A novel system for measuring in situ rates of photosynthesis and respiration of kelp

Macroalgae are important primary producers on temperate reefs worldwide, yet obtaining accurate measurements of in situ rates of photosynthesis and respiration for large seaweeds, particularly kelp, is challenging. Here we describe a novel and inexpensive photo respirometry system that can be used to generate photosynthetic-irradiance (P-E) curves for an entire adult kelp in situ. To demonstrate the application of this system, we (1) measured and compared P-E curves of the stipitate kelp Ecklonia radiata at 2 depths in northeastern New Zealand, and (2) compared laboratory-derived P-E curves to those obtained in situ. The system enclosed the lamina of the kelp and was sealed around the stipe, and oxygen evolution was recorded by an enclosed logger. By varying the irradiance in the chamber with shade cloth, and utilizing the variation in light associated with depth, the photosynthetic rates of an individual were measured over a range of irradiances in situ. Photosynthetic parameters of Ecklonia from field measurements varied with depth. P-E curves from individuals measured in situ differed from those measured in the laboratory, with maximum photosynthetic rates in the laboratory being 42 and 55% lower under natural and artificial light respectively. This system provides, for the first time, a method for generating P-E curves and estimating photosynthetic parameters for entire adult Ecklonia radiata individuals in situ. It can be adapted for use on other kelp and large seaweeds, and in tank-based experiments. Importantly, our findings also indicate that estimates of productivity based on laboratory-derived photosynthesis measurements likely underestimate kelp forest productivity.