Estuaries support diverse ecological communities which perform many important biological
functions and provide a variety of goods and services. As a result of direct and indirect human
impacts the worlds estuaries are currently being degraded at unprecedented rates and their
functionality and services are under threat. The conservation and sustainable use of estuarine
resources is of global significance due to their role in food production (e.g., aquaculture, nursery
habitats), mitigating climate change (e.g., carbon storage, coastal defence) and water quality (e.g.,
nutrient cycling). A key challenge in estuarine management is determining how to manage the many
ecosystem services simultaneously.
Bivalves are an important component of marine environments because they deliver many
ecosystem services and contribute markedly to the functioning of coastal ecosystems. Managers
must often balance economic value (e.g., harvest) with wider conservation objectives (e.g., coastal
birds) and there are often conflicts. Understanding interactions (i.e., trade-offs and synergies)
between ecosystem services is key to their sustainable management but knowledge is limited within
marine environments due to data scarcity and system complexity (e.g., non-linear, interactive,
extensive). Effective conservation and management strategies are regularly informed using
ecological models and depend heavily on understanding how environmental change impacts the
physiology and behaviour of organisms. Applying mechanistic models which link ecological
processes with ecosystem functions and services can help improve understanding.
Coastal birds rely heavily on intertidal invertebrate prey to maintain body condition, particularly
throughout winter and while travelling to and from breeding grounds. Human activities within
migration routes therefore require careful management to ensure they are sustainable and
minimise the negative impacts to ecological communities and functioning. Many coastal bird
populations are however experiencing long-term declines and the drivers are thought to be in part
associated with the disappearance and degradation of intertidal habitats. This thesis aimed to
assess the environmental impacts of harvesting the intertidal bivalve Mytilus edulis (Blue mussel),
specifically to coastal bird populations in Morecambe Bay.
Chapter three aimed to identify and develop a method for simulating the growth and development
of intertidal M. edulis in response to environmental conditions. A Dynamic Energy Budget (DEB)
model was developed to simulate the growth and development of intertidal M. edulis in response
to environmental conditions. The condition and growth rates of intertidal M. edulis in Morecambe
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Bay were observed to increase with decreasing elevation and a DEB growth model which was
modified to incorporate an energy conserving, intertidal adaption most accurately predicted the
relationship. Increases to the concentration of suspended particles, temperature and elevation
were most influential to individual growth.
Chapter four aimed to improve understanding of how to sustainably manage the ecosystem
services provided by intertidal M. edulis. A DEB population model for intertidal M. edulis was
developed to simulate the functioning of multiple ecosystem services simultaneously in response to
environmental conditions. It was used to quantify the net nutrient (nitrogen, phosphorus and
carbon) fluxes (i.e., source or sink) associated with the population, to assess the sensitivity of
services to environmental change and to identify potential synergies and trade-offs linked to
harvesting M. edulis. The population was predicted to serve as a sink for nutrients, particularly
carbon and mostly due to biodeposition and shell burial. Changes to the concentration of suspended
particles, temperature and pre-settlement mortality were most influential to all ecosystem service
rates. The harvest of market size M. edulis had a negligible but positive impact on the provision of
habitat provisioning (i.e., biomass) while the harvest of culture size individuals had an important
and negative effect. Strong synergies were identified between the provision of habitat provisioning
and the removal of nutrients and the effects of harvesting M. edulis subsequently extended to
regulatory services.
Chapter five aimed to assess the long-term risks of harvesting intertidal M. edulis to coastal bird
populations in Morecambe Bay. An Agent-Based Model (ABM) was developed for simulating both
the foraging behaviour of coastal birds and the dynamics of M. edulis populations in response to
environmental conditions. The model was used to assess the relative importance of M. edulis as a
resource for coastal bird populations in Morecambe Bay. Changes to the abundance of M. edulis in
Morecambe Bay were most influential to the annual body condition of the Somateria mollissima
(Common eider) population. The harvest of market size individuals was uncorrelated with the
condition of the Calidris canutus (Red knot), Haematopus ostralegus (Eurasian oystercatcher) and
Larus argentatus (Herring gull) populations, and weakly correlated (positive) with that of the S.
mollissima population. The harvest of culture size individuals was uncorrelated with the condition
of the C. canutus population, weakly correlated (negative) with that of both the H. ostralegus and
L. argentatus and strongly correlated (negative) with the condition of the S. mollissima population.
The negative correlations associated with harvesting culture size individuals became stronger when
supplementary bivalve (e.g., Macoma balthica) resources were low.