MEPS 515:97-109 (2014)  -  DOI: https://doi.org/10.3354/meps10974

ABSTRACT: Nitrogen is essential for algal productivity but often reaches limiting concentrations in temperate ecosystems. Increased water motion enhances nitrogen uptake by decreasing the thickness of the diffusion boundary layer surrounding algal surface tissue, allowing for increased nitrogen mass-transfer across this boundary. Macrocystis pyrifera forms large beds that span the water column and can alter the surrounding physical environment by creating bed-wide boundaries that may reduce current and wave propagation to the bed interior; reduced water motion may decrease mass-transfer rates and therefore alter nitrogen uptake. We investigated whether a water mass-transfer gradient across M. pyrifera beds exists by identifying 3 bed types likely to experience different water motion intensities (open, shoreline exterior and shoreline interior) and whether this gradient influenced heterogeneity in M. pyrifera growth and tissue status during low nitrogen (summer) and high nitrogen (winter) conditions. Gypsum dissolution suggested that mass-transfer significantly increased across beds; open bed dissolution rates were approximately 6% higher than the shoreline exterior, which exhibited mean dissolution rates 17% higher than the shoreline interior. Summer kelp growth, pigmentation, tissue %N and C:N paralleled mass-transfer, where exterior kelp exhibited higher values than interior kelp. The same trends did not exist during the winter, when ambient nitrogen concentrations were high, suggesting that mass-transfer is an important mechanism for nitrogen acquisition during limitation events. This study highlights mass-transfer variability across relatively small macroalgal beds and the corresponding effects on kelp growth and nitrogen status, which previously might have been assumed as uniform due to the general wave exposure.

KEY WORDS: Water motion · Algae · Nitrogen · Limitation · Hydrodynamic · Nutrient