NIELSEN, K.J.*; MENGE, B.A.; LUBCHENCO, J.: Fluorescing fronds: Snapshots of the physiological state of intertidal macrophytes

Disentangling the effects of multiple stressors on the physiological performance of organisms in the field is a major goal for physiological ecologists. Many ecological models invoke changes in physiological performance along environmental gradients as a major driver of the strength or sign of inter- and intra-specific interactions. The models predict that physiological performance declines along environmental stress gradients (e.g., tidal height) due to factors such as desiccation or temperature. Combining non-destructive physiological measurements with field experiments is necessary to test these models. We used in situ pulse amplitude modulation (PAM) fluorescence measurements of macroalgal fronds in conjunction with short-term manipulations to test the relative contributions of light, desiccation, and tidal height to the photosynthetic performance of a suite of intertidal macrophytes (Mazzaella splendens, M. flaccida, M. cornucopiae, Mastocarpus papillatus, Postelsia palmaeformis, Hedophyllum sessile, and Fucus gardnerii). Variable fluorescence declined, in some cases to zero, over morning low tides for both hydrated and desiccated fronds. In agreement with theory, variable fluorescence of desiccated fronds was significantly more depressed than for hydrated ones. Patterns in physiological performance over intertidal gradients within and among species agreed with environmental stress models. The results suggest that PAM fluorometry in combination with field experiments can be used to test community level, environmental stress models. Scaling physiological processes up and out of the lab in a model ecological system such as the rocky intertidal zone provides a unique opportunity to further our understanding of a complex biological system.