Characterising the biological uptake of CO2 across the Subtropical Frontal Zone

Abstract

Seawaters associated with the Subtropical Frontal Zone located off the eastern coast of New Zealand are of particular interest for studies of the global marine carbon cycle. The Southland Front represents the near-coastal portion of the Subtropical Frontal Zone, which is a major circumpolar oceanic system that marks the boundary between Subtropical and Sub-Antarctic water masses. The Southland Front is subject to unique interactions between the oceanic water masses and coastal processes and is characterised by high seasonal and spatial variabilities in surface partial pressure of CO2 (pCO2). These features and the proximity of the frontal zone to land provide an ideal natural laboratory for the marine carbonate system. The Munida Time Series Transect has collected high frequency marine carbonate data across the Southland Front since 1998. Historic data indicates considerable spatial, seasonal and interannual variability in pCO2, which is attributed to strong seasonal biological drawdown, thermodynamic mixing and wind events.

Chlorophyll-a (chl-a), net primary production (NPP) rates, macronutrient, and hydrographic data were sampled on a bi-monthly basis along the 65 km Munida Time Series Transect. Peak NPP rates were observed in the near-shore neritic water and modified-subtropical waters (STW) during the austral summer season of 2009/2010, with column integrated rates of 1.5 ± 0.2 g C m-2 d-1 and 1.6 ± 0.1 g C m-2 d-1 measured in December 2009 and January 2010, respectively. Phytoplankton primary production was also amplified (0.90 ± 0.08 g C m-2 d-1) during the December 2009 sampling period in the iron limited High Nutrient, Low Chlorophyll (HNLC) waters of the Sub-Antarctic (SASW) portion of the Southland Front. Measurements of relative depletions of macronutrient concentrations across the frontal system indicate that primary production is nitrogen limited in the neritic and STW waters, with primary production in the SASW co-limited by silicate and trace metal concentrations. Peak production rates measured in the Spring and Summer 2010 period for both the STW and SASW portions of the Southland Front were significantly less than the previous year, with integrated values of 0.7 ± 0.1 g C m-2 d-1 and 0.2 ± 0.2 g C m-2 d-1, respectively. This variation in primary production was likely caused by interannual differences in the physical structure of the Southland Front, including the front location, gradient strength, inherent optical properties, and subsequent transport and availability of growth limiting nutrients.

Coincident satellite derived chl-a observations were obtained for dates relating to sampling cruises along the Munida Time Series Transect in order to evaluate direct temporal matching pairs between remotely sensed and in situ chl-a concentrations. Although the retrieval and validation of global standard chl-a products from the ocean colour MODIS and MERIS sensors were limited due to cloud cover during the course of the study period, the performances of the empirical MODIS OC3M and MERIS Algal 1 global standard chl-a algorithms were relatively consistent with other New Zealand specific studies. These satellite estimates were within the international remote sensing detection goal of 35% of in situ chl-a concentrations. The semi-analytical MERIS Algal 2 systematically overestimated chl-a relative to in situ measurements by 500 – 800%. This overestimation was likely due to a lack of appropriate in situ matching data within the conditions optimised by the algorithm. This combined shipboard and remote sensing analysis produced a high resolution temporal and spatial biological profile of the frontal system.

The comparison of this biological profile to concurrent pCO2 data provided insight into the dynamic processes dictating air-sea CO2 exchange in the Southland Front. A numerical model was developed to isolate and compare the physico-chemical and biological processes dictating pCO2 variations over the 18 month sampling period. Biological processes (including organic production, degradation, and calcification processes) and thermodynamic processes were identified as the dominant processes in controlling seasonal pCO2sw trends. Other processes assessed by the numerical modelling included air-sea gas exchange and vertical entrainment. During the period from July 2009 to December 2010, all water masses in the Southland Front system were undersaturated in respect to atmospheric pCO2 and represented a net sink in atmospheric CO2. The undersaturation of these waters was observed by the dramatic minimum in measured pCO2 (approximately –Δ60 to 75 µatm) around late spring to summer. These minima coincided with peak biological activity as indicated by extreme deficits in macronutrient concentrations. Estimates of net community production (NCP) were derived from nutrient mass-balances between phosphate and carbon using several published nutrient ratios. The NCP results indicated that the Southland Front was generally autotrophic (NCP > 1) during the study period, with annual biological drawdown of carbon and nutrients dominating heterotrophic respiration processes.

This study provides the first inter-annual investigation of seasonal trends of phytoplankton abundance, distribution, and production for the Subtropical Frontal waters off the coast of Otago as sampled by the Munida Time Series Transect. The characterisation and quantification of the biological uptake of carbon dioxide relative to other processes within the marine carbonate system is required to resolve complex interactions between physical and biogeochemical processes within the dynamic Southland Front. The incorporation of this data from the Munida Time Series, along with other ocean time series observations, is essential for resolving complex interactions and processes throughout the global oceans and for detecting and predicting changes caused by anthropogenic climate change and ocean acidification.