In this project, researchers at the Monterey Bay Aquarium Research Institute and the University of Washington will construct and deploy Apex profiling floats equipped with ISUS optical nitrate sensors, Aanderaa oxygen optodes and Wet Labs ECO fluorometers. Two floats per year will be deployed at the HOT (Hawaii Ocean Time series) station, BATS (Bermuda Atlantic Time Series) station and at Ocean Station Papa in the Gulf of Alaska. Two additional float will be deployed each year in the Southern Ocean. These floats will provide time series observations of biogeochemical processes at these sites with temporal resolution that cannot be sustained by the current shipboard observations. With life spans for each float of four years, the float network will also provide significantly improved spatial resolution around each of the deployment sites near the end of this project.
These float-based sensors provide the potential to study a variety of processes. The project leaders recently demonstrated that profiling floats can provide time series observations of Net Community Production below the mixed layer based on dissolved oxygen measurements. The continuous measurements of NCP demonstrate that the euphotic zone of the subtropical ocean produces oxygen (net autotrophic) and that episodic events of high NCP are not required to sustain a positive autotrophic balance. These measurements thus may resolve a fundamental controversy as to whether the oligotrophic ocean is net heterotrophic or net autotrophic. Near the BATS site the floats will likely provide significantly improved understanding of the role of eddies in sustaining elevated primary production rates. At high latitudes, the floats will provide an unparalled view of the interaction of vertical mixing with timing of the spring bloom.
Two types of Broader Impacts will result from this work. The project will "stimulate and support the development and dissemination of next-generation instrumentation, multi-user facilities, and other shared research and education platforms." And secondly, by making the data from this research directly available in real time to the research and education, it will "advance discovery and understanding while promoting teaching, training and learning." Information about the project and the data stream it produces will also be made available on a dedicated web site.
Activities and Findings For this grant, the co-PI’s proposed to deploy 2 profiling floats equipped with chemical sensors per year for three years at the ocean time series stations HOT, BATS, Ocean Station Papa, and in the Southern Ocean. Twenty four total floats were to be deployed. At this point in time, we have deployed a total of twenty floats with funding from this grant: six at HOT, six in the Southern Ocean, three at BATS and four at Ocean Station Papa; one float was lost on deployment. The remaining 4 floats are in the final stages of assembly and will be ready for deployment shortly. All data from these floats is made publically available at www.mbari.org/chemsensor/floatviz.htm and also at http://runt.ocean.washington.edu as soon as they are received. A publication describing the hardware integration with profiling floats and which assesses the quality of the data that results is in review (Johnson et al., in review). The floats are producing a unique view of biogeochemical cycling at the time series sites. This has enabled us to produce fundamentaly new perspectives on important issues such as the metabolic balance of the ocean (Riser and Johnson, 2008), or the supply nutrients to the euphotic zone (Johnson et al., 2010; Ascani et al., 2012). The work is also laying the groundwork for a global, biogeochemical observing system, similar to Argo (Johnson et al., 2009). We continue to analyze the data returned by these floats and we reported the findings of these analyses in several publications (Johnson et al., 2009; Johnson et al., 2010) and in a number of presentations. One new paper is in press, one is submitted, and three papers are now in preparation. The paper in press (Ascani et al., 2012) integrates the data from four of our profiling floats equipped with nitrate sensors and deployed near Hawaii with a high resolution model to understand the sub-mesoscale processes that transport nitrate up from the base of the nitracline. The submitted manuscripts describes the methods used for nitrate sensor/float integration and nitrate sensor performance. The papers in preparation describe 1) the integration of float data with a one-dimensional model to study biogeochemical processes at Ocean Station Papa, 2) a study of biogeochemical dynamics near HOT from an almost 10 year long set of oxygen data obtained with profiling floats, including floats deployed in this proposal, and 3) a study of biogeochemical dynamics near BATS with a focus on the mechanisms that transport nitrate into surface waters. We also note that graduate students at Scripps Institution of Oceanography and at Georgia Tech are beginning to use the data sets that are returned by our floats and made available on the internet (T. Martz, SIO and T. Ito, Ga. Tech., personal communication). The data show that there is a clear drawdown in nitrate each summer with significant interannual variability in the extent of winter cooling and nitrate entrainment. We are using a numerical model, driven by local heat fluxes obtained from the NCEP Reanalysis data set to separate the impacts of ocean physics and biology on nitrate concentration. This allows us to determine the Net Community Production in this system each year. We now have a ten year record of oxygen concentrations measured with profiling floats deployed at HOT. The record begins with floats deployed by the UW and MBARI groups and continues with our joint effort. We have used this data set to assess rates of oxygen production and net community production in the upper ocean around Hawaii. The results are quite consistent with shipboard measurements. We also have records of temperature and nitrate measured by a profiling float deployed at the BATS station in the North Atlantic. The nitrate data clearly shows the late winter entrainment of nitrate into the mixed layer. This process is one of the hallmarks that differentiates the BATS and HOT time series. One of the underlying goals stated in the original proposal for this work was to determine if nitrate sensors on floats could resolve this process. The answer is clearly "yes". The floats also show interannual variability in the amount of nitrate entrained in the mixed layer that is clearly related to the depth of winter mixing. Following years with weak nitrate entrainment (2012), the nitracline appears to be pushed to greater depths, most likely by biological uptake. In summary, the data sets generated by the floats have provided an unparalleled view of nitrate and oxygen cycling in the ocean. We will keep analyzing these data sets as the floats continue to operate.
