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Phytoplankton:
Phytoplankton, the most abundant primary producers on earth are a free-floating aquatic microscopic diverse and polyphyletic group of mostly unicellular and colonial photosynthetic organisms (Falkwski, 1997) having a critical role in primary production, nutrient cycling, and food webs (Dawes 1998). In spite of constituting less than 1% of the Earth’s photosynthetic biomass, they make up a significant proportion of the global primary production (45% of annual net primary productivity; Field, 1998). Phytoplankton must be in the photic zone to entrap solar energy. A host of adaptations allow them to move into or to remain in the euphotic zone. Most phytoplankton species are motile and swim toward light; however, major movement is mostly through transport by water currents (Dawes 1998; Sandifer et al. 1980). Non-motile phytoplankton rely on physiological adaptations (production of mucilage and accumulation of lighter ions with a concomitant reduction of heavier ions or compounds), morphological characteristics (branching frustules, bladder-like cell shape, presence of gas-vaculoes), and physical factors (water viscosity, convection, wind-induced rotation) to reduce sinking rates (Dawes 1998).
Phytoplankton growth:
Phytoplankton growth outside the tropics is characterized by periodic oscillations. General patterns of phytoplankton dynamics in temperate water are well known (Reynolds, 1984; Sommer et al., 1986). Bloom events take place predominantly in spring and secondarily in autumn. A typical seasonal pattern phytoplankton in eutrophic and oligotrophic water is displayed in figure 7.
Phytoplankton diversity:
The apparent paradox of how unstructured marine environment offering little possibility for niche separation could support a high diversity has historically generated significant attention (Hutchinson, 1961). Scheffer (2003) proposed that ecological and environmental factors such as chaotic fluid motion, size-selective grazing, spatio-temporal heterogeneity, and environmental fluctuations continually interact such that the planktonic habitat fails to reach a static equilibrium favoring a single given species. The main global pattern of phytoplankton diversity is the latitudinal cline. With increasing latitude, diversity decreases. This pattern of diversity is shaped by the interplay of dispersal and competitive exclusion. Environmental variability modulates competitive exclusion – the largely uniform oligotrophic environment of the tropics allows more prolonged coexistence of competing species and the heterogeneous temperate zones favor the exclusion of slow growing species and homogeneous communities (Barton, 2010). On the other hand, lateral dispersal is prominent in regions of energetic ocean circulation that generate a collage of diversity hot spots on the global trend of latitudinal cline (Barton, 2010). Diversity and biomass of phytoplanktons have been shown to maintain a consistent unimodal relationship – intermediate biomass promotes the highest diversity while blooms harbor the lowest diversity (Irigoien, 2004). Surprisingly, no clear relation between phytoplankton and zooplankton diversity has been found (as shown in the figure below), while the biomass of zooplankton is an increasing saturating function of the phytoplankton biomass (Irigoien, 2004).
Phytoplankton comprise of microalgae and marine phototrophic eubacteria and archaebacteria. They include species from the following 8 major divisions: Cyanobacteria (blue-green algae), Chlorophyta (green algae), Prochlorophyta (prokaryotic picoplankton), Euglenophyta (euglenids and kinetoplastids), Pyrrhophyta (dinoflagellates), Cryptophyta (cryptomonads), Chrysophyta (golden algae), and Bacillariophyta (includes diatoms). Each group of phytoplankton shows characteristic colors, depending on the relative abundance of the resident photosynthetic pigments: green chlorophylls, yellow carotenes, or pink or blue phycobilins. Katz (2004) reported about 25000 morphologically distinct forms of phytoplankton, while at least 500 genera (Sournia, 1991) and 5000 marine species have so far been conclusively identified (Hallengraeff, 2003). Surprisingly, at least three times more planktonic species are known in the freshwater. This seems to be due to underestimation and lack of a focus on smaller species than true lack of diversity (Valout, 2001). In fact, only 40 picoplanktonic species are known – their classification is problematic and gradually being established (e.g., the new classes of Pelagophyceae and Bolidophyceae), and lastly they are not at all represented in standard cultures.
All phytoplankton do not share a common ancestor – successive events of endosymbiosis mark their evolutionary history and adaptive radiation (Simon, 2009). Based on cell-size phytoplankton are usually grouped into three categories. Picophytoplankton are the smallest, <2 micrometers (µm) in diameter and include the prochlorophytes and cyanobacteria; nanophytoplankton are intermediate sized, from 2-20 µm and include the flagellated cryptophytes, chrysophytes and prymnesiophytes; microphytoplankton are the largest and include those >20 µm in diameter and are made up mostly of diatoms and dinoflagellates. These two groups of diatoms and dinoflagellates generally comprises the bulk of a phytoplankton community (Figure 8)
Diatoms:
The diatoms are the most globally important planktonic primary producers in the ocean. Diatoms contain the chlorophyll-a and chl-c as well as a wide variety of carotenoids. Diatom cell shape follows one of two basic forms, radially symmetric centric or bilaterally symmetric pennate that bear a locomotory structure for gliding, called raphe. Pennate diatoms are often found on solid substrates, such as rocks, animals, or larger algae, while the centric forms are mostly pelagic. 29?Both types of diatoms have a SiO2 (silicon dioxide)-based bilayered external cell wall, called frustule, which is made up of a slightly larger epitheca fitting snugly over the hypotheca. The silicate frustule houses all the components of the cell. Surface of the frustule is generally decorated with fine lines, or striae, which are actually rows of tiny pores allowing exchanges across the frustule. The distribution of such morphological patterns is a key to the visual identification of diatom species.
