An Update of Primary Productivity and Chlorophyll,
a Twenty Year Database Analysis
Jennifer L. Sheldon, David M. Wolgast, James R. Wilkinson, Ralf Goericke
Scripps Institution of Oceanography
Introduction
Results
Discussion and Conclusions
Table 1 compares the previous regression analysis to last 10
years plus the updated 20 years data set and reveals a
decreased slope for regressions. This decrease can also be
seen in Figure 3 for mean observed primary productivity
for both decades. Spring and Summer seasons are seen in
Figure 1 with the highest slopes and thus the most activity,
and show the greatest change. The contour plots in Figure
4 demonstrate how the regression estimates use chlorophyll
to improve spatial coverage of productivity data in the
CalCOFI grid. Areas of observed productivity more
closely match expected productivity based on 10m Chl-a
data.
Database extraction for such a large data set is quick, easy
and powerful. Our comparison here of equations derived
from database queries revealed a decrease in the amount of
productivity per unit Chl-a for the last 10 years compared
to the previous 10 year study. Comparisons for seasons,
years and decades can be done quickly and with relative
ease. Use of this regression will allow better evaluations of
changes in California Current Ecosystem.
Primary production (PP) has been measured consistently on CalCOFI
cruises since 1984. Measurements are only made at ~15 stations per cruise,
compared to the 66 stations where other hydrographic and biological
parameters are determined. The resulting data set is difficult to extrapolate
to the whole CalCOFI study area. Mantyla et al. (1995) developed
relationships between chlorophyll a (Chl a), irradiance (L) and PP and
used these to predict PP at all 66 standard stations. They postulated that PP
can be estimated reliably from these parameters due to the success of the
regression analysis. The objective of this study was to update these
relationships and to use these routinely to estimate PP of the CalCOFI
study area.
Methods
Figure 2. Ten year regressions of observed primary productivity
data (P, mgC/m²/half-day), standardized to potential irradiance
(L, cal. X 10¯²/half-day) on the log of integral chjlorophyll
(Chl, mg/m²) for both decades.
Figure 1. Seasonal regressions of observed integral primary productivity on log chlorophyll a
from 1995-2006. Winter (Jan-Mar); Spring (April-June); Summer (July-Sept); Fall (Oct-Dec).
Mean Observed Integrated Primary Productivity
Integrated Pri Prod. mgC/m2/half-day
Primary productivity measurements are taken once a day. For these
experiments, the watercolumn is sampled in the late morning at six depths
cooresponding to the isolumes (97, 40, 10, 4, 1, 0.3%). Incubation bottles
are inoculated with 14C-bicarbonate at local apparent noon and incubated
for half of a light cycle in on deck incubators that are at sea surface
temperature. Experiments are terminated at Civil Twilight. Procedures
and calculations are performed as described in Mantyla, et.al 1995).
For this study, we used a regression from the last ten years to estimate
primary production (mgC/m²/half-day) from chlorophyll and potential
irradiance (L= Cosine approximation of the Smithsonian tabulated values
for total daily solar radiation per Julian day at 32.5N corrected for 75%
atmospheric transmission). These estimates are then used to generate
regression plots and contour maps for comparison with contours of ten
meter chlorophyll.
Table 1. Comparison of seasonal slopes of the regressions of observed integral
primary productivity on log chlorophyll a for the two ten year periods and
the twenty two year period combined Winter (Jan-Mar); Spring
(April-June); Summer (July-Sept); Fall (Oct-Dec). R² values are for the
respective data combined.
Future Studies
1400
1995-2006
700
0
1984-1994
Jan
Feb
Mar
Apr
May
June
July
Aug
Sept
Oct
Nov
Dec
Figure 3. Seasonal cycle of observed production values as cruise means
from for both decades.
Figure 4. Spatial patterns of chlorophyll and primary productivity on
CalCOFI Cruise 0604NH. The estimated integral primary productivity
map (middle) is derived from the regression from 1995-2006. The
observed integral primary Productivity map (bottom) is based on the 16
observed values.
Investigations should be made using the CalCOFI database to integrate nutrient levels to 200 meters in search of a correlation with the slackening of the nutricline proposed by McGowen et al. The interest and importance of the productivity of the region is leading to more powerful
tools such as the use of oxygen isotopes to give more time integrated values and thus a better estimate of the productivity of the region. Please see D. Munro poster this session. Additionally, the link between productivity and carbon flux makes changes in oxygen at depth another
area to be investigated. The decrease in productivity from 1994-2006 when compared to 1984-1994 results warrant additional work to compare our productivity to integral chl-a independent of incident light.
References
Mantyla, A.W., Venrick, E.L., and Hayward, T.L. Primary Production and Chlorophyll Relationships, Derived from Ten Years of CalCOFI Measurements. CalCOFI Rep., Vol 36, 1995.
McGowan, J.A., Bograd, S.J., Lynn, R.J., and Miller, A.J. The Biological Response to the 1977 Regime Shift in the California Current. Deep-Sea Research II 50 (2003) 2567-2582.
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