Specific primary production and phytoplankton cell size structure in an upwelling area off the coast of Chile (30°S)

Abstract:

Accounting for variability in oceanic primary production, which is generally attributed to physical forcing, requires a more integrated understanding through the quantification of biological parameters, such as phytoplankton cell size and physiology. An evaluation of predictions from allometric theory, where phytoplankton cell size should be inversely related to chlorophyll a specific primary production (mg C mg Chl a ^-1 h^-1), was conducted using cruise data collected within 200 km of the coast in the upwelling zone off Coquimbo, Chile (30°S) during July 1995, January and July 1996. Discrete water samples at the surface and up to 6 depth strata, selected according to light extinction coefficients, were size-fractioned ± 8 μm for chlorophyll a (B) and primary production (P) measurements.¶Overall, specific primary production (P^B) of the <8 μm fraction was significantly higher than P^B of the >8 μm fraction (4.48 versus 2.88 mg C mg Chl a ^-1 h^-1, P = 0.006 Wilcoxon test). Analysis of separate cruises indicated that during the July 1995 cruise, the negative relationship between phytoplankton size and P^B was not obtained. This result suggests that at smaller scales, the phytoplankton size versus P^B relationship could change according to an increase in the acquisition of resources, such as light and nutrients, particularly for larger sized cells. Alternatively, when a smaller size-range of cells is present, the variance explained by the relationship should decline.¶The distribution of all P^B values vs Cell Size Structure (CSSt), expressed as the proportion of the smaller phytoplankton fraction (Chl a <8 μm/total Chl a), shows that P^B remains around 4 mg C mg Chl a ^-1 h^-1 when >80% of the CSSt falls in the >8 μm size fraction, and may exceed 10.0 mg C mg Chl a ^-1 h^-1 when >50% of the CSSt is in phytoplankton with sizes <8 μm.¶The seasonality of alpha was independent of CSSt, therefore P variability in time was not always explained by CSSt. However correlations between P and Chl a were improved consistently by up to 5% when CSSt was incorporated into analyses.¶These results show that overall variability in production in this upwelling system is due to both Chl a and the size structure of the phytoplankton assemblage. However, our understanding of primary production variability by incorporating allometric considerations of cell size and physiology (i.e. alpha) is improved by contrasting different size and biomass ranges.