Dwight A. Dieterle

Red tides in the Gulf of Mexico: Where, when, and why?

[1] Independent data from the Gulf of Mexico are used to develop and test the hypothesis that the same sequence of physical and ecological events each year allows the toxic dinoflagellate Karenia brevis to become dominant. A phosphorus-rich nutrient supply initiates phytoplankton succession, once deposition events of Saharan iron-rich dust allow Trichodesmium blooms to utilize ubiquitous dissolved nitrogen gas within otherwise nitrogen-poor sea water. They and the co-occurring K. brevis are positioned within the bottom Ekman layers, as a consequence of their similar diel vertical migration patterns on the middle shelf. Upon onshore upwelling of these near-bottom seed populations to CDOM-rich surface waters of coastal regions, light-inhibition of the small red tide of ~1 ug chl l(-1) of ichthytoxic K. brevis is alleviated. Thence, dead fish serve as a supplementary nutrient source, yielding large, self-shaded red tides of ~10 ug chl l(-1). The source of phosphorus is mainly of fossil origin off west Florida, where past nutrient additions from the eutrophied Lake Okeechobee had minimal impact. In contrast, the P-sources are of mainly anthropogenic origin off Texas, since both the nutrient loadings of Mississippi River and the spatial extent of the downstream red tides have increased over the last 100 years. During the past century and particularly within the last decade, previously cryptic Karenia spp. have caused toxic red tides in similar coastal habitats of other western boundary currents off Japan, China, New Zealand, Australia, and South Africa, downstream of the Gobi, Simpson, Great Western, and Kalahari Deserts, in a global response to both desertification and eutrophication.

Carbon cycling in the upper waters of the Sargasso Sea: I. Numerical simulation of differential carbon and nitrogen fluxes

Abstract A complex ecosystem model is developed for the area around Bermuda in the Sargasso Sea. The model is physically driven by seasonal changes in spectral light, temperature, and water column mixing. Autotrophic growth is represented by four functional groups of phytoplankton. The groups have light and nutrient utilization characteristics that reflect those of Prochlorococcus , Synechococcus and Chromophycota species. The model includes differential carbon and nitrogen cycling, nitrification, and nitrogen fixation to effect realistic allochthonous and autochthonous nutrient sources to the euphotic zone. This simulation yields realistic seasonal and vertical (1) succession of phytoplankton functional groups’ biomass, productivity, and pigments; (2) profiles of dissolved inorganic carbon, nitrate, and ammonium; and (3) fluxes of carbon dioxide at the air–sea boundary and particulate carbon and nitrogen settling losses, when compared to the JGOFS BATS site. The addition of local nitrification, differential carbon and nitrogen remineralization, and nitrogen fixation removes the need for an unrealistically high upward vertical flux of nitrate to mimic the productivity and chlorophyll a stocks. The explicit numerical description of carbon and nitrogen utilization by heterotrophic bacteria simulated a population that was not nitrogen-limited in these waters. Instead, the heterotrophic bacteria community was limited by energy resources in the form of DOC, and was a nitrogen source for the autotrophic community through the excretion of excess NH 4 from the labile DOM energy source. Numerical descriptions of ecosystems based solely on nitrogen dynamics, or fixed carbon to nitrogen ratios, may yield an inaccurate prediction of carbon and nitrogen fluxes, and fail to properly predict the carbon cycle.

CO2 cycling in the coastal ocean. I – A numerical analysis of the southeastern Bering Sea with applications to the Chukchi Sea and the northern Gulf of Mexico

Abstract A quasi-two dimensional model of the carbon and nitrogen cycling above the 70m isobath of the southeastern Bering Sea at 57°N replicates the observed seasonal cycles of nitrate, ammonium, ΣCO2, pCO2, light penetration, chlorophyll, phytoplankton growth rate, and primary production, as constrained by changes in wind, incident radiation, temperature, ice cover, vertical and lateral mixing, grazing stress, benthic processing of phytodetritus and zooplankton fecal pellets, and the pelagic microbial loop of DOC, bacteria, and their predators. About half of the seasonal resupply of nitrate stocks to their initial winter conditions is derived from in situ nitrification, with the rest obtained from deep-sea influxes. Under the present conditions of atmospheric forcing, shelf-break exchange, and food web structure, this shelf ecosystem serves as a sink for atmospheric CO2, with storage in the forms of exported DOC, DIC, and unutilized POC (phytoplankton, bacteria, and fecal pellets). As a consequence of just the rising levels of atmospheric pCO2 since the the Industrial Revolution, however, the biophysical CO2 status of the Southeastern Bering Sea shelf may have switched over the last 250 years, from a prior source to the present sink, since this relatively pristine ecosystem has unergone little eutrophication. Such fluctuations of CO2 status may thus be reversed by the physical processes of : (1) reduction of atmospheric pCO2, (2) increased on welling of deep-sea ΣCO2, and (3) warming of shelf waters. Based on our application of this model to the Chukchi Sea and the Gulf of Mexico, about 1.0–1.2 gigatons C y-1 of atmospheric CO2 may now be sequestered by temperate and polar shelf ecosystems. When tropical systems are included, however, a positive net sink of only 0.6–0.8. × 1015g C y−1 may prevail over all shelves.

