revealed and can be at any stage when the experiments are performed. The labor
involved in just one measurement is substantial, so time-series at near-daily frequency
will require some further innovation. Another issue is that the quantity measured in
many experiments is change in chlorophyll concentration, which is relatively simple
but subject to changes not related to growth.
Some Comprehensive Regional Comparisons of
Phytoplankton Growth with Grazing
(^) Many attempts have been made to partition comprehensively both phytoplankton
production and the grazing of it according to the sizes of the responsible agents.
Studies on a quarterly basis in coastal Mediterranean waters by Fonda Umani and
colleagues (discussed in the next section) are an example. Landry et al. (2011)
assigned rates to size and taxonomic categories sampled and tested in the HNLC
eastern equatorial Pacific during cruises in December 2004 and September 2005. The
32-station series both crossed the equator at several longitudes (110° and 140°W) and
ran east–west at 0° and 0.5°N. Simplified dilution experiments (just full seawater and
64% seawater) were conducted at all stations with samples from eight light levels
(100% to 0.1% of E 0 ) down the water column. After 24-hour incubation at ocean
temperature in irradiance simulated for the depths of collection, the change in
chlorophyll content was evaluated for all bottles. Regressions of apparent growth vs.
dilution were very good, and the intercepts (growth rate, μ d−1, for phytoplankton) and
slopes (m d−1, mortality from grazers; essentially all being microzooplankton and
mostly protists, given the sample volumes) were compared (Table 9.5). Almost all
contributions to the vertical integrals of growth rate, μ, came between 100% and 1%
of E 0 , which agrees with a very old approximation of the lower boundary of the
euphotic zone. There was some photoinhibition (25%) above 50% of E 0 . Mortality
(grazing by heterotrophic protists) averaged 72% of increase, a fraction typical for
oligotrophic habitats in all oceans.
Table 9.5 Vertical integrals from eight depth profiles of two-point dilution experiment
results: phytoplankton growth rate, μ, and microzooplankton grazing rate, m, for 32
eastern tropical Pacific stations. Bold values of >100% are likely to represent
declining stocks; grazing can exceed growth.
(After Landry et al. 2011.)