TY - JOUR

T1 - Microplankton biomass and diversity in the Vietnamese upwelling area during SW monsoon under normal conditions and after an ENSO event

AU - Loick-Wilde, Natalie

AU - Bombar, Deniz

AU - Doan, Hai Nhu

AU - Nguyen, Lam Ngoc

AU - Nguyen-Thi, Anh Mai

AU - Voss, Maren

AU - Dippner, Joachim W.

PY - 2017/4

Y1 - 2017/4

N2 - Investigating microplankton biomass and diversity under different climatological conditions is key to the understanding of cascading effects of climate change on nutrient cycles and biological productivity. Here we have used data collected during two contrasting summers along the coast of Viet Nam to show how climatological-driven changes can have a significant influence on the distribution of microplankton communities and their biomass via its impact on nutrient concentrations in the water column. The first summer in July 2003 followed a weak El-Nino Southern Oscillation (ENSO) event and was characterized by weak coastal upwelling, in the second summer during July 2004, upwelling was normal. Very low silicate (SiO4) concentrations and SiO4:DIN ratios characterized the source water mass for upwelling in July 2004, and dynamic SiO4 to dissolved inorganic nitrogen ratios (SiO4:DIN) mainly below the Redfield-Brzezinski ratio and DIN to phosphate ratios (DIN:PO4 3−) below the Redfield ratio were a common feature off Viet Nam. Much higher particle concentrations and PSi/PC ratios during normal upwelling revealed major changes in the microplankton community structure among summers. Small dinoflagellates (10–20 µm) prevailed ubiquitously during reduced upwelling. During normal upwelling, the diatom Rhizosolenia sp. dominated the cell-carbon biomass in the silicate poor upwelling waters. Trichodesmium erythraeum dominated in the Mekong-influenced and nutrient depleted offshore waters, where it co-occurred with Rhizosolenia sp. Both species were directly associated with the much higher primary production (PP) and N2 fixation rates that were quantified in earlier studies, as well as with much higher diversities at these offshore sites. Along the coast, the correlation between Rhizosolenia sp. and PP rates was less clear and the factors regulating the biomass of Rhizosolenia sp. in the upwelling waters are discussed. The very low silicate concentrations in the source water mass for upwelling and the offshore deflection of the Mekong river plume likely triggered the observed ecological differences in the microplankton communities off Viet Nam in the normal upwelling season compared to the post ENSO conditions.

AB - Investigating microplankton biomass and diversity under different climatological conditions is key to the understanding of cascading effects of climate change on nutrient cycles and biological productivity. Here we have used data collected during two contrasting summers along the coast of Viet Nam to show how climatological-driven changes can have a significant influence on the distribution of microplankton communities and their biomass via its impact on nutrient concentrations in the water column. The first summer in July 2003 followed a weak El-Nino Southern Oscillation (ENSO) event and was characterized by weak coastal upwelling, in the second summer during July 2004, upwelling was normal. Very low silicate (SiO4) concentrations and SiO4:DIN ratios characterized the source water mass for upwelling in July 2004, and dynamic SiO4 to dissolved inorganic nitrogen ratios (SiO4:DIN) mainly below the Redfield-Brzezinski ratio and DIN to phosphate ratios (DIN:PO4 3−) below the Redfield ratio were a common feature off Viet Nam. Much higher particle concentrations and PSi/PC ratios during normal upwelling revealed major changes in the microplankton community structure among summers. Small dinoflagellates (10–20 µm) prevailed ubiquitously during reduced upwelling. During normal upwelling, the diatom Rhizosolenia sp. dominated the cell-carbon biomass in the silicate poor upwelling waters. Trichodesmium erythraeum dominated in the Mekong-influenced and nutrient depleted offshore waters, where it co-occurred with Rhizosolenia sp. Both species were directly associated with the much higher primary production (PP) and N2 fixation rates that were quantified in earlier studies, as well as with much higher diversities at these offshore sites. Along the coast, the correlation between Rhizosolenia sp. and PP rates was less clear and the factors regulating the biomass of Rhizosolenia sp. in the upwelling waters are discussed. The very low silicate concentrations in the source water mass for upwelling and the offshore deflection of the Mekong river plume likely triggered the observed ecological differences in the microplankton communities off Viet Nam in the normal upwelling season compared to the post ENSO conditions.

KW - ENSO

KW - Microplankton diversity

KW - N fixation

KW - Primary production

KW - Rhizosolenia sp.

KW - South China Sea

KW - Trichodesmium erythraeum

KW - Viet Nam

KW - Water mass

UR - http://www.scopus.com/inward/record.url?scp=85017402784&partnerID=8YFLogxK

U2 - 10.1016/j.pocean.2017.04.007

DO - 10.1016/j.pocean.2017.04.007

M3 - Journal article

AN - SCOPUS:85017402784

VL - 153

SP - 1

EP - 15

JO - Progress in Oceanography

JF - Progress in Oceanography

SN - 0079-6611

ER -