Pelagic biological processes along a salinity gradient in the Fly delta and adjacent river plume (Papua New Guinea)

Abstract

We measured planktonic processes (primary and bacterial production, respiration) and concentrations of suspended material, nutrients, chlorophyll a, floating macroparticulate matter, zooplankton and neuston in the delta and river plume of the Fly River in Papua New Guinea in July/August 1989 and February 1990. There appeared to be conservative mixing of dissolved nutrients across the salinity gradient, but there were seasonal differences in nitrate and phosphate concentrations in freshwater entering the estuary. Phosphate concentrations (>5 μM) in freshwater entering the estuary during July/August were higher than in most large tropical rivers. Concentrations of suspended solids in the estuary and plume ranged from 82 to 3312 mg l −1 in July/August and from 2 to 681 mg l −1 in February. Suspended solids were from 1.0 to 33.6% carbon, but the average for most stations was between 2 and 12%. There was an inverse relationship between per cent carbon and the suspended solid concentrations, similar to that observed in the Changjiang estuary. The mean C:N ratio for suspended solids for most stations was from 4 to 21 (mean 7.3). Mean floating macroparticulate matter biomass in the estuary was 80 mg C m −3 (2.2 mg N m −3). Mean chlorophyll a concentrations in estuary and plume stations ranged from 0.25 to 5.07 and from 0.32 to 0.73 μg l −1, respectively. Phytoplankton blooms (chlorophyll a up to 5.07 μg l −1) occurred just inside the mouth of the delta where suspended solids were ≤70 mg l −1. In February, net primary production in the oligohaline regions of the delta ranged from 22 to 94 mg C m −2 day −1, but highest production (up to 693 mg C m −2 day −1) was recorded at the most seaward stations in the delta. In the plume, in situ production ranged from 227 to 340 mg C m −2 day −1. Community respiration (0.8–36 g C m −2 day −1) across the salinity gradient was typically 1–2 orders of magnitude higher than in situ production. Bacterial production rates ranged from 0 to 128 μg C l −1 day −1 (grand mean 18.8) indicating that bacterial respiration could account for ∼2–49% of total pelagic respiration. Zooplankton biomass ranged from < 1 to 623 mg m −3 and correlated with chlorophyll a and particulate matter. The Fly differs from other large tropical rivers in that maximum in situ primary production occurs where suspended loads are ∼70 mg l −1. This is similar to the Huanghe system, and is possible because of the relatively high concentrations of nitrate and phosphate in the waters of the Fly delta. The dominance of respiration over primary production across the river plume and the quantities of allochthonous carbon exported from the river and mangrove forests in the delta indicates that benthic standing stocks and secondary production on the inner Papuan shelf are supported mainly by detritus advected from the Fly and the other large rivers entering the Gulf of Papua.