Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Christopher D. Stallings, Ph.D.

Co-Major Professor

Susan Bell, Ph.D.

Committee Member

Ernst Peebles, Ph.D.

Committee Member

Joshua Kilborn, Ph.D.

Committee Member

David Kimbro, Ph.D.

Keywords

herbivory, intertidal zones, macroalgal beds, mangroves, oyster reefs, positive interactions

Abstract

A core aim of landscape ecologists as well as conservation and management practices has been to understand how processes that structure communities vary as a function of landscape context. Landscape heterogeneity (i.e. landscape composition, configuration) and fine-scale habitat characteristics can influence ecological interactions across habitat patches at a range of scales. Therefore, the main objective of this work is to apply a landscape ecology perspective to understand how seascape heterogeneity can influence demographic rates, community patterns, and ecological processes. To accomplish this overall goal, I conducted a literature review on oyster reefs from a seascape ecology perspective (Chapter 1) and I carried out three independent research studies (Chapters 2-4) using observational and experimental approaches.

In Chapter 2, I assessed demographic rates of oysters in reefs adjacent to various habitat types in a transition zone. In northeast Florida, the dominant coastal habitat transitions from Smooth Cordgrass (Sporobolus alterniflorus; temperate species) to Black Mangroves (Avicennia germinans; tropical species). These two foundation species may affect the demographic rates of the Eastern Oyster (Crassostrea virginica), another foundation species commonly located adjacent to them. Therefore, I deployed juvenile oysters in cage experiments comprising three levels of predator exposure on (a) oyster reefs bordering Smooth Cordgrass, (b) reefs bordering Black Mangroves, and (c) isolated oyster bars, to quantify survival and growth rates north and south in the Guana Tolomato Matanzas National Estuarine Research Reserve. Additionally, I analyzed three datasets with information on the abundances of oysters, associated organisms, potential predators, and other biotic (e.g., chlorophyll a) and abiotic factors (e.g., salinity, dissolved oxygen) across the seascape. The results of this chapter indicated that neighboring habitats and regional variation in the seascape may influence demographic rates of oysters.

Although oyster demographic rates across intertidal zones have been examined in oyster reefs at higher latitudes, far less is known about them at subtropical locations where desiccation stress is expected to be greater due to higher temperature. Furthermore, little is known about oyster demographic rates when in association with a macrophyte intertidal habitat, which may reduce desiccation stress and positively influence oysters at higher intertidal zones. In Chapter 3, I measured demographic rates of oysters along intertidal zones on oyster reefs and on oyster clusters associated to Red Mangrove prop roots in Tampa Bay, Florida. This study addressed: (a) how do demographic rates of the Eastern Oyster vary along intertidal zones (bottom, middle, and top) on oyster reefs and on prop roots and (b) whether the association of oysters with mangroves may reduce desiccation stress, thus positively influencing oysters at higher intertidal zones. I found oysters on prop roots to be at higher densities and had higher survival. Consistent with density and survival, water loss (a proxy for desiccation stress) was lower on the prop roots, suggesting that the mangrove canopy may have provided a positive effect on oysters. This chapter contributes to our understanding of fine-scale zonation patterns on two biogenic habitats that exist in close association.

In tropical seascapes, beds of benthic macroalgae occur naturally interspersed within or nearby other habitats, but it is unclear what roles they play to support marine fauna. Even less is known about how the introduction of non-native macroalgal habitats (e.g., macroalgal farms) into tropical seascapes may affect ecological processes that influence ecosystem function and its comparison to seascapes with natural macroalgal beds. To address this knowledge gap, in Chapter 4, I surveyed fish assemblages and deployed macroalgal assays to quantify herbivory within naturally-occurring macroalgal habitats, macroalgal farms, as well as at varying distances in the seascape near Mafia Island, Tanzania. The results showed that macroalgal beds had a higher species richness of fish and lower herbivory, while higher herbivory occurred in farmed seascapes likely to the farms attracting herbivores. This chapter advances our understanding of the effects that alteration of tropical seascapes (due to the introduction of farms) may have on patterns of community assembly and ecological processes.

Overall, the findings of this dissertation suggested that neighboring habitat patches can influence demographic characteristics of organisms, but the responses may be contextual upon location in the seascape due to variation in regional factors. Moreover, fine-scale variation in biotic and abiotic factors in intertidal habitats can affect demographic rates of organisms and the presence of other foundation species may influence these patterns. Lastly, seascape alterations can influence patterns of community assembly and ecological processes thus affecting ecosystem structure and function. As seascapes continue to be altered due to climate change (e.g., poleward movement of foundation species) and anthropogenic activities (e.g., farming practices), studies that assess the ecological responses of such changes will improve our understanding on the cascading effects within ecosystems and the services they provide.

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