The effects of climate change on macroalgal growth, trophic interactions and community structure

Global climate change is threatening the structure, function, and health of ecosystems. While factors of climate change have been studied extensively over the past few decades, most research has focused on the response of single organisms or populations; as our ecosystems are comprised of complex interactions and relationships, it is of critical importance to understand how entire communities are going to be impacted by climate change. Ocean acidification (a by-product of increased atmospheric carbon dioxide, CO2), and nutrient loading are two major forces of global change that are projected to have detrimental impacts on coastal marine species and ecosystems. Most work on ocean acidification has focused on the response of calcifying organisms, where the changes in ocean chemistry associated with acidification enhance shell dissolution and impair growth. However, while calcifying species are expected to exhibit negative responses to acidification, primary producers, like macroalgae, are expected to flourish. Both ocean acidification and nutrient loading can stimulate the growth and productivity of opportunistic, fast-growing, ephemeral macroalgae at the expense of foundational species such as corals, seagrasses, and long-lived, perennial macroalgae (i.e. kelps). As a result, these ecosystems will likely undergo major shifts in structure, function, and diversity. Few studies have investigated the interactive effects of ocean acidification and nutrient loading, particularly in terms of community response and trophic interactions. Despite increasing the growth rates of macroalgae, the presence and diversity of herbivores within an ecosystem has the potential to control this expected algal growth. The research described in this dissertation aims to: 1) quantify the combined effects of ocean acidification and nutrient loading on the growth, tissue quality, and competition of two abundant macroalgal species with different life histories; 2) test whether or not an abundant grazer can enhance consumption of macroalgae under future conditions of acidification and nutrients, promoting community resilience; 3) describe the impact of ocean acidification on the growth and diversity of reef-associated turf algal communities. Using a laboratory mesocosm design, the response of Ulva (an ephemeral, opportunistic green alga) and Fucus (a long-lived, perennial brown alga) to the interaction of two levels of ocean acidification and two levels of nutrients was tested. Individual, field-collected algal thalli were placed in flow-through seawater systems with one of four experimental conditions: high pCO2 (~1100 µatm) or background pCO 2 (~390 µatm) and high nutrients (200 µM TN) or low nutrients (10 µM TN), in a fully factorial design. Three experiments were run: the first two investigated the response of Ulva and Fucus in monocultures; the third tested the response of both species cultured together (biculture). Growth rates and tissue quality (via carbon to nitrogen ratios, C:N) were measured after 21 days of exposure to treatments. Ocean acidification and nutrient loading significantly increased the growth in Ulva, where growth rates under high pCO2 and high nutrients were about 3X greater than those grown under ambient conditions, with the environmental factors appearing to have an additive impact on Ulva growth. Growth rates of Fucus were unaffected by environmental conditions. Both species exhibited an increase in tissue quality as a result of decreased C:N when exposed to high nutrients. Response variables were compared between monoculture and biculture experiments for both species. Growth rates of Ulva and Fucus were unaffected by the treatment culture, but tissue C:N of Fucus was significantly higher when grown with Ulva, indicating potential resource competition, where Ulva outcompetes Fucus. The enhanced growth exhibited by Ulva supports previous work indicating the enhanced growth of opportunistic algal species under future climate conditions. While this could be troubling for species inhabiting coastal ecosystems (such as seagrasses, non-bloom forming macroalgae, and fish), grazers may hold the key to mitigating algal growth and keeping ecosystems in balance. Consumption rates and feeding preferences of a common marine snail ( Littorina littorea) were tested using the same experimental design and environmental parameters detailed above. Snails were placed in treatment mesocosms for seven days and were given a choice of Ulva and Fucus. High pCO2 levels significantly reduced macroalgal consumption by about 50% and snails switched from a mixed algal diet to feeding exclusively on Ulva. Respiration rates for L. littorea were measured, and under high pCO2 respiration was significantly reduced. Artificial food trials were run to help explain the diet preference change. No difference was found comparing consumption of the artificial food, pointing to the preference shift being driven by algal tissue toughness. Reef ecosystems have been well studied with respect to ocean acidification. This research shows that corals are overtaken by fleshy macroalgae and fast-growing turf algae. Here, we tested the response of turf algal communities to three levels of ocean acidification. Natural turf communities growing on coral rubble from the Great Barrier Reef, Australia were collected and exposed to ambient, medium, and high pCO2 treatments in mesocosms for 41 days. Communities were assessed for biomass and genus diversity. Biomass of turf communities was significantly higher under high pCO2. Turf community evenness and diversity significantly increased under high pCO2. This change in community structure is likely due to the decline in abundance of Polysiphonia (a filamentous, branched red algae). The results indicate that enhanced turf growth under conditions of acidification will aid in the growth and expansion of macroalgae at the expense of corals in reef ecosystems. Changes in turf diversity should inform how larger macroalgal communities may be structured in the future. This research highlights the success that opportunistic macroalgae and turf algae will have under future climate conditions. The success of macroalgae, however, comes at the expense of other critical and foundational species within a community. In addition, macroalgal communities are likely to undergo assemblage shifts as well, due to species-specific responses to environmental change, where species gain a competitive advantage. Top-down forces, such as grazing, may protect against changes in community structure and macroalgal assemblages, but only if grazers are not negatively impacted directly by environmental change. If grazers exhibit decreased consumption and are unable to keep up with anticipated algal growth, this will ultimately enhance macroalgal abundance in coastal ecosystems