V. Montecino et D. Quiroz, Specific primary production and phytoplankton cell size structure in an upwelling area off the coast of Chile (30 degrees S), AQUAT SCI, 62(4), 2000, pp. 364-380
Accounting for variability in oceanic primary production, which is generall
y 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 theo
ry, where phytoplankton cell size should be inversely related to chlorophyl
l a specific primary production (mg C mg Chl a(-1) h(-1)), was conducted us
ing cruise data collected within 200 km of the coast in the upwelling zone
off Coquimbo, Chile (30 degreesS) during July 1995, January and July 1996.
Discrete water samples at the surface and up to 6 depth strata, selected ac
cording to light extinction coefficients, were size-fractioned +/- 8 mum fo
r chlorophyll a (B) and primary production (P) measurements.
Overall, specific primary production (P-B) Of the <8 <mu>m fraction was sig
nificantly higher than P-B of the >8 mum fraction (4.48 versus 2.88 mg C mg
Chl a(-1) h(-1), P = 0.006 Wilcoxon test). Analysis of separate cruises in
dicated 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 an
d nutrients, particularly for larger sized cells. Alternatively, when a sma
ller size-range of cells is present, the variance explained by the relation
ship 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 <mu>m/to
tal 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 mum 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 <mu>m The seasonality of alpha was independent of CSSt, therefore P varia
bility in time was not always explained by CSSt. However correlations betwe
en P and Chl a were improved consistently by up to 5% when CSSt was incorpo
rated 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 as
semblage. However, our understanding of primary production variability by i
ncorporating allometric considerations of cell size and physiology (i.e. al
pha) is improved by contrasting different size and biomass ranges.