Halifax: 19th - 21st October, 2015

VITALS
Ventilation, Interactions and Transports Across the Labrador Sea

Biology Team

Objectives

To understand how climate-sensitive physical processes (e.g. warming, convection and restratification, nutrient supply) affect rates of primary production, respiration and key nitrogen cycling steps along the east-west gradient in water properties across the Labrador Sea. This stems from the working hypothesis that the impact of biological processes on gas fluxes varies spatially, seasonally and inter-annually according to the depth of winter convection, biological productivity and the functional composition of photosynthetic primary producers (PPP).

Background

The vital functions of marine organisms link climate, global nutrient cycles and the fluxes of oxygen and CO2 within the ocean and across the air-sea interface (Fig. 3). In the western Arctic and subarctic Atlantic, the annual production of dissolved oxygen (DO) and fixation of CO2 by PPP are constrained by nitrogen (N) supply (Tremblay and Gagnon 2009). Vertical mixing and diffusion during fall/winter months inject variable amounts of the N-nutrient nitrate (NO3-) into the euphotic zone. This nitrate combines with advective and atmospheric N sources to drive the production of “new” organic matter, by phytoplankton, especially during blooms, whose timing and magnitude varies greatly across the Labrador Sea (Frajka-Williams and Rhines 2010). Temporary oceanic carbon storage ensues via the export of particulate matter (i.e. the biological pump) and is generally favored when large diatoms are the dominant PPP rather than coccolithophores, Phaeocystis and flagellates (Bopp et al. 2005; Kim et al. 2011).

PPP and dissolved oxygen sustain the growth and respiration of consumers (e.g., bacteria, zooplankton), whose activity releases CO2 and recycles organic N into simple reduced forms (NR) that fuel “regenerated” primary production within the euphotic zone. In deeper waters, nitrifying chemotrophic primary producers (CPP) obtain energy by oxidizing the NR released from the bacterial decomposition of detritus (Ward, 2011). These processes lead to oxygen consumption (Kortzinger et al., 2008), CO2 and N2O production and the replenishment of oceanic nitrate stores. The extent to which the Labrador Sea acts as a net sink or source of oxygen and CO2 depends on the balance between production and respiration (i.e. net community production, NCP) and the processes that physically redistribute and ventilate these gases. Respiration is expected to increase more rapidly than primary production as cold climates warm (Yvon-Durocher et al. 2010), possibly leading to a decline in NCP. Biological rates are expected to change seasonally, vertically and laterally across the Labrador Sea, where water masses originating from the high Arctic (west side) and the Atlantic (east side) exhibit distinct nutrient ratios (Deutsch and Weber 2012), temperatures and mixing regimes.

Methods

To meet our objective we will integrate remote-sensing based estimates of primary production with ship-based measurements of NCP (underway), biological rates (experimental incubations) made along the AR7W line during expeditions of the CCGS Hudson in May 2014, 2015 and 2016. This work will build on the state variables already measured by DFO's AZOMP program. The data will also be used by the other teams to 1) understand spatial (ship surveys/gliders) and temporal (moorings) changes in oxygen and pCO2, 2) interpret the historical AR7W database, and 3) constrain the ecosystem model to assess future scenarios.

Remote-sensing

Chlorophyll-a (an index of phytoplankton biomass) and gross primary production will be estimated using a new semi-analytical approach developed specifically for Arctic waters (Tremblay et al., 2011, Belanger et al., 2012) and refined for the Labrador Sea using VITALS field observations of optical properties and primary production (see below) and the vertical distribution of chlorophyll fluorescence recorded by the moorings. The contribution of coccolithophores (Balch et al. 2005) (and possibly other functional group or size classes of algae, Nair et al. 2008; Lubac et al. 2008) to PPP biomass will be assessed from spectral reflectance.

Ship-based (Underway)

Samples from the upper mixed layer will be obtained using the CCGS Hudson’s water intake and directly analyzed for core properties (nutrients, temperature, chlorophyll) and NCP using the O2/Ar method with equilibrator inlet mass spectrometry (Cassar et al. 2009). This measurement integrates biological processes over the time scale of a week. The data will also be used to provide spatial context for the discrete sampling and to ground truth remote-sensing based estimates of primary production. Discrete determinations of O2/Ar will calibrate the EIMS (Hamme, Gases team).

Ship-based (Discrete)

Measurements of optical properties and the collection of water samples will be performed at strategic locations (e.g. mooring sites, Arctic outflow in the west, Atlantic inflow in the west). Water samples will be taken in different water layers with a Rosette equipped with a CTD and sensors for oxygen, nitrate, irradiance and fluorescence. Nutrients will be measured using standard automated colorimetric procedures (nitrate, nitrite, phosphate, silicate; BIO collaborators) and the manual fluorometric method (ammonium; Tremblay, Holmes et al. 1999). The stable isotopic composition of N and O in nitrate will be determined using the bacterial method (Sigman et al. 2001; Granger and Sigman 2009) to assess its origin (Pacific or Atlantic-derived) and recent cycling history (Montoya 2007) (Granger). BIO collaborators will measure particulate organic carbon, chlorophyll a, phytoplankton assemblage composition based on pigments (High-Performance-Liquid Chromatography; HPLC) and microbial abundance (flow-cytometry of SYBR-green stained samples, Li et al. 2009). Additional samples will be preserved for the identification of phytoplankton functional groups using molecular approaches and classical microscopy, with special care given to the preservation and identification of coccolithophores (Beaufort et al. 2011).

