Predicting macroalgal responses to multiple environmental changes using functional relationships from uptake mechanisms to growth rates

Chair: Heidi Pethybridge

Janet E. Kübler(1), Steve Dudgeon(1)

1 CSUN, Northridge, CA 91330-8303, USA

Background
Fleshy macroalgae are hypothesized to benefit from several aspects of ocean climate change (carbon fertilization, eutrophication and warming) and macroalgal blooms are suggested to be increasing in frequency and severity globally. Here, we investigate the combinations of environmental conditions that could lead to large changes in macroalgal production on temperate coasts.

Methods
We combined models and multifactor growth experiments to develop response norm surfaces for photosynthesis and growth of macroalgae to changing pCO2, temperature, light intensity and nitrogenous nutrient supplies. Macroalgal species were selected to represent simple (Plocamium cartilagineum) and complex (Ulva spp.) modes of inorganic carbon uptake, the latter with known scope for acclimation.

Findings
We find that both species of algae used in our experiments had nonlinear photosynthetic and growth responses to increasing pCO2 over the range 344 to 1053 µatm with declining growth rates above 500-700 µatm). Those responses were further modulated by interactions with light supply, temperature and nutrient availability. High photon flux density and nutrient enrichment increased growth rates. Model results, based on additive effects of data for individual factor studies in the published literature very closely predicted the responses measured in multifactor culture experiments.

Conclusions
Relatively few of the combinations of multiple aspects of ocean climate change resulted in strongly elevated growth rates for either species. The scope for temperate macroalgae to benefit from warming and inorganic carbon supply is limited by light availability under many current conditions. Coastal eutrophication is likely to exacerbate light limitation to attached benthic macroalgae. Productivity and growth rates, of the macroalgae studied here, are predicted to decline due to over fertilization with inorganic carbon by the end of this century.