Biological Oceanography

(^)
(^) Some data for choosing appropriate parameters and initial conditions for the model
were derived from year-around sample series collected in the 1970s from weather
ships occupying Ocean Station “P” (50°N, 145°W) (Figs. 4.8a to 4.8d). Those
included temperature, daily irradiance, and mixed-layer depth data. Some starting
values were also derived from weather-ship data, chlorophyll and nitrate values in
particular. Some parameters were taken from the data (Fig. 4.8e) of an observational
program in the 1980s (Miller 1993). Those include the P vs. E relation (a very steep
initial slope, α, of P vs. E) and an effect by which the concentration of ammonium,
favored for uptake, reduces nitrate uptake (Wheeler & Kokkinakis 1990).
(^) The original model was written in Fortran (six to eight pages), with outputs as time-
series (Figs. 4.9a to 4.9d) of chlorophyll, herbivore stock, nitrate, ammonium, detrital
nitrogen, and sundry other variables. The calculation not only reproduces a seasonally
flat chlorophyll time-series at the right level and with the right annual total of primary
production, it also gets the low-level, short-term, inverse oscillations of chlorophyll
and ammonium about right in terms of amplitude and period. The Frost (1993) model
remains a good representation of the basic production processes in an HNLC system,
although Strom et al. (2000) pointed out some problems. In particular, the model
depends upon a grazing threshold for microzooplankton to make HNLC conditions
persist. Unfortunately, a threshold is consistently not found in experimental work.
However, Leising et al. (2003) have shown that the effect of thresholds can be
mimicked by increasing the half-saturation constant for grazing (the variable “f” in the
ΔH/Δt equation above). It remains to be seen whether that is the case for
microherbivores in the subarctic Pacific. That is the value of models; they suggest
logically feasible hypotheses for testing at sea.
Fig. 4.9 Output from the subarctic Pacific model. (a) Integrated chlorophyll in the
upper 50 m compared to field data; (b) phytoplankton production rate; (c)
Microherbivore standing stock, H, in the mixed layer. The cycle of primary
productivity appears in this second trophic level, not in the phytoplankton; (d) Mixed
layer detritus, D.
(^) (After Frost 1993.)