Variability in Microphytobenthos Biomass and Carbon Isotopic Values in Shallow Coastal Waters of the Northern Gulf of Mexico

Estuaries and inshore coastal waters of the northern Gulf of Mexico (GOM) are highly productive systems supporting diversity of life, including important fisheries species (e.g., Minello et al. 2003). Salt marshes and seagrass meadows are formed by conspicuous and high-biomass primary producers, long considered important at the base of coastal food webs (Teal 1962). However, the inconspicuous primary producers, phytoplankton and microphytobenthos (MPB, single-celled micro-algae on the sediment surface) are also important in these systems, having been shown to support a variety of consumers (Currin et al. 1995, 2011, Galvan et al. 2008). While disentangling MPB biomass and productivity rates is logistically challenging, there are many studies which suggest both phytoplankton and MPB represent a potentially large portion of primary production in these systems due to the rapid turnover rates (Sullivan and Moncreiff 1988, Blanchard et al. 2002)

Silicon biogeochemistry in the open-ocean surface waters : insights from the Sargasso Sea and equatorial Pacific

Diatoms are a ubiquitous group of plankton responsible for 20-40% of oceanic primary production, and a higher fraction of organic matter export to the ocean interior. Diatoms actively transport dissolved inorganic silicon into their cells, and through the process of silicification (i.e. biogenic silica production) they build tough and intricate shells, known as frustules. With a global distribution and the ability to persist in high numerical abundances, diatoms dominate the biological cycling of Si in the oceans. The biogeochemical cycling of Si has been well studied in the coastal ocean, specifically in upwelling regions where diatoms are generally the dominant phytoplankton group. However, much less is known about the role diatoms play in the open ocean; which comprises the vast majority of the oceanic surface area. Outside of the Southern Ocean, only 11 studies (prior to 2003) directly examined surface-water Si biogeochemistry in the open ocean. Current knowledge about Si biogeochemistry in the open ocean suffers from what can only be described as gross under-sampling. This dissertation reports on the surface-water Si biogeochemistry in two open-ocean regions: the northwestern Sargasso Sea and the eastern equatorial Pacific. The three research chapters are linked by the examination of spatial or temporal variability in surface-water Si biogeochemistry. Chapters 2 and 4 examine scales of temporal variability in Si biogeochemistry in the Sargasso Sea. The results demonstrate that biogenic silica concentrations, and presumably Si biogeochemical processes, vary on daily, seasonal, multi-year (e.g. ~3-4 years), and decadal time scales. In Chapter 3 data were gathered over a ~2.6x106 km2 area (i.e. larger than the Bering Sea) in the equatorial Pacific. Within that area biogenic silica production showed little spatial or temporal variability. Additionally, the estimated contribution to new production (productivity supporting the export of organic matter to the ocean interior) by diatoms in this region was 4-10 times higher than the diatom contribution to total autotrophic biomass

The Influence of Juncus-rhizosphere Dissolved Organic Matter on Coastal Plankton Communities

Many aquatic plants produce copious amounts of dissolved organic matter (DOM) which enters surrounding waters and potentially stimulates planktonic activity. In the northern Gulf of Mexico, Juncus roemarianus (i.e. black needlerush) is a dominant marsh grass species residing in coastal zones and barrier islands. The below-ground biomass i.e. rhizosphere, can be consistently submerged, serving as a potential source of DOM to the surrounding waters. The lability and possible stimulatory effect of J. roemarianus DOM was examined for three plankton communities collected within the discharge region of Mobile Bay and adjacent waters of Gulf Shores, Alabama (less affected by Mobile Bay). DOM within the pore water surrounding the J. roemarianus was extracted, concentrated, and added to the field communities along with positive (i.e. addition of labile organic matter) and negative (i.e. no additions) controls. In the Mobile Bay experiment, the DOM addition stimulated increased autotrophic biomass and heterotrophic activity well above that observed in the negative controls. However, experiments utilizing Gulf Shores water showed little to no stimulation. Our results suggest that J. roemarianus DOM addition may stimulate planktonic activity; however, the degree of enhancement is likely controlled by the community composition and water properties (e.g. nutrient availability)

Diatom control of the autotrophic community and particle export in the eastern Bering Sea during the recent cold years (2008–2010)

The southeastern Bering Sea has exhibited shifts in climate since the start of the 21st century. The regional climate shifts are manifested in the duration and areal extent of seasonal sea-ice coverage. During a recent cold period (2008–2010) with extensive spring sea-ice cover over the southeastern shelf of the Bering Sea, a total of 77 water column and 24 sediment trap profiles were collected over the shelf and shelf break and analyzed for autotrophic pigment concentrations and elemental (carbon, nitrogen, phosphorus, and silicon) concentrations in suspended and exported particulate material. These results are used to establish the seasonal succession of the autotrophic community and the control that both phytoplankton and zooplankton exert on export production. In spring (April to mid-June), total chlorophyll a (TChl a) concentrations were generally low (i.e., \u3c 1 μg L–1); however, localized phytoplankton blooms near the marginal ice zone (MIZ) lead to elevated spring average TChl a concentrations (i.e., \u3e5 μg L–1). In summer (mid-June to late July), photic zone chlorophyll a concentrations were typically \u3c1 μg L–1 over the shelf and at the shelf break. Diatoms represented the greatest contribution to TChl a (regional averages of 71%–96% in spring and 25%–75% in summer) and autotrophic biomass in spring and summer. This algal class also represented 50%–99% of TChl a associated with particles sinking from the photic zone. The relatively high proportion of phaeophorbide a in sediment trap material indicates that sinking of zooplankton fecal pellets facilitate the export of particles through the water column. Further, zooplankton grazing may be an important process that returns regenerated nutrients to the water column based on the elemental composition of suspended and sinking particles. In colder than average years, the emergence of diatom blooms in the spring MIZ supports the production of abundant large zooplankton, which are a primary food source for juvenile pelagic fishes of economically important species. Therefore, processes in colder than average years may be essential for the transfer of particulate organic carbon from the surface waters and the success of the economically important pelagic fisheries

