Dean A. Stockwell

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.

Mass development of the planktonic diatom Proboscia alata over the bering sea shelf in the summer season

During most of the vegetation season from late May to early September, the large-sized diatom alga Proboscia alata forms local patches with high abundances and biomasses in different oceanographic domains of the eastern Bering Sea shelf. The average abundance and biomass of the species in these patches amounts to 0.7 × 106 cells/l and 5 g WW/m3, respectively, for the layer of 0–25 m, while the corresponding estimates for the layer of the maximal species concentrations are 4.0 × 106 cells/l and 38 g WW/m3 (1.6 g C/m3). These levels of abundance and biomass are typical of the spring diatom bloom in the region. The outbursts of P. alata mass development are an important element of carbon cycling in the pelagic zone of the shelf area in the summer season. The paradox of the P. alata summertime blooms over the middle shelf lies in their occurrences against the background of the sharp seasonal pycnocline and the deficiency in nutrients in the upper mixed layer. The duration of the outbursts in the P. alata development is about two weeks and the size of the patches with high abundances can be as large as 200 km across. Degradation of the P. alata summertime outbursts may occur during 4–5 days. The rapid sinking of the cells through the seasonal pycnocline results in an intense transport of organic matter to the bottom sediments. One of the possible factors responsible for the rapid degradation of the blooms is the affect on the population by ectoparasitic flagellates. At the terminal stages of the P. alata blooms, the share of infected cells may reach 70–99%.

Effect of nutrient amendments on the summer phytoplankton dynamics on the Bering Sea shelf under experimental conditions

Experiments on nutrient and iron amendments were performed with phytoplankton on the eastern shelf of the Bering Sea in June 2000 and August 2001. The nutrient amendments (NO3, NH4, SiO4, NO3 + SiO4, NH4 + SiO4, and Fe + NO3) increasing their initial concentrations by ∼20 μM were put into test bottles 10 l in volume each. With iron addition (Fe or Fe + NO3), its concentration increased by 5 nM. The experiments performed showed that the main nutrient that limited the phytoplankton development was nitrogen. Regardless of the composition of the dominant algae in the background community, the amendments caused massive development of diatoms. The intense growth was characteristic for diatoms of both the spring and spring-summer assemblages. At high abundances of Phaeocystis pouchetii or of the coccolithophore Emiliania huxleyi in the natural water, nitrogen-containing amendments caused an intense growth of these species, along with the massive development of diatoms. In the case of the diatom prevalence in the initial sample, the intensities of the utilization of NO3 and NH4 in combination with SiO4 in the course of the experiment were 1.7 and 3 times as high as their intensities with no silicon amendments. Likewise, NO3 + SiO4 and NH4 + SiO4 mixed amendments caused an increase in the silicon assimilation by a factor of 4–5 as compared to pure silicon amendments. During one of the experimental series in which both diatoms and Phaeocystis pouchetii actively developed, virtually complete nitrogen utilization (90–99.8%) in 4–5 days was observed for both the NO3 and NH4. The addition of silicon and iron only caused no significant growth of the phytoplankton abundance. It was assumed that the destruction of the seasonal thermocline and the supply of nutrients into the surface layer as a result of strong wind forcing might cause a phytoplankton bloom in the summer time and result in the much pronounced qualitative and quantitative spatial heterogeneity of the phytoplankton characteristic of the eastern shelf of the Bering Sea.

The biochemical composition of phytoplankton in the Laptev and East Siberian seas during the summer of 2013

The Laptev and East Siberian seas which are generally viewed as terrestrial organic matter (TerrOM)-dominated seas, are among the least biologically understood regions in the Arctic Ocean. During the summer of 2013, however, the TerrOM signature was negligible in our samples. We investigated the biochemical composition (carbohydrates [CHO], proteins [PRT], and lipids [LIP]) of phytoplankton-dominated particulate organic matter in order to improve our understanding of the physiological status of resident phytoplankton. Our chlorophyll-a values and the presence of SCMs and resting spores were associated with a cessation of the phytoplankton bloom. Despite the low inorganic nitrogen nutrients in the water column, the cellular PRT (39%) were comparable to CHO (42%) contents and the inorganic (dissolved nitrogen:dissolved phosphate) and organic (PRT:CHO) indices did not indicate a nitrogen stress of phytoplankton metabolism. Altogether, the phytoplankton were likely in a growth transition from the exponential to the stationary phase, resulting in CHO-dominated cells with moderate PRT. By comparing our biochemical analyses with the LIP-dominated (> 50%) ones in the Chukchi Sea (the summers of 2011 and 2012), we conclude that more severe nitrogen-limited conditions occurred in the Chukchi Sea. In a quality aspect, we suggest that consumers which feed on LIP-rich phytoplankton could have an advantage to overwinter while those feeding on CHO-rich phytoplankton will gain energy efficiently in a short term. Therefore, the biochemical composition of phytoplankton could be a valid integrator of surrounding environments in which phytoplankton grow and can be a good indicator of their nutritional value.

First in situ estimations of small phytoplankton carbon and nitrogen uptake rates in the Kara, Laptev, and East Siberian seas

Carbon and nitrogen uptake rates by small phytoplankton (0.7–5 μm) in the Kara, Laptev, and East Siberian seas in the Arctic Ocean were quantified using in situ isotope labeling experiments; this research, which was novel and part of the NABOS (Nansen and Amundsen Basins Observational System) program, took place from 21 August to 22 September 2013. The depth-integrated carbon (C), nitrate (NO−3 ), and ammonium (NH + 4 ) uptake rates by small phytoplankton ranged from 0.54 to 15.96 mg C m−2 h−1, 0.05 to 1.02 mg C m−2 h−1, and 0.11 to 3.73 mg N m−2 h−1, respectively. The contributions of small phytoplankton towards the total C, NO−3 , and NH + 4 varied from 25 % to 89 %, 31 % to 89 %, and 28 % to 91 %, respectively. The turnover times for NO−3 and NH + 4 by small phytoplankton found in the present study indicate the longer residence times (years) of the nutrients in the deeper waters, particularly for NO−3 . Additionally, the relatively higher C and N uptake rates by small phytoplankton obtained in the present study from locations with less sea ice concentration indicate the possibility that small phytoplankton thrive under the retreat of sea ice as a result of warming conditions. The high contributions of small phytoplankton to the total C and N uptake rates suggest the capability of small autotrophs to withstand the adverse hydrographic conditions introduced by climate change.