AME 25:293-300 (2001)  -  doi:10.3354/ame025293

ABSTRACT: Abundances of planktonic thraustochytrids and bacterioplankton were investigated in the Seto Inland Sea, Japan, with reference to concentrations of potential growth substrates such as dissolved organic carbon (DOC) and particulate organic carbon (POC) concentration and of phytoplankton (chlorophyll a concentration). Thraustochytrid and bacterioplankton cells were stained with acriflavine and acridine orange, respectively, and directly counted by epifluorescence microscopy. Thraustochytrids occurred in the water column at a density of 0.9 x 10² to 2.1 x 10⁴ cells l^-1, with an overall average (±SD) of 3.1 ± 4.0 x 10³ cells l^-1. Bacterioplankton were found in the water column at a density of 1.1 x 10⁹ to 6.6 x 10¹⁰ cells l^-1, with an overall average of 4.8 ± 7.0 x 10⁹ cell l^-1. Thraustochytrid abundance was significantly correlated primarily to POC concentration (r = 0.547, p < 0.001) and secondarily to chlorophyll a concentration (r = 0.318, p < 0.001). By contrast, bacterioplankton concentration was significantly correlated primarily to chlorophyll a concentration (r = 0.498, p < 0.001) and secondarily to DOC concentration (r = 0.236, p = 0.002) but negatively to POC concentration (r = 0.143, p = 0.068). Phytoplankton-derived POC accounted for a certain fraction (only ambiguously estimated at 20 to 60%) of total POC, and about 20% of POC was ascribable to bacterioplankton biomass. The remaining POC was assumed to be non-phytoplanktonic, non-bacterioplanktonic sources such as phytodetritus and terrigenous matter. Therefore, it is suggested that bacterioplankton abundance may depend primarily on phytoplankton-derived DOC whereas thraustochytrid abundance is more influenced by non-phytoplankton, non-bacterioplankton POC. That is, planktonic degraders, thraustochytrids and bacterioplankton, decompose substrates of different origins and sizes. The potential ecological importance of the thaustochytrids as active degraders of POC and as an alternative food source is discussed.

KEY WORDS: Thraustochytrid · Fungoid protist · Bacterioplankton · Abundance · Particulate organic carbon · Dissolved organic carbon · Chlorophyll a