| Summary: | Marine ecosystem engineers are species that modify the physical environment. Often, this is via the dense aggregation of many individuals leading to the formation of biogenic habitat. Reduced fluid mixing within these habitats can allow the alteration of seawater chemistry as the by-products and substrates of biological processes like photosynthesis, respiration, and/or calcification accumulate or are consumed, respectively. Habitats dominated by photosynthetic habitat-formers have received much attention in recent years for their ability to locally ameliorate stress associated with ocean acidification as photosynthesis removes carbon dioxide from the seawater. However, less understood are the ways non-photosynthetic ecosystem engineers shape seawater chemistry at small spatial scales. Lacking photosynthetic ability, habitats formed by these organisms would be expected to increased carbon dioxide concentrations, and thus, increase chemical stress for resident organisms. Likewise, these habitat-formers often calcify, or form calcium carbonate shells or skeletons, which can alter alkalinity, another component of the carbonate chemistry system. Given the diversity of communities that rely on habitats formed by non-photosynthetic ecosystem engineers, understanding the chemical parameters to which organisms respond and the extent to which those parameters are altered within these habitats is of utmost importance as oceans change in the future. Throughout this dissertation, I explore the biological and physical drivers of altered chemistry within aggregations of heterotrophic ecosystem engineers and identify the particular chemical parameters that drive organismal response to altered seawater chemistry. In Chapter 1, I detail experiments where I measured the chemical modification that can occur within mussel beds. I show that chemical gradients are a balance between biological processes, like respiration and calcification, and physical processes, particularly increased mixing induced by higher seawater velocity. In ...
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