More Sophisticated Models, Subarctic Pacific
Ecosystem Dynamics
(^) Several models, still NPZ models, but all with somewhat more sophistication, have
been developed to examine the so-called “Subarctic Pacific Problem” (Frost 1993;
Fasham 1995; Denman & Peña 1999; Denman et al. 2006). The oceanic Gulf of
Alaska, in particular, has strong seasonal cycles of phytoplankton growth rates that,
unlike those in the oceanic North Atlantic, do not result in cycling of phytoplankton
stock, particularly as measured by chlorophyll concentration. The Frost paper,
summarized here, has the advantage (much to be commended in modeling reports) of
providing the mechanics of the model in sufficient detail that a person with moderate
skill can program it from the equations provided. Some of the other models highlight
additional features of the system.
(^) The goal is to model the essential relations among phytoplankton, grazers and
nutrients that characterize the subarctic Pacific (discussed in more detail in Chapter
11). In the oceanic sectors of the Gulf of Alaska and west almost to Japan, there are
no phytoplankton blooms. Instead, there is a low amplitude oscillation of
phytoplankton stocks keeping chlorophyll-a between about 0.15 and 0.65 μg liter−1
(Fig. 11.5) with occasional run-ups to almost 1.0 μg liter−1, but very rarely more than
that. Levels are somewhat higher to the west, seaward of Sakhalin and Hokkaido. Just
as important, nitrate in the surface mixed layer remains greater than 6 μM right
through the year; it never drops to a value low enough to limit phytoplankton growth.
There is an annual cycle in surface nitrate (Fig. 4.8), between 17 μM in March and
about 7 μM in July, but it is not depleted. Pelagic ecosystems with both of these
features are referred to as high-nitrate, low-chlorophyll (HNLC) systems. As
discussed in Chapters 3 and 11, Martin et al. (1989) suggested that the key factor
keeping phytoplankton stocks low is probably limitation of the growth rate of larger
phytoplankton by the very low availability of iron. Their iron-enrichment experiments
showed large phytoplankton eventually blooming in containers with added iron, but
not in those without it. The phytoplankton that are present are growing rapidly, one
doubling per day or more, but they are small. The effect of this (Miller et al. 1991a &
b) is that phytoplankton are then susceptible to grazing by protozoans. Protozoans, in
turn, can grow about as fast as phytoplankton, which enables them to hold the
phytoplankton stocks within narrow limits.
Fig. 4.8 Input data for the Frost subarctic Pacific model. (a) Daily irradiance from
weather-ship radiometer data. (b) Mixed-layer depth from daily CTD casts. (c)
Mixed-layer temperature. (d) Annual nitrate cycle based on data for many years, and