Natural environmental gradients alter community composition and ecosystem functioning on coral reefs
Bernas, D. (2023). Natural environmental gradients alter community composition and ecosystem functioning on coral reefs. MSc Thesis. California State University: Northridge. xi, 89 pp.
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Document type: Dissertation
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| Abstract |
Environmental variability caused by natural gradients in marine ecosystems promotes shifts in biodiversity and ecosystem functioning. Coral reefs experience a multitude of natural and anthropogenic environmental gradients affecting presence and abundance of both community species and functional traits. Shifts in functional diversity can lead to changes in community metabolism (net ecosystem production and calcification [NEP and NEC]) as the altered community interacts with its environment. In these studies, I tested the impact of a common but under-investigated natural environmental gradient on coral reefs, submarine groundwater discharge (SGD). SGD is the expulsion of terrestrial fresh or recirculated seawater onto a marine environment and is associated with decreasing temperatures, salinity, and pH with increasing nutrients and variable carbonate chemistry. I quantified changes in species and functional diversity and broad community composition along an SGD gradient in Moorea, French Polynesia to understand how this suite of biogeochemical variables impacts reef communities. I then directly tested the effect of SGD on growth of common species along this reef, as well as the interactive effect of SGD exposure and community composition differences on ecosystem metabolism. Richness of species and functional entities along the SGD gradient displayed a polynomial relationship, with peak richness at moderate SGD. Functional community composition (presence and abundance) increased along the variability gradient, while species composition was insignificant, highlighting the utility of testing multiple biodiversity metrics on community response to environmental shifts. I observed physiological differences between high and low SGD exposure of different species of benthic reef taxa, where growth of Porites rus, Valonia fastigiata, and Halimeda opuntia decreased in high SGD. Differences in community identity resulted in changes to ecosystem functioning, while environmental exposure of SGD had no effect on photosynthesis or calcification. However, community assemblages that are not adapted for the high variability of SGD exhibited a possible stress response through elevated respiration in the high SGD exposure treatment. These studies showed the direct and indirect impacts of SGD through shifts in diversity and community composition that led to changes in ecosystem functioning, as well as physiological changes of certain reef taxa in a highly variable biogeochemical environment. SGD is present across multiple ecosystems globally and provides a unique opportunity to study community responses across a variety of ecosystems. As global ecosystems are increasingly impacted by environmental variability and new disturbance pressures, we can look at community responses to multivariable environmental shifts already occurring naturally within SGD gradients to understand how additional variability may alter ecosystem stability. |
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