Physical disturbance creates bacterial dominance of benthic biological communities in tropical deltaic environments of the Gulf of Papua

Unlike many reactive continental shelf mud deposits in temperate regions, bacteria and microfauna rather than macrofauna typically dominate benthic biomass and activities over large areas of the Gulf of Papua (GoP) deltaic complex, Papua New Guinea. During mid NW monsoon periods (Jan–Feb), macrofaunal densities at Gulf stations were relatively low ( 260 - 1270 m - 2 ), large macroinfauna were absent (upper ∼ 25 cm), and small ( < 0.5 mm ), surface deposit-feeding polychaetes and tubiculous amphipods were dominant, reflecting a frequently destabilized seabed and high sedimentation/erosion rates. Although frequent physical disturbance generally inhibits development of macrobenthic communities, some regions of the Gulf deposits are periodically colonized and extensively bioturbated during quiescent periods, as shown by preserved biogenic sedimentary structures. Bacterial inventories integrated over the top 20 cm were extremely variable within each sub region of the clinoform complex. A possible bimodal pattern with bathymetric depth and distance offshore may occur: lowest-inventories within the sandy, proximal Fly River delta, an open Gulf inner topset zone ( ∼ 10–20 m) having sites of relatively high inventories, an open Gulf mid-topset region with intermediate values and less extreme variation, and the outer topset—upper foreset zone ( ∼ 40–50 m) where highest values are attained ( 7.6 × 10 11 cells cm - 2 ). Various measures of microbial activity, including measures proportional to the cellular rRNA content and the proportion of dividing cells, indicate extremely productive populations over the upper 1-m of the seabed throughout the Gulf of Papua region. Bacterial biomass (0–20 cm) including data of Alongi et al. (1991, 1992, 1995) varied from a low of 1.6 ± 0.3 gC m - 2 in intertidal mud banks to a high of 190 ± 38 gC m - 2 in the topset—foreset zone. Macrofaunal biomass did not exceed 2 gC m - 2 in any sampled region, ranging from 0.009 ± 0 to 1.1 ± 0.08 gC m - 2 with no obvious correlation with bathymetric depth (1–63 m). Meiofaunal biomass was generally an order of magnitude lower than macrofaunal biomass. Relatively elevated bacterial biomass and high turnover rates are consistent with high measured rates of benthic remineralization, presumably reflecting the rapid response time of bacteria to physical reworking, the associated entrainment of organic substrate, and flushing of metabolites. Solute exchange is also enhanced below the directly mixed surface region, possibly producing ‘far field’ stimulation of microbes in underlying deposits. Physical reworking and reoxidation of sediments between ∼ 10 and 50 m water depth maintain suboxic, nonsulfidic conditions in the upper ∼ 0.5–1 m despite active microbial communities and high benthic remineralization rates.