Effect of tidal currents and tidal stage on estimates of bed size in the kelp Nereocystis luetkeana

Aerial or overhead photography is a technique commonly used to measure spatio-temporal variability in surface canopy kelps. To evaluate potential biases in this technique, we used elevated (shore-based) oblique angle photography coupled with in situ measurements of currents and tides to sample 4 independent Nereocystis luetkeana beds at sites adjacent to San Juan Island (Washington State, USA) in 2004 and 2005. Data were collected at high frequency (every 10 min) to quantify the effects of tidal height and tidal currents on the amount of N. luetkeana visible at the surface. Effects of tidal height were highly significant in all 6 time series. Current significantly affected apparent bed size in 5 of the 6 time series, although the magnitude of the effect varied among sites. N. luetkeana beds decrease in apparent size as current velocity increases and as tidal level increases. Currents can fluctuate rapidly, changing by many 10s of cm s(-1) over periods as short as 10 min, due in large part to interactions of tides with local topography and bathymetry close to shore, where kelp beds are typically found. As a result, currents can introduce significant variability to estimates of kelp population size collected by remote sensing techniques.