Carlos E. Del Castillo

Bio-optical properties and ocean color algorithms for coastal waters influenced by the Mississippi river during a cold front

During the passage of a cold front in March 2002, bio-optical properties examined in coastal waters impacted by the Mississippi River indicated that westward advective flows and increasing river discharge containing high concentrations of nonalgal particles contributed significantly to surface optical variability. A comparison of seasonal data from three cruises indicated spectral models of absorption and scattering to be generally consistent with other coastal environments, while their parameterization in terms of chlorophyll (Chl) alpha concentration showed seasonal variability. The exponential slope of the colored dissolved organic matter (CDOM) averaged 0.0161+/-0.00054 nm(-1) and nonalgal absorption averaged 0.011 nm(-1) with deviations from general trends observed due to anomalous water properties. Although the phytoplankton specific absorption coefficients varied over a wide range [0.02 to 0.1 m2 (mg Chl)(-1) at 443 nm] being higher in offshore surface waters, values of phytoplankton absorption spectra at the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) wave bands were highly correlated to modeled values. Particulate scattering characteristics were similar to observations for other coastal waters, while backscattering ratios were on average lower in phytoplankton-dominated surface waters (0.011+/-0.003) and higher in low Chl near-bottom waters (0.0191+/-0.0045). Average percent differences in remote sensing reflectance Rrs derived from modeled and in-water radiometric measurements were highest in the blue wave bands (52%) and at locations with more stratified water columns. SeaWiFS estimates of Chl and CDOM absorption derived using regional empirical algorithms were highly correlated to in situ data.

Multidecadal increase in North Atlantic coccolithophores and the potential role of rising CO2

Passing an acid test Calcifying marine organisms will generally find it harder to make and maintain their carbonate skeletons as increasing concentrations of atmospheric CO2 acidify the oceans. Nevertheless, some types of organisms will be damaged more than others, and some may even benefit from higher CO2 levels. Coccolithophores are a case in point, because their photosynthetic ability is strongly carbon-limited. Rivero-Calle et al. show that the abundance of coccolithophores in the North Atlantic has increased by up to 20% or more in the past 50 years (see the Perspective by Vogt). Thus, this major phytoplankton functional group may be able to adapt to a future with higher CO2 concentrations. Science, this issue p. 1533; see also p. 1466 Coccolithophores may be able to adapt to a world with higher levels of carbon dioxide. [Also see Perspective by Vogt] As anthropogenic carbon dioxide (CO2) emissions acidify the oceans, calcifiers generally are expected to be negatively affected. However, using data from the Continuous Plankton Recorder, we show that coccolithophore occurrence in the North Atlantic increased from ~2 to more than 20% from 1965 through 2010. We used random forest models to examine more than 20 possible environmental drivers of this change, finding that CO2 and the Atlantic Multidecadal Oscillation were the best predictors, leading us to hypothesize that higher CO2 levels might be encouraging growth. A compilation of 41 independent laboratory studies supports our hypothesis. Our study shows a long-term basin-scale increase in coccolithophores and suggests that increasing CO2 and temperature have accelerated the growth of a phytoplankton group that is important for carbon cycling.

Optically determined sources of allochthonous organic matter and metabolic characterizations in a tropical oligotrophic river and associated lagoon

Abstract The Cinaruco River, an oligotrophic ecosystem in the Venezuelan llanos (savanna), has strong seasonal hydrology and supports large populations of ecologically diverse fishes. The relative contributions of autochthonous and allochthonous production sources that support high stocks of secondary consumers are undetermined in this river. We used excitation–emission fluorescence spectroscopy and absorption spectra of dissolved organic matter to infer degradation of leaf material originating from the surrounding gallery forest. During the low-water period, a large fraction of fluorescent organic matter contained in leaves degraded quickly in river water and was an important allochthonous contribution of C to the system. However, the fluorescence signature of dissolved organic matter in lagoons was different from that of the main river channel during the falling-water period, suggesting that other sources of C were present. Allochthonous organic matter clearly fueled microbial respiration during the falling-water period, but our in-water experiments using light–dark bottle methods indicated that autochthonous production was an important supplementary C source in shallow nearshore waters. During the low-water and falling-water periods, water-column primary production in nearshore waters ranged from 150 to 500 mg C m−2 d−1 and was 2× greater than community respiration, i.e., the nearshore component of the water column was net autotrophic. Benthic primary production in nearshore areas where light reached the sediments ranged between 350 and 500 mg C m−2 d−1 and was about equal to community respiration. Primary production was probably limited by the availability of dissolved inorganic N, which sometimes was below detection limits of ˜0.5 μM (mean ˜0.25 μM). Our results support the idea that autotrophic production is an important C source in neotropical rivers.

Examining organic carbon transport by the Orinoco River using SeaWiFS imagery

[1] The Orinoco River is the fourth largest in the world in terms of water discharge and organic carbon export to the ocean. River export of organiccarbon is akey component of the carbon cycle and the global carbon budget. Here, we examined the seasonal transport of organic carbon by the Orinoco River into the eastern Caribbean using the conservative relationship of colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) in low salinity coastal waters influenced by river plumes. In situ measurements of CDOM absorption, DOC, and salinity were used to develop an empirical model for DOC concentration at the Orinoco River Plume. Satellite remote sensing reflectances were used with empirical models to determine DOC and Particulate organic carbon (POC) river transport. Our estimates of CDOM and DOC significantly correlated with in situ measurements and were within the expected ranges for the river. Total organic carbon transport by the Orinoco River during the period of 1998 to 2010 was 7.10 � 10 12 gCy � 1 , from 5.29 � 10 12 gCy � 1 of DOC and 1.81 � 10 12 gCy � 1 of POC, representing � 6% increase to previous published estimates. The variability in organic carbon transport responded to the seasonality in river flow more than to changes in organic carbon concentration in the river. Our results corroborate that is possible to estimate organic carbon transport using ocean color data at global scales. This is needed to reduce the uncertainties of land–ocean carbon fluxes.