Simulation of carbon‐nitrogen cycling during spring upwelling in the Cariaco Basin

Coupled biological-physical models of carbon-nitrogen cycling by phytoplankton, zooplankton, and bacteria assess the impacts of nitrogen fixation and upwelled nitrate during new production within the shelf environs of the Cariaco Basin. During spring upwelling in response to a mean wind forcing of 8 m s−1, the physical model matches remote-sensing and hydrographic estimates of surface temperature. Within the three-dimensional flow field, the steady solutions of the biological model of a simple food web of diatoms, adult calanoid copepods, and ammonifying/nitrifying bacteria approximate within ∼9% the mean spring observations of settling fluxes caught by a sediment trap at ∼240 m, moored at our time series site in the basin. The models also estimate within ∼11% the average 14C net primary production and mimic the sparse observations of the spatial fields of nitrate and light penetration during the same time period of February-April. Stocks of colored dissolved organic matter are evidently small and diazotrophy is minimal during spring. In one summer case of the model with weaker wind forcing, however, the simulated net primary production is 14% of that measured in August-September, while the predicted detrital flux is then 30% of the observed. Addition of a cyanophyte state variable, with another source of new nitrogen, would remedy the seasonal deficiencies of the biological model, attributed to use of a single phytoplankton group.

Nitrogen exchange at the continental margin: A numerical study of the Gulf of Mexico

Abstract A two-layered baroclinic circulation model and a 21-layered biochemical model are used to explore the consequences of Loop Current-induced upwelling and terrestrial eutrophication on “new” production within the Gulf of Mexico. During a quasi-annual penetration and eddy-shedding cycle of the Loop Current, the simulated seasonal changes of incident radiation, wind stress, and surface mixed layer depth induce an annual cycle of algal biomass that corresponds to in situ and satellite time series of chlorophyll. The simulated nitrate fields match those of shipboard surveys, while fallout of particulate matter approximates that caught in sediment traps and accumulating in bottom sediments. Assuming an f ratio of 0.06–0.12, the total primary production of the Gulf of Mexico might be 105–210g C m −2 y −1 in the absence of anthropogenic nutrient loadings, i.e. 2–3 fold that of oligotrophic regions not impacted by western boundary currents. Less than 25% of the nitrogen effluent of the Mississippi River may be stored in bottom sediments, with most of this input dispersed in dissolved from beneath the pycnocline, after remineralization of particulate detritus within several production cycles derived from riverine loading. At a sinking rate of 3m d −1 , however, sufficient phytodetritus survives oxidation in the water column to balance estimates of bottom metabolism and burial at the margins.

Predictive Ecological Modeling of Harmful Algal Blooms

We have thus far constructed a series of coupled ecological/physical models to predict the origin and fate of harmful algal blooms of the toxic dinoflagellate, Gymnodinium breve, on the West Florida shelf. We find that (1) the maximal population growth rate of G. breve must be ∼0.80 day−1 during initiation of a red tide, but as little as 0.08 day−1 during its decay, (2) diatoms dominate when estuarine and shelf-break supplies of nitrate are made available to a model community of small and large diatoms, coccoid cyanophytes and Trichodesmium, non-toxic and red-tide dinoflagellates, microflagellates, and coccolithophores, (3) a numerical recipe for large red tides of G. breve requires DON supplies, mediated by iron-starved, nitrogen-fixers, while small blooms may persist on sediment sources of DON, (4) selective grazing must be exerted on the non-toxic dinoflagellates, and (5) vertical migration of G. breve in relation to seasonal changes of summer downwelling and fall/winter upwelling flow fields determines the duration and intensity of red tide landfalls along the barrier islands and beaches of the west coast of Florida, once other losses are specified. Given poorly known initial and boundary conditions and the expense of shipboard monitoring programs, however, bio-optical moorings or remote sensors are the most likely sources of model validation and improved HAB forecasts. Accordingly, the bio-optical implications of our ecological models must be included in future simulation analyses of HABs on both the West Florida shelf and within other coastal regions. Of particular importance for initiation of the coupled biophysical models is an improved understanding of the relationship of remotely sensed surface signals to shade-adapted dinoflagellates, aggregating below the first optical depth of the water column.

