Physical mechanisms of nutrient supply and controlling factors of phytoplankton growth in the Arafura Sea

Abstract

The Arafura Sea is a relatively shallow basin (< 200 m deep) located in the eastern part of the Indonesian archipelago. Its border on the west is a deeper basin, the Banda Sea, with an average depth of ∼5000 m. Previous studies have shown that circulation in this particular region is mainly influenced by monsoonal winds, which are the northwest (NW) and southeast (SE) monsoons. In addition, previous studies have shown that upwelling is a characteristic phenomenon in this region, induced by the SE monsoon from June to August. During this period, the lower sea surface temperature and elevated chlorophyll-a concentrations are observed near the coast of Papua (northern Arafura Sea). Recent studies have suggested that these features are due to the upwelling of cold and nutrient-rich water masses originating from deeper layers of the Banda Sea.

The present study aims to investigate the mechanism of nutrient supply and its implication on phytoplankton distribution in further detail using a numerical model. A three-dimensional (3D) biogeochemical model ECOHAM (ECOsystem model HAMburg) is utilized in this study. The model domain extends from 122°–139°20’ E and 1°48’–14°19’ S, covering the Arafura and the Banda Sea regions. ECOHAM is forced by ocean current fields derived from HAMSOM (HAMburg Shelf Ocean Model), river run-off, atmospheric nitrogen deposition, wind stress, and solar radiation. Moreover, the initial and boundary values of biogeochemical variables are derived from WOA and GCOMS. Finally, the model results are validated against the in-situ nutrient measurements and satellite-derived chlorophyll-a concentration.

The simulated nutrient (i.e., nitrate and phosphate) concentrations show a good fit with observations, especially in the upper 200 m. Besides nutrients, the simulation overestimates surface chlorophyll-a concentrations in the northern Arafura Sea, but it still represents the seasonal variation quite well. Furthermore, the sensitivity test reveals that a 10% change in temperature factor Q10 for phytoplankton can significantly changes the Redfield net primary production by up to 25%.

This modeling study suggests a different mechanism of nutrient supply between the shallow region in the northern part (Sahul Shelf) and the continental slope area of the Arafura Sea. In the Sahul Shelf, nitrate is primarily transported to the near-surface layer via vertical mixing, which is stronger during the SE monsoon, compared to the NW monsoon. On the other hand, nitrate supply in the continental slope area is mainly regulated by advection. During the NW monsoon, the simulation reveals the horizontal intrusion of nitrate-rich water masses from the eastern Banda Sea in the layer above the nitracline (a layer in which the nitrate concentration increases rapidly with increasing depth). By contrast, during the SE monsoon, the vertical advection transports nitrate to the layer above the nitracline, which is confirmed by nitrate budget analysis.

Furthermore, this study shows that phytoplankton growth is mainly regulated by nitrogen availability. In the Sahul Shelf, the seasonal variations of phytoplankton production and zooplankton grazing indicate the bottom-up control in June-August and top-down control in October-December in the zooplankton-phytoplankton system. In the continental slope area, nitrate concentration in the near-surface layer is depleted, suggesting a strong nitrate limitation, especially for diatoms. In this region, non-diatom production is higher than for diatom because non-diatoms take up ammonium more effectively.