Vertical profiles of basic apparent and inherent optical properties will be obtained to support the remote-sensing work (Bélanger, Babin). The spectral absorption coefficient of phytoplankton will be measured using the spectrophotometric method (Rottgers and Gehnke 2012). Spectral downwelling irradiance and upwelling radiance will be acquired at 19 wavelengths across the ultraviolet-visible-near infrared domains using a compact optical profiling system (C-OPS, Biospherical Instruments). Underway, hyperspectral, above-water measurements of remote sensing reflectance (Rrs) will be collected at the bow of the ship using a HyperSAS (Satlantic). Values of Rrs and diffuse attenuation coefficients (Kd) will be calculated following NASA ocean optics protocols (Mueller et al. 2003). Atmospheric optical depth will be determined using a sun photometer (MicroTops) under clear sky conditions to evaluate the quality of the atmospheric corrections applied to ocean color data. These measurements will increase the likelihood for direct ground-truthing of satellite-derived reflectance.

Rates of gross and net primary production will be estimated in vivo using the 14C technique in temperature-controlled deck and laboratory incubators (Babin, Tremblay). In the laboratory, photosynthesis-irradiance curves will be done in light-gradient incubators and used to 1) assess the impact of physical forcing and biological acclimation on productivity, and 2) provide parameters (e.g. P*max) for the remote-sensing algorithm and ecosystem model. Deck incubations will be used to obtain daily estimates of net primary production and to assess the amount of 14C passing into dissolved organic carbon (DOC) as well as the particulate organic (POC) and inorganic (PIC = calcification by coccolithophores) carbon pools (Gosselin et al., 1997, Paasche and Brubak, 1994) (Tremblay, Babin).

The assimilation of different N sources (N2, ammonium, nitrate) will be estimated in deck incubators with 15N-labeling techniques (Mohr et al., 2010, Montoya et al., 1996, Tremblay et al., 2006) and used to estimate rates of new and regenerated primary production (to be compared with NPP). Incubations will be terminated by filtration and the amount and isotopic enrichment of particulate organic N on filters determined by Isotope-Ratio-Mass-Spectrometry. Filtrates will be preserved used to assess ammonium recycling (NR) and nitrification with the double-diffusion technique (Raimbault and Garcia, 2008). Nitrification will be measured by comparing pre and post-incubation determinations of the 15N/14N and 18O/16O of nitrate (Christman et al. 2011). DIC consumption by nitrifyers will be measured in the dark during incubations with 14C-bicarbonate (Dore and Karl 1996). Bacterial production will be estimated with the tritiated method (Nguyen et al. 2012) (Maranger). NCP will be measured from oxygen time courses in the light (using a Fibox 3 with sensor spots). Community respiration (CR) by the organisms present in sampling bottles will be estimated from oxygen changes in the dark, and net primary production (NPP) by difference between the light and dark treatment (Nguyen et al. 2012). These methods are usually sensitive enough to measure rates in the upper water column, but will be complemented by in-vivo ETS (Electron Transport System) measurements in deeper waters (Martinez-Garcia et al. 2009; Teira et al. 2010).

Mesozooplankton respiration and ammonium excretion will be estimated (collaboration with Head) in vitro using animals collected from vertical net tows in the upper 100 m. Additional experiments will also test the combined effects of temperature and different organic carbon substrates on respiration as both should influence N recycling and C respiration and NR in warming regions. Since nitrifying CPP in subsurface waters rely mainly on NR, we will test the use of 15N-based estimates of ammonium recycling and nitrification as independent proxies for CR, to be compared with in-vivo ETS measurements.

Glider and Float Based

Beneath the mixed layer, oxygen concentrations along the quasi-Lagrangian trajectories of the gliders and floats provide a measure of the remineralization rates and the associated export productivity. In addition, optical backscatter reveals the penetration depth of sinking particles. Both the export productivity calculated from the subsurface oxygen measurements and the penetration depth of sinking particles, are related to the strength of the biological pump.

Deliverables

The results will provide direct rate measurements of major biological processes affecting carbon, oxygen, and N2O flux in the Labrador Sea. The experimental approach, as well as the spatial and inter-annual components of the work, will reveal the dependency of these rates on temperature and the processes that supply nutrients and organic carbon as growth substrates for PPP and bacteria. This data will be intrumental in interpreting the distributions and changes of bulk properties (oxygen, pCO2, N2O) measured from the ship, by moorings and gliders. They will be used to adjust the parameterization of the BLING biogeochemical model to the work area. Improvement and ground-truthing of the remote-sensing algorithm of primary production will insure its reliability and usefulness for monitoring change in future years, providing an additional tool to DFO's monitoring arsenal. Advances in our fundamental understanding of the interactions between the biological and solubility pumps will improve our ability to predict future climate change.