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Neal, BP, Condit C, Liu G, dos Santos S, Kahru M, Mitchell BG, Kline DI.  2014.  When depth is no refuge: cumulative thermal stress increases with depth in Bocas del Toro, Panama. Coral Reefs. 33:193-205.   10.1007/s00338-013-1081-6   AbstractWebsite

Coral reefs are increasingly affected by high-temperature stress events and associated bleaching. Monitoring and predicting these events have largely utilized sea surface temperature data, due to the convenience of using large-scale remotely sensed satellite measurements. However, coral bleaching has been observed to vary in severity throughout the water column, and variations in coral thermal stress across depths have not yet been well investigated. In this study, in situ water temperature data from 1999 to 2011 from three depths were used to calculate thermal stress on a coral reef in Bahia Almirante, Bocas del Toro, Panama, which was compared to satellite surface temperature data and thermal stress calculations for the same area and time period from the National Oceanic and Atmospheric Administration Coral Reef Watch Satellite Bleaching Alert system. The results show similar total cumulative annual thermal stress for both the surface and depth-stratified data, but with a striking difference in the distribution of that stress among the depth strata during different high-temperature events, with the greatest thermal stress unusually recorded at the deepest measured depth during the most severe bleaching event in 2005. Temperature records indicate that a strong density-driven temperature inversion may have formed in this location in that year, contributing to the persistence and intensity of bleaching disturbance at depth. These results indicate that depth may not provide a stress refuge from high water temperature events in some situations, and in this case, the water properties at depth appear to have contributed to greater coral bleaching at depth compared to near-surface locations. This case study demonstrates the importance of incorporating depth-stratified temperature monitoring and small-scale oceanographic and hydrologic data for understanding and predicting local reef responses to elevated water temperature events.

Nevison, CD, Keeling RF, Kahru M, Manizza M, Mitchell BG, Cassar N.  2012.  Estimating net community production in the Southern Ocean based on atmospheric potential oxygen and satellite ocean color data. Global Biogeochemical Cycles. 26   10.1029/2011gb004040   AbstractWebsite

The seasonal cycle of atmospheric potential oxygen (APO similar to O-2 + 1.1 CO2) reflects three seasonally varying ocean processes: 1) thermal in- and outgassing, 2) mixed layer net community production (NCP) and 3) deep water ventilation. Previous studies have isolated the net biological seasonal signal (i.e., the sum of NCP and ventilation), after using air-sea heat flux data to estimate the thermal signal. In this study, we resolve all three components of the APO seasonal cycle using a methodology in which the ventilation signal is estimated based on atmospheric N2O data, the thermal signal is estimated based on heat flux or atmospheric Ar/N-2 data, and the production signal is inferred as a residual. The isolation of the NCP signal in APO allows for direct comparison to estimates of NCP based on satellite ocean color data, after translating the latter into an atmospheric signal using an atmospheric transport model. When applied to ocean color data using algorithms specially adapted to the Southern Ocean and APO data at three southern monitoring sites, these two independent methods converge on a similar phase and amplitude of the seasonal NCP signal in APO and yield an estimate of annual mean NCP south of 50 degrees S of 0.8-1.2 Pg C/yr, with corresponding annual mean NPP of similar to 3 Pg C/yr and a mean growing season f ratio of similar to 0.33. These results are supported by ocean biogeochemistry model simulations, in which air-sea O-2 and N2O fluxes are resolved into component thermal, ventilation and (for O-2) NCP contributions.

Nommann, S, Sildam J, Noges T, Kahru M.  1991.  Plankton Distribution During a Coastal Upwelling Event Off Hiiumaa, Baltic Sea - Impact of Short-Term Flow Field Variability. Continental Shelf Research. 11:95-108.   10.1016/0278-4343(91)90037-7   AbstractWebsite

The evolution of an upwelling event and the associated plankton distribution off Hiiumaa island, northeastern Baltic Sea, is traced through continuous registration of wind and currents, consecutive CTD and chlorophyll fluorescence surveys and underway shipboard measurements of near-surface temperature and particle concentration over a 10-day period in June 1986. The earlier mesoscale pattern of warmer (13-degrees-C) near-shore waters containing higher chlorophyll concentration was drastically changed as the wind turned from SW to NNW and increased up to 12 m s-1, which resulted in the offshore Ekman transport of the warmer coastal water and upwelling of cold (6-7-degrees-C) phytoplankton-poor deeper water along the coastal slope. A relatively fast biological response to the upwelling resulted in the form of enhanced primary production and 4-7-fold increase of the standing crop of some phytoplankton populations (mainly dinoflagellates) within 4 days (at the upwelling frontal boundary). It is shown that a persistent, moderate wind is favourable to sustain a local phytoplankton bloom while keeping the vertical transport of the deeper nutrient-rich water still going but being not powerful enough to stir away the growing phytoplankton. Our measurements confirm the importance of coastal upwellings for the productivity of the Baltic near-shore ecosystems in the summer stage.