and deepen the euphotic zone (but without swirling phytoplankton down to
darker layers), allowing an increase in excess of daily grazing.
5 Oscillations could occur as a manifestation of a trophic cascade (Dagg et
al. 2009). An increase or decrease in mesozooplankton (copepods,
euphausiids, salps) would decrease or increase protozoans, releasing or
cropping down phytoplankton, initiating a phytoplankton (and fluorescence)
oscillation that would progressively dampen, much as in the data shown
(Fig. 11.5). That just pushes the uncertainty along the causal chain to a new
uncertainty, but there are many mechanisms to initiate stock increases and
decreases among the predators of microherbivores.(^) Of course, more than one, or even all, of these mechanisms may operate in different
intervals, but grazers almost always catch up to phytoplankton stock increases within
a few weeks, limiting stock measured as chlorophyll well below 1.0 μg liter−1. Strom
et al. (2000) pointed out the problem that there is a relatively high lower limit to these
oscillations. In more oligotrophic subtropical waters, where major nutrients as well as
required trace metals are driven to near-vanishing levels, chlorophyll stays below 0.1
μg liter−1 for very long intervals. Why do subarctic Pacific levels stay above that
seemingly false bottom? Possibly the complexity of the microbial food-web includes
a self-limiting activity: e.g. larger microheterotrophs eating more of the smaller ones
as the pico- and nanophytoplankton are reduced. We do not know in any fundamental
way how microherbivore grazing is regulated.
(^) The reality and importance of iron limitation in the subarctic Pacific and other
HNLC areas have been conclusively demonstrated by enrichment with iron of
modestly large, typically 10 × 10 km, patches of open ocean. Martin and Fitzwater’s
(1988) initial demonstration of iron limitation of larger phytoplankton types was
based on adding iron in nanomolar amounts to incubation containers maintained on a
ship’s deck. This left open the possibility that something about the containment
interfered with alga–grazer or alga–nutrient interactions. Field iron-enrichment
studies in the subarctic Pacific were carried out primarily by Canadian (“SERIES”,
Harrison et al. 2006) and Japanese (“SEEDS I & II”, e.g. Tsuda 2005)
oceanographers, morally supported by the North Pacific Marine Sciences
Organization (“PICES”). SERIES involved stirring acidic ferric chloride solution (in
seawater with some SF 6 as a patch tracer) with a ship’s wake into an 8.5 × 8.5 km
patch of ocean centered on a drifter buoy starting at 50.14°N, 144.75°W. A second
iron addition was made 7 days after the first, covering the then N–S elongate patch
shape. By day 26 the patch was ∼35 × 10 km with some side lobes. Iron was raised at
least initially after each addition to ∼2.4 nM, and the response of phytoplankton was
dramatic, if somewhat slow and prolonged. Chlorophyll rose to ∼5 μg liter−1 in the
patch center by day 18, with dominance of relatively large, pennate diatoms
(Marchetti et al. 2006). Over the same interval, silicate was drawn down from 15 to 1