CO2 cycling in the coastal ocean. II. Seasonal organic loading of the Arctic Ocean from source waters in the Bering Sea

Abstract A Lagrangian model of water parcel transit along a 2850-km trajectory from the 80-m isobath of the southeastern Bering Sea to the same depth of the northwestern Chukchi Sea replicates the major seasonal features of nitrogen and carbon cycling on these shelves. Spring-summer extraction of nitrate from the Bering and Chukchi water columns and of CO 2 from the atmosphere is followed by fall-winter storage of ammonium and DOC near the shelf-break of the Canadian Basin. Here, the memory of a simulated seasonal range in water parcel contents of 0.2–13.0 μ-at NO 3 l −1 , 2056–2125 μg-at ΣCO 2 l −1 , 0.3–3.3 μg-at NH 4 l −1 , and 67–134,μg-at total marine DOCl-l −1 exiting the Chukchi Sea, is evidently maintained in the halocline of the adjacent Canadian Basin at depths of ∼ 75 m during summer and ∼ 125 m during winter. Based on these properties of imported water parcels, estimated rates of nitrification, DOC oxidation, and ΣCO 2 evolution in the Canadian Basin suggest (1) a residence time of ∼ 10 y for shelf waters of Pacific origin in the halocline, (2) production of POC within the overlying ice-covered slope waters may indeed be 10-fold larger than first estimates made in the deeper Basin during the 1950s, (3)∼ 81% of all of the DOC within Bering Strait is of marine origin from prior production cycles in the SBS, and (4) over 50% of the color signal seen by satellite above these waters is of DOC origin, rather than from phytoplankton pigments.

Carbon cycling in the upper waters of the Sargasso Sea : II. Numerical simulation of apparent and inherent optical properties

A mathematical framework to incorporate spectral apparent and inherent optical properties into a one-dimensional ecological simulation (EcoSim) of the Sargasso Sea is developed. The simulation includes equations for spectral algal particulate absorption, in the form of 4 functional groups of phytoplankton, colored degradational matter (CDM), in the form of 2 classes of colored dissolved organic carbon (CDOC), and downwelling diffuse attenuation coefficients (Kd’s) at a 5 nm resolution. Particulate absorption responds to changes in phytoplankton species biomass, pigmentation, and photo-adaptation. CDM absorption responds to concentration changes of labile and relict CDOC. Kd’s respond to changes in spectral total absorption, backscattering, and the average cosine of downwelling photons. The spectral bio-optical outputs provide an additional means of validating an ecological simulation. The vertical and seasonal changes in the diffuse attenuation coefficient at wavelengths of 412, 442, 467, 487, 522, and 567 nm compare well with in situ measurements. There appears to be an underestimation of CDM by the EcoSim that is reflected in the simulated absorption and attenuation at 412 nm. The particulate absorption slope between 412 and 487 nm suggests an overestimation of chlorophyll b and photoprotective carotenoid concentrations from Prochlorococcus functional groups. Simulated Kd(522) and Kd(567) appear to be higher than observations and may result from the exclusion of the inelastic scattering process (i.e. Raman scattering and CDM fluorescence). Results suggest that CDM absorption does not co-vary with particulate absorption. Comparison with a six year time-series of CZCS data suggests that CDM interference of the estimated CZCS chlorophyll a may have been pre-valent in the late spring and the early fall.

A simulation analysis of the fate of phytoplankton within the Mid-Atlantic Bight

Abstract A time-dependent, three-dimensional simulation model of wind-induced changes of the circulation field, of light and nutrient regulation of photosynthesis, of vertical mixing as well as algal sinking, and of herbivore grazing stress, is used to analyse the seasonal production, consumption, and transport of the spring bloom within the Mid-Atlantic Bight. The models case (c) of a 58-day period in February–April 1979, simulated “new” primary production, based on just nitrate, with a mean of 0.31 g C m−2 day−1 over the whole model domain. About 57% of this total shelf carbon fixation was removed by herbivores, with 21% lost as export, either downstream towards Cape Hatteras or offshore to slope waters, after the first 58 days of the spring bloom. Extension of the model for another 22 days of case (c) increased the mean export to 27%, while variation of the models parameters in eight other cases led to a range in export from 8 to 38% of the average “new” primary production. Spatial and temporal variations of the simulated algal biomass, left behind in the shelf water column, reproduced chlorophyll fields sensed by satellite, shipboard, and in situ instruments.

Satellite detection of phytoplankton export from the mid-Atlantic bight during the 1979 spring bloom

Abstract Analysis of CZCS imagery confirms shipboard and in situ moored fluorometer observations of resuspension of near-bottom chlorophyll within surface waters (1–10 m) by northwesterly wind events in the mid-Atlantic Bight. As much as 8–16 μg Chl l −1 are found during these wind events from March to May, with a seasonal increase of algal biomass until onset of stratification of the water column. Rapid sinking or downwelling apparently occurs after subsequent wind events, however, such that the predominant surface chlorophyll patterns is ≈0.5–1.5 μg l −1 over the continental shelf during most of the spring bloom. Perhaps half of the chlorophyll increase observed by satellite during a wind resuspension event represents in situ production during that 4–5 day interval, with the remainder attributed to accumulation of algal biomass previously produced and temporarily stored within near-bottom water. Present calculations suggest that at least 10% of the primary production of the spring bloom may be exported as ungrazed phytoplankton carbon from mid-Atlantic shelf waters to those of the continental slope.