[Eoas-seminar] Biogeochemistry Seminar Nov. 11, 2019, Rm. 327 OSB 12:15-13:15

[eoas-seminar at lists.fsu.edu]

Biogeochemistry Seminar Nov. 11, 2019
Rm. 327 OSB 12:15-13:15

Speaker: Zhou Liang 
Title: Dissolved Organic Phosphorus (DOP) Distributions in the Eastern Indian Ocean and Subtropical South Pacific Ocean

Abstract:
Significant rates of export production and nitrogen fixation occur in oligotrophic gyres in spite of low inorganic nutrient concentrations in surface waters. Prior work suggests that dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are important nutrient sources when inorganic nutrients are scarce. In particular, DOP has been shown to be an important P source for diazotrophs who may be better suited to using low concentrations of organic vs. inorganic P. Prior modeling work has also suggested that DOP dominating the P pool in the euphotic zone is important for supporting export production in oligotrophic gyres. However, validation of such models is limited by the number of upper ocean DOP concentration measurements, especially in the South Pacific and Indian Oceans. Here, we present measurements of DOP concentration from the 2016 GO-SHIP I08S and I09N meridional transect in Eastern Indian Ocean, and DON and DOP concentration measurements from the 2017 GO-SHIP P06 zonal transect in the South Pacific Subtropical Gyre (SPSG). Together with DOC and DON concentration measurements from prior occupations of the same GO-SHIP lines we evaluated changes of DOC: DON: DOP concentration ratio stoichiometry in euphotic zone across these two transects to infer regions of preferential DON and/or DOP production and consumption. Specifically, DOC and DOP accumulate in the eastern part of SPSG, but a significant increase in DOC to DOP concentration ratio in euphotic zone from 225: 1 to 493: 1 from east to the core of gyre on west is observed.  In the Eastern Indian Ocean, DOP concentration in euphotic zone is high in equatorial region and Southern Ocean but low in Bay of Bengal and subtropical gyre. DOC to DOP concentration ratio in the euphotic zone increases from 159: 1 to 192:1 from equatorial region to the gyre and then decreases to 120: 1 in the Southern Ocean. Also, a similar trend for DOC to DON concentration ratio in the euphotic zone along the Eastern Indian Ocean meridional transect is observed. These stoichiometric shifts in upper ocean DOC: DON: DOP ratios are considered in the context of ocean circulation and upwelling and downwelling patterns in the Indian Ocean and SPSG, phosphate stress, as well as prior observations of the distribution of nitrogen fixation, especially in the western tropical South Pacific.