The development of phosphate sensors suitable for long-term in situ deployments in natural waters, is essential to improve our understanding of the distribution, fluxes, and biogeochemical role of this key nutrient in a changing ocean. Here, we describe the optimization of the molybdenum blue method for in situ work using a lab-on-chip (LOC) analyzer and evaluate its performance in the laboratory and at two contrasting field sites. The in situ performance of the LOC sensor is evaluated using hourly time-series data from a 56-day trial in Southampton Water (UK), as well as a month-long deployment in the subtropical oligotrophic waters of Kaneohe Bay (Hawaii, USA). In Kaneohe Bay, where phosphate concentrations were characteristic of the dry season (0.13 ± 0.03 µM, n = 704), the in situ sensor accuracy was 16 ± 12% and a potential diurnal cycle in phosphate concentrations was observed. In Southampton Water, the sensor data (1.02 ± 0.40 µM, n = 1,267) were accurate to ±0.10 µM relative to discrete reference samples. Hourly in situ monitoring revealed striking tidal and storm derived fluctuations in phosphate concentrations in Southampton Water that would not have been captured via discrete sampling. We show the impact of storms on phosphate concentrations in Southampton Water is modulated by the spring-neap tidal cycle and that the 10-fold decline in phosphate concentrations observed during the later stages of the deployment was consistent with the timing of a spring phytoplankton bloom in the English Channel. Under controlled laboratory conditions in a 250 L tank, the sensor demonstrated an accuracy and precision better than 10% irrespective of the salinity (0-30), turbidity (0-100 NTU), colored dissolved organic matter (CDOM) concentration (0-10 mg/L), and temperature Grand et al.In situ Lab-On-Chip Phosphate Sensor (5-20 • C) of the water (0.3-13 µM phosphate) being analyzed. This work demonstrates that the LOC technology is mature enough to quantify the influence of stochastic events on nutrient budgets and to elucidate the role of phosphate in regulating phytoplankton productivity and community composition in estuarine and coastal regimes.

Here, we present a new in situ microfluidic phosphate sensor that features an improved "phosphate blue" assay which includes polyvinylpyrrolidone in place of traditional surfactants-improving sensitivity and reducing temperature effects. The sensor features greater power economy and analytical performance relative to commercially available alternatives, with a mean power consumption of 1.8 W, a detection limit of 40 nM, a dynamic range of 0.14-10 μM, and an infield accuracy of 4 ± 4.5%. During field testing, the sensor was continuously deployed for 9 weeks in a chalk stream, revealing complex relations between flow rates and phosphate concentration that suggest changing dominance in phosphate sources. A distinct diel phosphorus signal was observed under low flow conditions, highlighting the ability of the sensor to decouple geochemical and biotic effects on phosphate dynamics in fluvial environments. This paper highlights the importance of high resolution in situ sensors in addressing the current gross under-sampling of aquatic environments.

Atmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition (i.e., wet + dry deposition) in remote regions. In this work, we discuss the upper ocean distribution of dissolved Fe and Al in the eastern Indian Ocean along a 95°E meridional transect spanning the Antarctic margin to the Bay of Bengal. We use the mixed layer concentration of dissolved Al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates. Except in the northern Bay of Bengal where the Ganges-Brahmaputra river plume contributes to the inventory of dissolved Al, the surface distribution of dissolved Al along 95°E is remarkably consistent with the large-scale gradients in mineral dust deposition and multiple-source regions impacting the eastern Indian Ocean. The lowest total dust deposition fluxes are calculated for the Southern Ocean (66 ± 60 mg m À2 yr À1 ) and the highest for the northern end of the south Indian subtropical gyre (up to 940 mg m À2 yr À1 at 18°S) and in the southern Bay of Bengal (2500 ± 570 mg m À2 yr À1 ). Our total deposition fluxes, which have an uncertainty on the order of a factor of 3.5, are comparable with the composite atmospheric model data of Mahowald et al. (2005), except in the south Indian subtropical gyre where models may underestimate total deposition. Using available measurements of the solubility of Fe in aerosols, we confirm that dust deposition is a minor source of dissolved Fe to the Southern Ocean and show that aeolian deposition of dissolved Fe in the southern Bay of Bengal may be comparable to that observed underneath the Saharan dust plume in the Atlantic Ocean.

A high-resolution section of dissolved iron (dFe) and aluminum (dAl) was obtained along~95°Ein the upper 1000 m of the eastern Indian Ocean from the Antarctic margin (66°S) to the Bay of Bengal (18°N) during the U.S. Climate Variability and Predictability (CLIVAR) CO 2 Repeat Hydrography I08S and I09N sections (February-April 2007). In the Southern Ocean, low concentrations of dAl (<1 nM) reflect the negligible dust inputs impacting the water masses subducted in the circumpolar domain. The low dAl concentrations characterizing the Southern Ocean terminate near 45°S, probably because of the advection of subtropical water masses that received dust and sedimentary inputs in their formation region. Our subsurface dFe data near the southern Kerguelen Plateau were significantly higher than historical observations in other Indian sectors of the Southern Ocean. We surmise that the offshore advection of dFe-rich waters along the western flank of the southern Kerguelen plateau and enhanced vertical mixing could contribute to this elevated subsurface dFe inventory. Elevated subsurface particulate and dFe levels downstream of the northern Kerguelen Plateau may reflect long-range lateral transport from the plateau's sediments and/or remineralization inputs. At the northern edge of the south Indian subtropical gyre, the deposition of Australian dust, possibly combined with the advection of dAl-enriched waters from the Indonesian Throughflow, creates a region of elevated dAl in the upper 400 m but without a corresponding enrichment in dFe. In the northern Indian Ocean, the South Equatorial Current constitutes a remarkable biogeochemical front separating the oxygen-rich and dFe-poor subtropical gyre waters from the dFe-rich and oxygen-depleted waters of the northern Indian Ocean. By tracing the accumulation of macronutrients and dFe along the advective pathway of Indian Central Water, we show that the central waters of the northern Indian Ocean receive excess dFe in addition to that produced by remineralization inputs. The resuspension of shelf sediments and release of pore waters probably contribute to the elevated dFe and dAl levels observed below the highly stratified upper layers of the Bay of Bengal.