The effect of temporal undersampling on primary production estimates

Abstract

Annual primary production estimates for specific oceanic regions have typically been made using a variety of measures of productivity spaced, at best, several weeks apart Primary productivity in the oceans is known to be extremely episodic. It is hypothesized here that primary production data with a temporal resolution of several weeks have a high potential for error due to undersampling. In the present analysis, time series of gross primary productivity were calculated using time series of photosynthetically available radiation and chlorophyll a concentration as input to an optical production model. The input data are of minute scale resolution and were gathered during a number of moored experiments. These took place over the past 5 years at several oceanic sites. The minute scale productivity time series were integrated to form time series of daily estimates of gross production. These range in duration from 40 to 260 days. The time series exhibit several regimes characteristic of oceanic primary productivity, such as phytoplankton blooms, productivity pulses associated with advected water masses, steady state growth, and development of a subsurface productivity maximum. The presence of these features makes our time series ideal for investigating (1) the sensitivity of annual production estimates to the timing of the sample set and (2) the error introduced by undersampling inherent in coarser sampling methods. It was found that distinct pulses of productivity generate the greatest error and that high variability leads to large errors, even for well‐resolved sampling intervals. The maximum percent error due to undersampling was found to be 85%. Additionally, up to a fourfold range between the maximum and minimum estimates of average daily production was found over all sampling intervals. Finally, the maximum expected range (300 g C m−2 yr1) and the expected standard deviation (±42 g C m−2 yr1) for annual water column production were determined at a Sargasso Sea site for which long‐term productivity time series were available at four depths within the euphotic zone.