CARIACO Research Articles

Total dissolved atmospheric nitrogen deposition in the anoxic Cariaco basin

Atmospheric deposition of total dissolved nitrogen (TDN) is an important source of nitrogen for ocean primary productivity that has increased since the industrial revolution. Thus, understanding its role in the ocean nitrogen cycle will help assess recent changes in ocean biogeochemistry. In the anoxic Cariaco basin, the place of the CARIACO Ocean Time-Series Program, the influence of atmospherically-deposited TDN on marine biogeochemistry is unknown. In this study, we measured atmospheric TDN concentrations as dissolved organic (DON) and inorganic (DIN) nitrogen (TDN = DIN + DON) in atmospheric suspended particles and wet deposition samples at the northeast of the basin during periods of the wet (August–September 2008) and dry (March–April 2009) seasons. We evaluated the potential anthropogenic N influences by measuring wind velocity and direction, size-fractionated suspended particles, chemical traces and by performing back trajectories. We found DIN and DON concentration values that ranged between 0.11 and 0.58 μg-N m−3 and 0.11–0.56 μg-N m−3 in total suspended particles samples and between 0.08 and 0.54 mg-N l−1 and 0.02–1.3 mg-N l−1 in wet deposition samples, respectively. Continental air masses increased DON and DIN concentrations in atmospheric suspended particles during the wet season. We estimate an annual TDN atmospheric deposition (wet + particles) of 3.6 × 103 ton-N year−1 and concluded that: 1) Atmospheric supply of TDN plays a key role in the C and N budget of the basin because replaces a fraction of the C (20% by induced primary production) and N (40%) removed by sediment burial, 2) present anthropogenic N could contribute to 30% of TDN atmospheric deposition in the basin, and 3) reduced DON (gas + particles) should be a significant component of bulk N deposition.

Environmental control of the dominant phytoplankton in the Cariaco basin: a hierarchical Bayesian approach

We develop a hierarchical Bayesian model linking the abundance of individual phytoplankton species with over a decade (1995–2011) of environmental data from the Cariaco Ocean Time Series Program in the Cariaco Basin, Venezuela, to characterize how phytoplankton respond to environmental forcing. Temperature, salinity, irradiance, and macronutrient concentrations account for 39% of the variation in log cell abundance across 67 species. Individual phytoplankton taxa varied widely in their response to these environmental variables. A principal component analysis of the environmental response profiles clearly distinguishes the responses of diatoms and dinoflagellates to environmental forcing. Phytoplankton abundance primarily varied with temperature, pH, and irradiance, with salinity and macronutrient concentrations acting as secondary drivers. In the aggregate, our results demonstrate that environmental changes, whether short-term or a result of climate change, should be expected to have dramatic consequences on the taxonomic composition of phytoplankton communities.

Vertical fluxes of particulate biogenic material through the euphotic and twilight zones in the Cariaco Basin, Venezuela

Surface-tethered particle interceptor traps (PITs) were deployed at 50 and 100m (1–3 days) on ten occasions in the Cariaco Basin between March 2007 and November 2009 to measure the settling fluxes of biogenic particles at 50m (the base of the euphotic zone—Ez) and 100m. Fluxes at these two depths were compared to concurrent fluxes estimated with moored sediment traps at 150, 225 and 410m from the CARIACO Ocean Time-Series program. We measured particulate organic carbon (POC), particulate organic nitrogen (PON), calcium carbonate, biogenic silica and terrigenous material concentrations in samples collected with both the drifting and moored traps. We also estimated the fluxes of foraminifera shells and coccolithophore cells at 50 and 100m using drifting traps samples. Surface chlorophyll a and primary production observations during each sampling period were examined to quantify the relationship between the magnitude and geochemical composition of the vertical flux and overlying production. Surface chlorophyll a concentrations and primary production rates were highest during months of upwelling (2.58−1.35mgm−3 and 3.6−1.4g Cm−2d−1, respectively). The fluxes of POC, PON, calcite and silica measured during the upwelling season (December–May) were typically higher than during the period of non-upwelling (August–November), when surface waters are more strongly stratified. POC fluxes measured with the drifting traps (50 and 100m) varied between 0.95 (upwelling) and 0.14gm−2d−1 (non-upwelling), compared with those from the moored traps (150, 225 and 410m) which ranged from 0.21 to 0.01gm−2d−1. Similarly, the fluxes of biogenic opal in the upper 100m ranged from 1.12 and 0.18gm−2d−1, and those at greater depths varied from 0.27gm−2d−1 during upwelling to values near zero during stratification periods. The fluxes of POC, PON, calcite and silica in the upper 100m decreased by an order of magnitude at the depth of the oxic–anoxic interface (>200m). The sinking organic matter collected with the floating traps within the upper 100m was significantly correlated with surface chlorophyll a concentrations (r=0.68, p<0.05) as a result of close coupling between the flux of biogenic particles and primary production. In contrast, there was no clear relationship between surface chlorophyll/primary production and fluxes measured below 150m depth.

Bio-optical characteristics of Cariaco Basin (Caribbean Sea) waters

Bio-optical properties of marine waters of the Cariaco Basin (southeastern Caribbean Sea) were assessed monthly between 1995 and 2005 as part of the CARIACO Ocean Time Series program. Temporal changes in light quality and penetration were caused by seasonal variation in the concentration of three major optical constituents, namely phytoplankton, detrital particles, and colored dissolved organic matter (CDOM). All constituents showed higher absorption coefficients during the upwelling season (January–May) compared to the rainy season (June–November). Both the absorption coefficient due to CDOM ( a g (440)) and due to phytoplankton ( a ph (440)) had similar contributions to total absorption of light during the upwelling season ( a ph (440)=0.062±0.042 m −1, a g (440)=0.065±0.047 m −1). In contrast, a g (440) dominated light absorption during the rainy season ( a ph (440)=0.017±0.011 m −1, a g (440)=0.057±0.031). This led to an overestimate in SeaWiFS-derived chlorophyll concentrations during the rainy season, of between 7% and 45%. The detrital component, a d (440), typically showed the smallest contribution ( a d (440)=0.021±0.014 m −1 during upwelling and 0.007±0.001 m −1 during the rainy season). There was no clear relationship between the various optically active components in time. During the upwelling season the chlorophyll-specific absorption coefficient, a p h ⁎ (440), was nearly half the value observed during the rainy season due to changes in the package effect (explaining ∼40% of the variability) and in accessory pigment composition as a result of species succession (explaining ∼60% of the variability). The euphotic zone depth (depth of the 1% photosynthetic active radiation (PAR) level) was typically shallower during the upwelling season (36.7±12.3 m) than during the rainy season (47.9±13.5 m) due to the onset of a shallower and stronger phytoplankton bloom. During upwelling, the highest chlorophyll- a concentrations (Chl>1 mg m −3) were observed in the upper 25 m with primary production rates exceeding 1800 mgC m −2 d −1. During the rainy season, a deep chlorophyll maximum (DCM, concentrations between 0.2 and 0.8 mg m −3) was observed between 35 and 55 m, with low (<0.2 mg m −3) Chl concentrations above this depth and primary production values of ∼990 mgC m −2 d −1. The DCM occurred immediately above the seasonal thermocline and around the 1% PAR light level. During the upwelling season, no DCM was observed.