Primary and bacterial productivity of tropical seagrass communities in the Gulf of Carpentaria, Australia

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Abstract
Plant and bacterial productivities were compared at different seasons in 3 communiites of seagrass in the Gulf of Carpentaria, Australia, in 1985. Seagrass biomass and shoot density were very variable, with no seasonal trend. The gross primary productivity of the seagrasses in each of the communiites (Syringodium isoetifolium with Cymodocea serrulata in a bay, Halodule uninervis at a river mouth, and Thalassia hemprichii with Cymodocea rotundata on a reef flat) were determined seasonally from rates of lacunal gas production. Productivity varied seasonally, being lowest in winter (July). Values ranged from 0.6 to 1.0 g C m-2 d-1 for C. serrulata, 0.3 to 8.1 for S. isoetifolium, 0.9 to 3.5 for H. uninervis, 0.2 to 0.4 for T. hemprichii and 0.1 to 1.0 for C. rotundata. Gross community primary productivity, measured from diurnal changes in oxygen concentration in the water column, ranged from 3.3 in winter to 9.3 g C m-2 d-1 in summer at the bay site, 2.7 to 4 at the river mouth and 3.3 to 8.4 on the reef flat. Bacterial productivity was determined using the rate of tritiated thymidine incorporation into DNA. Most (90 to 95%) bacterial biomass was produced in the sediment. Between winter and summer, total bacterial productivity (including water column) ranged from 1.0 to 4.8 g C m-2 d-1 in the bay, 0.6 to 2.5 at the river mouth and 0.6 to 3.7 on the reef flat. Bacterial productivity averaged 43% (range 10 to 90%) of gross primary productivity, and thus would account for about half of the primary production if their growth efficiency were 50%. Animals appeared to have an impact on bacteria at the sediment surface in summer, when specific growth rates and productivity were high, but numbers were low.