Masters Thesis

Relationships Between a Branch-Forming Crustose Coralline Alga, Associated Small Motile Invertebrates, and Water Flow

Coral reefs are among the most diverse and threatened marine ecosystems. The ability of primary producers, such as coral and macroalgae, to create and modify habitat is key to promoting species coexistence and ecosystem functioning, even amid disturbances. Crustose coralline algae (CCA) are important bioengineers, offering protection from physical forces and predation, and are susceptible to local and global stressors, such as pollution and climate change. Yet, assemblages associated with CCA have received little attention in coral reef ecosystems. Lithophyllum kotschyanum is an abundant, structurally complex species of CCA in the shallow subtidal on the north shore of Moorea, French Polynesia. In the austral winter of 2019, I quantified Lithophyllum-associated assemblages collected from three sites in both high and low water flow environments. A model selection framework was used to rank the relative importance of three drivers on assemblage composition: algal host morphology (interstitial volume, branch density, and thallus rugosity), algal mortality, and water flow. Assemblages were dominated by habitat-generalist cryptofaunal taxa and comprised up to 78 individuals and 14 different taxa per algal host. Patterns in assemblage organization were best explained by algal morphology. Algal morphology varied within low- and high- flow environments and correlated with assemblage patterns that were specific to the body sizes of associated cryptofauna. Medium- and large- bodied crustaceans were more abundant as interstitial volume increased between algal branches, while numbers of small-bodied crustaceans and echinoderms, capable of conforming to algal interstices, increased with algal branch density and rugosity. This study demonstrates how changes in CCA structural features could have implications on the taxonomic and size composition of cryptofauna assemblages. In addition to providing physical structure, biogenic hosts, like CCA, modify the chemical environment directly above their surfaces in a thin, laminar zone called the diffusive boundary layer (DBL). The chemistry and thickness of the DBL is essential to host metabolism but the role of associated cryptofaunal invertebrates in its development remains poorly understood. In Oct-Nov 2020, I investigated the interactive effects of cryptofaunal presence and water flow rate, a known determinant of DBL thickness and chemistry, on the DBL surrounding L. kotschyanum. Oxygen and pH profiles were measured on algal branches and interstitial spaces under three flow speeds, dark and light, and with and without associated cryptofauna. DBL thickness, oxygen concentrations and pH at the algal surface, and oxygen flux were compared across flow speed and cryptofaunal assemblage presence. Effects of cryptofaunal presence were limited to interstitial spaces in the light condition and depended on flow speed. With cryptofauna present, the DBL was thinner and surface pH was closer to ambient pH, but only in zero flow. Cryptofauna remained within the refuge of algal interstitial spaces in the light where, in zero flow, DBL thicknesses were greater than the mean maximum lengths of the most abundant taxa. Cryptofauna additionally increased oxygen flux (net photosynthesis) in interstitial spaces but this was specific to high assemblage biomass and high algal branch density. This effect on algal photosynthesis illustrates a previously unknown mutualism where invertebrates gain shelter while facilitating the productivity of more structurally complex hosts through the modulation of the algal diffusive boundary layer. Collectively, these findings highlight that CCA habitat is important to the coexistence of cryptofauna across low- and high- flow environments but whether these relationships benefit CCA hosts is context-dependent. Outcomes were based on the flow environment, cryptofaunal biomass, and CCA structural complexity. The maintenance of these two groups, CCA and cryptofauna, are important to coral reef biodiversity and ecosystem functioning and these studies offer insight into the relationships between them.

Items in ScholarWorks are protected by copyright, with all rights reserved, unless otherwise indicated.