Vertical distribution of Macrocystis pyrifera nutrient exposure in southern California

We examined water column temperature time series profiles for several years at two locations in a single kelp (Macrocystis pyrifera) forest to characterize the alongshore variability of the nutrient climate that giant kelp is exposed to and compare it to the response of giant kelp. The differences in nutrient climate are due to differential alongshore vertical variations in temperature, a well-established proxy of nitrate, due to the topographically induced internal wave dynamics within the kelp forest. We observed the greatest temperature variability during summer and most of this variability occurred near the surface. The 14.5 °C isotherm, indicating the presence of nitrate, ranged the entire vertical extent of the water column, and was shallowest during the winter and in the southern portion of the kelp forest. Predicted water column integrated nitrate varies from 0 μmol NO3−/m2 to 431 μmol NO3−/m2 yielding a time series daily average of 0.12 gN/m2day (North La Jolla) and 0.18 gN/m2day (South La Jolla). Redfield conversion of these values puts the time series daily average for carbon production (upper limit) between 0.8 and 1.2 gC/m2day for the north and south parts of the bed respectively, and shows considerable variation at several time scales. Giant kelp in the southern portion of the forest exhibited greater stipe densities (a proxy for kelp production) than individuals in the northern portion, corresponding with the alongshore nutrient climate variability. The depth of the nutricline varied by up to 10 m over time scales as short as hours. Variability was greatest at diurnal and semi-diurnal frequencies, with shallower water column depths showing greatest variability. These depth-specific variations in temperature and nutrient exposure may have biologically important consequences for M. pyrifera especially during low nutrient seasons.

► We quantified the depth-specific physical environment of Macrocystis pyrifera.
► Temperature time series profiles revealed within kelp bed differences.
► Primary production upper limits are estimated from water column integrated nitrate.
► Meter scale vertical change in nutrient availability can occur over a few hours.
► Kelp response to a variable environment at different scales is discussed.