Proxies of community production derived from the diel variability of particulate attenuation and backscattering coefficients in the northwest Mediterranean Sea
Résumé
A 6 yr time series of high frequency inherent optical property (IOP) measurements in the Mediterranean was used to derive information on the diel and seasonal variability of particulate production. Empirical relationships between particulate attenuation (c(p)), particulate backscattering (b(bp)), and particulate organic carbon allowed calculation of estimates of net community production (NCP), daytime NCP (NCPd), and gross community production (GCP) from the diel variations in either IOP. Similar seasonal variations and good correlation (r = 5 0.83, p < 0.001) between daily means of c(p) and b(bp) were observed, yet differences in the timing and amplitude of their diel cycles led to significant differences in their derived production metrics. Best agreement was obtained during bloom proliferation (44.1-66.7% shared variance) when all three production estimates were highest, while worst was during bloom decline. Best overall correlation was found for NCPd. Accordingly, only c(p)-derived estimates reproduced predicted seasonal variations in community production and seasonality of ``traditional'' chlorophyll-based primary production models. Analysis of the diel cycles of ``real-time'' net community production (NCPh), determined from the first-derivative of either c(p) or b(bp), revealed ca. fourfold to eightfold lower daytime NCPh, twofold to fivefold lower daily maximum NCPh, and twice as much intraseasonal variability relative to the mean amplitude of diel variations for b(bp). Although the timing of maximum of c(p)-derived NCPh was consistently prior to solar noon, significant seasonal differences in the timing of maximum b(bp)-derived NCPh was observed. Particulate backscattering may be used to infer biogeochemical properties, while greater understanding of the diel cycles of b(bp) is needed before b(bp) can be used to investigate daily community production.