Monsoon-driven biogeochemical dynamics in an equatorial shelf sea:
time-series observations in the Singapore Strait
Abstract
Coastal tropical waters are experiencing rapid increases in
anthropogenic pressures, yet coastal biogeochemical dynamics in the
tropics are poorly studied. We present a multi-year biogeochemical time
series from the Singapore Strait in Southeast Asia’s Sunda Shelf Sea.
Despite being highly urbanised and a major shipping port, the strait
harbours numerous biologically diverse habitats, and is a valuable
system for understanding how tropical marine ecosystems respond to
anthropogenic pressures. Our results show strong seasonality driven by
the semi-annual reversal of ocean currents: dissolved inorganic nitrogen
(DIN) and phosphorus varied from ≤0.05 µmol l-1 during the intermonsoons
to ≥4 µmol l-1 and ≥0.25 µmol l-1, respectively, during the southwest
monsoon. Si(OH)4 exceeded DIN year-round. Based on nutrient
concentrations, their relationships to salinity and coloured dissolved
organic matter, and the isotopic composition of NOx-, we infer that
terrestrial input from peatlands is the main nutrient source. This input
delivered dissolved organic carbon (DOC) and nitrogen, but was notably
depleted in dissolved organic phosphorus. In contrast, particulate
organic matter showed little seasonality, and the δ13C of particulate
organic carbon (-21.0 ± 1.5‰) is consistent with a primarily
autochthonous origin. Diel changes in dissolved O2 varied seasonally
with a pattern that suggests that light availability controls primary
productivity more than nutrient concentrations. However, diel changes in
pH were greater during the southwest monsoon, when remineralisation of
terrestrial DOC lowers the seawater buffer capacity. We conclude that
terrestrial input results in mesotrophic conditions, and that the strait
might be vulnerable to further eutrophication if nutrient inputs
increase during seasons when light availability is high. Moreover, the
seasonality of diel pH variation suggests that coral reefs exposed to
terrestrial organic matter in the Sunda Shelf may be at significant risk
from future ocean acidification.