360
Chapter 11: The Pacific Ocean
for altered nutrient flux pathways and for the trophic structure of the pelagic ecosystem,
even up to higher levels. This has been characterized as involving selection for N 2 -fixing
cyanobacteria, including Trichodesmium, and a shift from large cells to Prochlorococcus and other very small autotrophs. Increased bacterial heterotrophic activity is also
indicated. However, one should not infer from this discussion that the domain shift
totally eliminated diatoms from the phytoplankton. As Scharek et al. (1999) showed,
there remained in 1995 a highly structured diatom community at ALOHA, with distinct
assemblages in the DCM and the mixed layer. Summer increases in numbers mainly
involved lightly silicified species, such as Hemiaulus hauckii and Masogloia woodiana.
In this context, we should note the presence in this province of autotrophic cells
such as the diatom Hemiaulus hauckii and also Rhizoselenia cyclindricus and R. hebetata
that have nitrogen-fixing symbionts, principally the cyanophyte Richelia intracellularis.
Venrick (1974) surveyed the occurrence and potential productivity of this symbiosis at
the CLIMAX site and found it to be most significant in summer; Venrick computed
a potential carbon fixation rate (37–77 mg C m
−2 day-) that was about 30–60% of the
difference between total winter nonbloom and total summer bloom production at this
station. Villareal et al. (1993) have investigated diatom “mats” composed of several species
of Rhizoselenia that are very abundant at CLIMAX, containing >90% of all biogenic silica
here; the mats appear to have the ability to modify their density from negative to positive
buoyancy, and shuttle between the upper euphotic zone and the nitrate-replete deeper
zone. It is suggested that these mats may be an important vector of new nitrogen inputs
to the euphotic zone, representing as much as 50% of the required flux.
Profiles of microplankton biomass (mostly protists, monads, flagellates, and naked
dinoflagellates) do not have a subsurface maximum corresponding to the DCM but rather
a broad depth range of relatively high abundance (0–100 m 5–10 mg C liter
−1 ) above a
deeper zone of lower abundance (100–200 m 1–5 mg C liter
−1 ). Separation of vertical
habitat occurs also among mesozooplankton (Ambler and Miller, 1987), some species
of which specialize in the DCM (presumably those that consume larger cells), whereas
others preferentially occupy the upper mixed layer where primary production rates are
maximal. In the western part of the province at 28
N 136
E, Tsuda et al. (1989) have
observed that grazing by microplankton in the DCM (70–120 m) contributes 60–100% of
total consumption and that this is balanced by production within a time scale of several
days. It was reported that mesozooplankton contribute only < 5% of all consumption;
at shallower locations than the DCM, these organisms are largely secondary predators,
consuming microplankton, though at the DCM they contributed to the consumption of
algal cells. This statement must represent a great simplification of the taxon-by-taxon food
selection of mesozooplankters concerning which there is a very large body of knowledge;
it would be foolhardy to attempt to review this for each province because the common
threads that runs through these studies concern (i) the diversity of the ranges of diet
characteristic even of congeneric species and (ii) the flexibility of the response of each
growth stage of each species to the ambient food environment in which it finds itself.
A strong seasonal signal in the abundance of all size fractions (0.2–20 mm) of the
mesozooplankton can be detected despite the extreme internal incoherence of the data,
with a seasonal doubling of both day and night abundance and biomass during summer
months; interannual variability also occurs and a long-term doubling of biomass was
observed in the CLIMAX-ALOHA time series (Landry et al., 2001). This increase involves
such forms as the harpacticoid Macrosetella gracilis, associated with mats of N 2 -fixing
Trichodesmium that attain maximal biomass in boreal summer.
The major consumers of the very small, dominant phytoplankton cells—at least after
the domain shift—remain protists, with which the mesozooplankton cannot compete.