Primer Extension Capture: Targeted Sequence Retrieval from Heavily Degraded DNA Sources

We present a method of targeted DNA sequence retrieval from DNA sources which are heavily degraded and contaminated with microbial DNA, as is typical of ancient bones. The method greatly reduces sample destruction and sequencing demands relative to direct PCR or shotgun sequencing approaches. We used this method to reconstruct the complete mitochondrial DNA (mtDNA) genomes of five Neandertals from across their geographic range. The mtDNA genetic diversity of the late Neandertals was approximately three times lower than that of contemporary modern humans. Together with analyses of mtDNA protein evolution, these data suggest that the long-term effective population size of Neandertals was smaller than that of modern humans and extant great apes

Drivers of diatom production and the legacy of eutrophication in two river plume regions of the northern Gulf of Mexico

In the northern Gulf of Mexico (nGoM), the Louisiana Shelf (LS) and Mississippi Bight (MB) subregions are influenced by eutrophication to varying degrees. Despite recognition that dissolved silicon may regulate diatom productivity in the nGoM, there is only one published data set reporting biogenic silica (bSiO2) production rates for each subregion. We report that bSiO2 production rates on the LS and MB are high and appear to be controlled by different nutrients among seasons. Despite exceptional upper trophic level biomass regionally, which suggests significant primary production by diatoms (as in other systems), gross euphotic-zone integrated bSiO2 production rates are lower than major bSiO2 producing regions (e.g. upwelling systems). However, when normalizing to the depth of the euphotic zone, the bSiO2 production rates on the LS are like normalized rates in upwelling systems. We suggest local river-plume influenced hydrography concentrates diatom productivity within shallow euphotic zones, making production more accessible to higher trophic organisms. Comparison of rates between the LS and MB suggest that the fluvial nitrate within the LS stimulates bSiO2 production above that in the MB, which has a smaller watershed and is less eutrophic (relatively). Beyond understanding the factors controlling regional bSiO2 production, these data offer the most comprehensive Si-cycle baseline to date as the LS and MB will likely exchange freely in the mid to late century due to land subsidence of the Mississippi River delta and/or sea-level rise

Техніки графіки

Робоча програма навчальної дисципліни «Техніки графіки» для студенів спеціальності 023 «Образотворче мистецтво*, декоративне мистецтво, реставрація». Освітній рівень перший (бакалаврський) (2 курс, 3 семестр

Benthic silicon cycling in the Arctic Barents Sea: a reaction–transport model study

Over recent decades the highest rates of water column warming and sea ice loss across the Arctic Ocean have been observed in the Barents Sea. These physical changes have resulted in rapid ecosystem adjustments, manifesting as a northward migration of temperate phytoplankton species at the expense of silica-based diatoms. These changes will potentially alter the composition of phytodetritus deposited at the seafloor, which acts as a biogeochemical reactor and is pivotal in the recycling of key nutrients, such as silicon (Si). To appreciate the sensitivity of the Barents Sea benthic system to the observed changes in surface primary production, there is a need to better understand this benthic–pelagic coupling. Stable Si isotopic compositions of sediment pore waters and the solid phase from three stations in the Barents Sea reveal a coupling of the iron (Fe) and Si cycles, the contemporaneous dissolution of lithogenic silicate minerals (LSi) alongside biogenic silica (BSi), and the potential for the reprecipitation of dissolved silicic acid (DSi) as authigenic clay minerals (AuSi). However, as reaction rates cannot be quantified from observational data alone, a mechanistic understanding of which factors control these processes is missing. Here, we employ reaction–transport modelling together with observational data to disentangle the reaction pathways controlling the cycling of Si within the seafloor. Processes such as the dissolution of BSi are active on multiple timescales, ranging from weeks to hundreds of years, which we are able to examine through steady state and transient model runs. Steady state simulations show that 60 % to 98 % of the sediment pore water DSi pool may be sourced from the dissolution of LSi, while the isotopic composition is also strongly influenced by the desorption of Si from metal oxides, most likely Fe (oxyhydr)oxides (FeSi), as they reductively dissolve. Further, our model simulations indicate that between 2.9 % and 37 % of the DSi released into sediment pore waters is subsequently removed by a process that has a fractionation factor of approximately −2 ‰, most likely representing reprecipitation as AuSi. These observations are significant as the dissolution of LSi represents a source of new Si to the ocean DSi pool and precipitation of AuSi an additional sink, which could address imbalances in the current regional ocean Si budget. Lastly, transient modelling suggests that at least one-third of the total annual benthic DSi flux could be sourced from the dissolution of more reactive, diatom-derived BSi deposited after the surface water bloom at the marginal ice zone. This benthic–pelagic coupling will be subject to change with the continued northward migration of Atlantic phytoplankton species, the northward retreat of the marginal ice zone and the observed decline in the DSi inventory of the subpolar North Atlantic Ocean over the last 3 decades