivalve, Nuttallia obscurata, on sediment characteristics in the intertidal area.  The varnish clam, N. obscurata, was first reported in the early 1990s and is believed to have been introduced via ballast waters into Vancouver Harbour, British Columbia (BC) from its native habitat in the Northwest Pacific.  It has since spread rapidly reaching up to Smith Sound, BC in the north and down to Coos Bay, Oregon in the south at densities reported as high as 800 individuals/m².  I applied a 3-tiered approach, which consisted of a field survey (Tier I), a density manipulation experiment (Tier II) and a microcosm experiment (Tier III).  From my field survey, I found a trend of increasing organic matter content with increasing bivalve densities at mid sediment depths (3-6 cm).  An increase in organic matter content could lead to over-enrichment of sediments, which could lead to anoxia that would result in the change of the redox potentials of the sediments.  I found a trend of higher organic matter with high varnish clam densities (Tier II).  I also found significantly higher ammonium levels with high densities (Tier II) and showed that the increase in ammonium was due to the excretory processes of the varnish clam (Tier III).  Nitrogen is a limiting nutrient in marine systems, with ammonium being the form of nitrogen that is preferentially uptaken by phytoplankton.  A change in the flux of nitrogen as well as an accumulation of organic matter would result in drastic changes to ecosystem functioning in the intertidal.  Given that varnish clams are often found with the economically important Manila clams, Tapes philippinarum, the results of my research could have consequences for the aquaculture industry on the BC coast.

Parasitism is the most common animal lifestyle yet surprisingly little is known about the role parasites play in the ecosystems in which they are embedded. This thesis examines the causes and consequences of the dispersal of a marine ectoparasite, the salmon louse (Lepeophtheirus salmonis), in an area of intensive salmon aquaculture where the transmission of lice from farmed salmon may have consequences for the dynamics of adjacent wild salmon populations. Salmon lice are capable of leaving one host in search of another as they approach sexual maturity; while previously thought to be an artefact of the confined conditions characteristic of experiments and salmon farms I show that this is common in nature with at least 50% of lice on juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon moving from one host to another as they mature. I demonstrate that the underlying drivers of this dispersal shift with ontogeny from competition for resources to access to mates. Movement among hosts increases the probability that a louse interacts with multiple hosts potentially increasing the extent of louse transmission from farmed to wild salmon and the spread of other pathogens if lice can act as vectors. In addition, the movement of lice among hosts may have important consequences for predator-prey interactions between salmonids during early marine life. Coho salmon smolts (O. kisutch) selectively prey upon infected pinks. I show that sea lice transfer from pinks to coho during these interactions and as a result coho experience a 2 to 3 fold increase in parasite exposure over what they would otherwise experience through passive exposure to infective larvae from farms. To test for a population level response to this increase in louse exposure I examined coho population dynamics spanning a region of exposure to lice from infected pink salmon and salmon farms and show that populations exposed to recurrent infestations were depressed 7 fold relative to adjacent unexposed populations. These findings highlight the ecosystem context in which louse transmission from farmed to wild fish occurs and suggest that species interactions and parasite behaviour may cause the effects of parasite transmission from farmed to wild fish to propagate up marine food webs with broader consequences than previously appreciated.