Despite this, small copepods remain abundant and represent 80% of all zooplankton
biomass by day and 77% at night (Landry et al., 2001). Chaetognaths and larger crustacea
(euphausiids, decapods, amphipods) dominate the larger size fractions. I refer you to the
Chapter 11: The Pacific Ocean
for altered nutrient flux pathways and for the trophic structure of the pelagic ecosystem,
even up to higher levels. This has been characterized as involving selection for N 2 -fixing
cyanobacteria, including Trichodesmium, and a shift from large cells to Prochlorococcus and other very small autotrophs. Increased bacterial heterotrophic activity is also
indicated. However, one should not infer from this discussion that the domain shift
totally eliminated diatoms from the phytoplankton. As Scharek et al. (1999) showed,
there remained in 1995 a highly structured diatom community at ALOHA, with distinct
assemblages in the DCM and the mixed layer. Summer increases in numbers mainly
involved lightly silicified species, such as Hemiaulus hauckii and Masogloia woodiana.
In this context, we should note the presence in this province of autotrophic cells
such as the diatom Hemiaulus hauckii and also Rhizoselenia cyclindricus and R. hebetata
that have nitrogen-fixing symbionts, principally the cyanophyte Richelia intracellularis.
Venrick (1974) surveyed the occurrence and potential productivity of this symbiosis at
the CLIMAX site and found it to be most significant in summer; Venrick computed
a potential carbon fixation rate (37–77 mg C m
−2 day-) that was about 30–60% of the
difference between total winter nonbloom and total summer bloom production at this
station. Villareal et al. (1993) have investigated diatom “mats” composed of several species
of Rhizoselenia that are very abundant at CLIMAX, containing >90% of all biogenic silica
here; the mats appear to have the ability to modify their density from negative to positive
buoyancy, and shuttle between the upper euphotic zone and the nitrate-replete deeper
zone. It is suggested that these mats may be an important vector of new nitrogen inputs
to the euphotic zone, representing as much as 50% of the required flux.
Profiles of microplankton biomass (mostly protists, monads, flagellates, and naked
dinoflagellates) do not have a subsurface maximum corresponding to the DCM but rather
a broad depth range of relatively high abundance (0–100 m 5–10 mg C liter
−1 ) above a
deeper zone of lower abundance (100–200 m 1–5 mg C liter
−1 ). Separation of vertical
habitat occurs also among mesozooplankton (Ambler and Miller, 1987), some species
of which specialize in the DCM (presumably those that consume larger cells), whereas
others preferentially occupy the upper mixed layer where primary production rates are
maximal. In the western part of the province at 28
N 136
E, Tsuda et al. (1989) have
observed that grazing by microplankton in the DCM (70–120 m) contributes 60–100% of
total consumption and that this is balanced by production within a time scale of several
days. It was reported that mesozooplankton contribute only < 5% of all consumption;
at shallower locations than the DCM, these organisms are largely secondary predators,
consuming microplankton, though at the DCM they contributed to the consumption of
algal cells. This statement must represent a great simplification of the taxon-by-taxon food
selection of mesozooplankters concerning which there is a very large body of knowledge;
it would be foolhardy to attempt to review this for each province because the common
threads that runs through these studies concern (i) the diversity of the ranges of diet
characteristic even of congeneric species and (ii) the flexibility of the response of each
growth stage of each species to the ambient food environment in which it finds itself.
A strong seasonal signal in the abundance of all size fractions (0.2–20 mm) of the
mesozooplankton can be detected despite the extreme internal incoherence of the data,
with a seasonal doubling of both day and night abundance and biomass during summer
months; interannual variability also occurs and a long-term doubling of biomass was
observed in the CLIMAX-ALOHA time series (Landry et al., 2001). This increase involves
such forms as the harpacticoid Macrosetella gracilis, associated with mats of N 2 -fixing
Trichodesmium that attain maximal biomass in boreal summer.
The major consumers of the very small, dominant phytoplankton cells—at least after
the domain shift—remain protists, with which the mesozooplankton cannot compete.
Despite this, small copepods remain abundant and represent 80% of all zooplankton
biomass by day and 77% at night (Landry et al., 2001). Chaetognaths and larger crustacea
(euphausiids, decapods, amphipods) dominate the larger size fractions. I refer you to the