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Description
Interactions between fleshy and coralline algae can create a myriad of dynamics at the base of marine ecosystems, influencing nutrient availability and habitat characteristics for higher trophic levels. Rhodoliths create complex ecosystems that support a high number of marine taxa in waters worldwide that can be susceptible to environmental changes. Climate change conditions brought upon by increases in ocean warming (OW) and ocean acidification (OA) and the combination of both (greenhouse conditions, GH) can be especially detrimental to coralline algal productivity and calcification abilities. To examine how three dominant fleshy algal species in Catalina Island rhodoliths (Lithothamnion australe), L. australe productivity and calcification rates were measured when in the presence of Zonaria farlowii, Dictyota flabellata, and Dictyopteris undulata and then were compared to L. australe productivity by itself. Productivity was measured under four climate experimental treatments that represent differences between present-day (ambient), future OA (decreased by 0.3 pH units), future OW (increased by 2°C), and combined or GH (OA + OW) conditions. To do this, Catalina Island macroalgal relative proportion cover was quantified using photo quadrats that were analyzed using CoralNet, a machine-learning software. To examine productivity shifts, L. australe individuals and individuals of one species of fleshy macroalgal species at a time were placed in laboratory mesocosms separately and in combination while being exposed to the four climate treatments. To examine shifts in calcification of L. australe in the presence or absence of fleshy macroalgae and under the four climate treatments, mesocosm water was collected and titrations on total alkalinity (TA) were conducted to calculate calcification net rates. Photo-physiology was quantified as shifts in dissolved oxygen to measure net primary productivity (NPP) and respiration. Results indicate that fleshy algal association across all three macroalgal species increases L. australe NPP and respiration rates, and that GH conditions have a marginal negative impact on L. australe, reducing NPP and increasing respiration. Calcification was greatest under acidified treatments but did not shift when L. australe was in the presence of fleshy algae. These results suggest that algal-algal association within rhodolith beds can both positively and negatively affect L. australe productivity.
