BATS is the long-term time-series study of biogeochemical cycles in the western North Atlantic Ocean near Bermuda. BATS is one of two oceanographic time-series sites that have been supported by a large interdisciplinary and international program ( JGOFS). The other time-series (Hawaii Ocean Time-series, HOT) is located in the North Pacific Ocean, near Hawaii. The purpose of both time-series studies is to understand the role of the oceans in large-scale processes of global change, especially their role in affecting greenhouse gases such as CO₂.

Long-term time series are a powerful tool for investigating ocean biogeochemistry and its effects on the global carbon cycle. The seasonal, interannual and longer-scale dynamics of carbon and nutrient cycles in the upper ocean control ecosystem productivity, the net exchange of carbon dioxide between the atmosphere and the ocean, and the distribution of many elements in the sea. Understanding the overall carbon cycle requires that we understand each of its component processes. The focus of the BATS and HOT efforts is to improve understanding of the “time-varying” components of the ocean carbon cycle, related biogenic elements of interest (e.g., nitrogen, phosphorus), and identifying the relevant physical, chemical and ecosystem properties responsible for this variability. Within this context, there are a number of continuing goals and objectives for the BATS research program, including:

On societally relevant timescales (e.g., decades to centuries), oceanic biological processes sequester large quantities of atmospheric carbon, modulating the concentrations of CO₂ in the lower atmosphere (IPCC 1990; IPCC 2001). Prior to the beginning of BATS and HOT in 1988, our understanding of the controls and modulation on the ocean carbon cycle was based on several long-standing paradigms. Subsequent research from both time-series sites have significantly contributed to changes and challenges to our paradigms about ocean biogeochemical processes. For example:

Primary Production, Particle Export and the Biological Carbon Pump. Prior to the JGOFS and related programs, the gravitational flux of particulate organic carbon (POC) from the euphotic zone was thought to dominate biological carbon sequestration (i.e., the Biological Pump of Carbon) to the deep ocean. Export production was scaled to rates of primary production (PP; e.g., Eppley and Peterson, 1979) and, gravitational flux scaled (exponentially decreasing with depth) to near surface sediment trap measurements (e.g., Martin et al., 1987). In contrast to expectations, however, no meaningful relationships have been found between PP and POC flux at either the BATS or HOT sites (e.g., Karl et al., 2001c). Instead, export pathways that were poorly quantified a decade ago -- export of dissolved organic carbon (DOC), “active carbon transport” by diel migrant zooplankton and food web influences on the partitioning of primary production between POC and dissolved organic carbon (DOC) -- have emerged as significant terms in the biological pump of carbon to the ocean interior (e.g., Carlson et al., 1994; Steinberg et al., 2000). The question originally posed by the time-series sites, “what controls carbon export flux”, is therefore still an open and active area of BATS research.

Historically, biologists and geochemists have been at odds about whether nitrogen or phosphorus proximately or ultimately limits primary production in the world’s ocean (e.g., Codispoti 1989; Falkowski et al. 1998; Tyrrell 1999). Leaving aside the now demonstrated importance of iron from this discussion (e.g., Coale et al. 1996), Michaels et al. (2001) have hypothesized that there are feedback-dependent oscillations between N- and P-limited states mediated by the activity of nitrogen-fixers. The dominant oceanic N₂-fixing species were believed to be Trichodesmium spp. and the diatomsymbiont Richelia, but new molecular biological techniques employed at HOT and in the tropical Atlantic have found a great diversity of single-celled prokaryotes that possess and express the genes for N₂-fixation (Zehr et al. 2001). The presence of these additional diazotrophs may further enhance the important role of N₂-fixers in the global N cycle, and has significant implications for the coupling of phosphorus and nitrogen cycles in the ocean. At BATS, the presence of excess nitrate relative Redfield proportions of phosphorus in the thermocline has been attributed to nitrogen fixation (e.g., Michaels et al., 1996; Gruber et al., 1997; Hood et al., 2001), but contradictory evidence remains about it's quantitative significance (e.g., Orcutt et al., 2001; Hansell et al., 2004). Studies on the controls and magnitude of new production in the oligotrophic gyre of the North Atlantic will continue through BATS and related projects, for example, to focus on nitrogen fixation, variability of excess nitrate (e.g., Bates and Hansell, 2004) and non-Redfield C:N:P production and remineralization processes, mesoscale and submesoscale forcing/modulation of new and export production, and iron cycling and availability.

During the last decade, complex coupling between ocean physics and modes of climate variability (such as North Atlantic Oscillation, NAO; El Niño-Southern Oscillation, ENSO) has increasingly been demonstrated. Only more recently has coupling between ocean biogeochemical dynamics/biological community structure and mode of climate variability been demonstrated, for example, in the Pacific Ocean (Chavez et al. 2003). In the North Atlantic, significant correlative relationships between NAO and the interannual variability of many biogeochemical properties (e.g., mixed layer depth, SST, PP, nutrients; Bates 2001a; Oschlies 2001) have been shown. An inverse correlation has also been found at BATS between the NAO index and the relative abundance of haptophytes (e.g., coccolithophores; Lomas and Bates, 2004) during the winter/spring bloom. Given the potential importance of this taxonomic group in the mineral ballasting of POC fluxes (e.g., Armstrong et al. 2002) greater understanding of population dynamics is clearly necessary. The importance of the determining the coupling and feedbacks between atmospheric/climate forcing and ocean biology/biogeochemistry remains an important component of ocean research.

Both BATS and HOT time-series have documented the oceanic uptake of anthropogenic CO₂ and the increase in CO₂ over time. It was expected that the CO₂ content of the surface ocean should increase at a rate (e.g., ~1 µmoles kg^-1yr^-1) equivalent to equilibration with the increase in atmospheric CO₂ each year. However, in the North Atlantic subtropical gyre, the CO₂ content of the subtropical mode water (lying between the seasonal and permanent thermoclines) has increased at double the rate (e.g., ~2.2 µmoles kg^-1 yr^-1; Bates et al. 2002). This has been related to changes in ocean mixing and NAO variability, and importantly, the oceanic sink of anthropogenic CO₂ was higher (up to 0.24 Pg C yr^-1) during the BATS period relative to earlier decades.