Monsoon-driven biogeochemical dynamics in an equatorial shelf sea: time-series observations in the Singapore Strait

Abstract

M. (2022). Monsoon-driven biogeochemical dynamics in an equatorial shelf sea: time-series observations in the Singapore Strait.

The Validity of Optical Properties as Tracers of Terrigenous Dissolved Organic Carbon during Extensive Remineralization in Coastal Waters

“…We have shown previously that there is a seasonal change in biogeochemistry in the Singapore Strait driven by the monsoonal current reversal (Martin et al, 2022;Martin et al, 2021;Zhou et al, 2021). The present study extends the timeseries data by one more year (2020-2021) and examines additional optical parameters (S320-412, FI, BIX, HIX, and C2 from PARAFAC analysis)…”

“…In contrast to these optical measures associated with tDOM, neither S320−412 nor BIX were related to tDOC%, and indeed showed little variation throughout our timeseries. The Singapore Strait does not appear to experience clear seasonality in phytoplankton production (Martin et al, 2022), and the production and microbial processing of autochthonous DOM are likely closely coupled yearround, with a relatively refractory marine DOM pool (Zhou et al, 2021). While our data thus cannot evaluate how well these two optical properties can trace the presence of freshly produced autochthonous DOM, our results do show that in the absence of fresh autochthonous DOM inputs they show fairly stable values even as the DOM pool varies between terrestrial and marine DOM sources.…”

“…Because we lack optical data from the peatland-draining rivers on Sumatra that form the riverine endmember for our study site (Martin et al, 2022;Zhou et al, 2021), we complement our analysis using two other data sources. For CDOM absorption (as a440), we calculated the dischargeweighted average for the main peatland-draining rivers based on Wit et al (2018) and Siegel et al…”

Pathways and consequences of dissolved organic matter transformation in tropical coastal waters

“…During the SW monsoon there is acidification of the waters reaching Singapore, due to the remineralization of terrestrial dissolved organic carbon from fluvial inputs from the Sumatran peatlands and the subsequent remineralization of terrestrial organic carbon (Mayer & Pohlmann, 2014;Siegel et al, 2019;Zhou et al, 2021). Generally, the SW monsoon has lower salinity (≤ 30 SW, ~33 NE), higher DOC (75 -109 µmol L -1 SW, ~65 µmol L -1 NE), lower pH (7.85 -7.9 SW, 7.95 -8.03 NE), higher pCO2 (550 -700 µatm SW, < 550 µatm NE), higher Nitrate + Nitrite (0.613 -5.6 µmol L -1 SW, <0.05 -3.21 µmol L -1 NE) and higher DIP (0.089 -0.342 µmol L -1 SW, <0.016 -0.210 µmol L -1 NE) than the NE monsoon (Zhou et al, 2021;Martin P., Moynihan, M. A., Chen, S., et al, 2021). concomitantly with a Valeport FastCTD profiler.…”

“…Nutrient addition treatments were amended with 5 µmol L -1 nitrate (KNO3, Sigma-Aldrich) and 0.3 µmol L -1 phosphate (KH2PO4, Fisher Scientific), at concentrations similar to natural variability in nitrate (5 µmol L -1 ; Martin P., Moynihan, M. A., Chen, S., et al, 2021) and with phosphate addition equivalent to the Redfield ratio of 16N:1P to avoid inducing N or P limitation through nutrient additions. Silicic acid (DSi) was not added because data from on-going time-series measurements indicate that DSi in the Singapore Strait is always at levels considered replete (mean = 5.4 ± 2.6 µmol L -1 from October 2017 to June 2019, unpublished data).…”