Dinoflagellates:
The dinoflagellates are often brown in color and noticeably luminescent. Approximately half the dinoflagellates are strictly heterotrophic and lack chlorophyll, additionally the majority of the chl-containing species are mixotrophic, i.e. they carry out photosynthesis and consume bacteria or other phytoplankton. Dinoflagellates are sometimes naked but usually have a porous cellulose cell wall, fitted with an equatorial groove that contains a ribbon flagellum. This groove compartmentalizes the dinophyte’s ornate cell wall into the epicone and hypocone. A second groove perpendicular to the equatorial groove houses a longitudinal flagellum, used for movement.
Prymnesiophyceae
Another major group of eukaryotic marine nanophytoplankton is constituted by the Prymnesiophyceae, whose presence in marine waters is globally observed (e.g., the bloom-forming coccolithophorid Emiliania huxleyi). They all contain a pair of flagella and a thin filamentous peg-like appendage, called the haptonema. In contrast to dinoflagellates and diatoms, they are generally not present in the freshwater. Prymnesiophyceae cells are covered by organic scales that can be calcified (coccoliths). Phaeocystis, a genus of Prymnesiophyceae produces colonies whose polysaccharide matrix induces foam on the beaches of the North Sea (Lancelot et al., 1987). Toxic species, such as Chrysochromulina polylepis, may also bloom sporadically, as was the case in 1988 of Norway (Vaulot, 2001).
Microphytobenthos:
Another important primary producer of the coastal regions are the benthic microalgae. These organisms inhabit the top few centimeters of the substrate layers (mud or sand) of marine sediment, given sufficient light for photosynthesis (MacIntyre et al. 1996, MacIntyre and Cullen 1996,Charpy and Charpy-Roubaud 1990). Benthic microalgaehave an important role as a food source for higher trophic levels in shallow water as well as estuarine food webs (MacIntyre et al. 1996, Sorokin 1991, Charpy and Charpy-Roubaud 1990, Kang, 2003). Stable isotope tracer analysis has demonstrated that a host of benthic consumers including omnivores, suspension feeders and deposit feeders mostly rely on benthic microalgae for food (Sullivan, 1990). The cohesive nature of the benthic microalgae reduces resuspension and erosion of sediment layers, therefore promoting the stability of benthic habitats (Miller et al. 1996, Williams et al.1985). Mucilaginous films containing acidic “Extracellular Polymeric Substances” (EPS) (Paterson et al., 1990; Delgado et al., 1991; De Brouwer & Stal, 2001) glue these thin, dense microbial mats. The amount of EPS excretion by diatoms is often related to the rate of primary production (Cadée & Hegemann, 1974). By forming biofilms benthic diatoms modulate nutrient fluxes across the sediment–water interface – they function as an active biofilter and generally reduce the flow of inorganic nutrients into the pelagic zone (Facca et al. 2002; Nicholson, 1999; Sigmon and Cahoon 1997). Contrastingly, they also directly provide organic carbon to phytoplankton systems (Brandini, 2001). MPB can even determine the community structure of overlying pelagic phytoplankton assemblages by influencing the release of dissolved silica (Conley et al. 1993, Sigmon and Cahoon 1997). They can also exert an indirect impact on nitrate fluxes as oxygenation of surface sediments during photosynthesis can alter coupled nitrification–denitrification processes (sundback, 2000).
MPB GROWTH:
Sediment-dwelling MPB covers 70% of world’s shelf regions (Emery, 1968). In a typical temperate bay (Onslow bay, North Carolina), 80% of the Chl-a related biomass was shown to reside in the sediment (Cahoon, 1990). In shallow coastal ecosystems, Chl-a related algal biomass in an integrated water column sample has systematically been shown to be much lower than Chl-a related algal biomass in sediments (Cahoon, 1990). The biomass of MPB could vary dramatically – reported values range from 85 mg Chl–a m-2 in a temperate bay in France to 1153 mg Chl-a m-2 in the Great Barrier Reef of Australia (Guarini, 1998; Woelfel, 2010).
A primary factor which is universally shaping the distribution and growth of MPB is light availability. As only the upper 0.2-2 mm of sediment generally has sufficient penetration by light, the distribution of benthic microalgae is restricted to this relatively thin surface layer (Wolff, 1979; MacIntyre et al., 1996). The texture and relief of the sediment surface and its organic content also determine the vertical distribution of MPB communities. Within less than one centimeter of depth, the sediment properties change from fully oxygenated to anoxic conditions and pH, sulphide, irradiance, and nutrients also show strong vertical variability (Joergensen et al., 1983; Wiltshire, 1992; Wiltshire, 1993). As the top layers of the sediment represent a zone with such remarkably strong physicochemical gradients, most benthic microalgae show adaptive diurnal and tidal rhythms of vertical migration, moving in response to light, tide cycles, desiccation, predation and resuspension (Admiraal et al., 1984; Pinckney & Zingmark, 1991; Paterson et al., 1998). The speed at which MPBs migrate vertically is typically low – from 10 to 27 mm h-1 (Hopkins; 1963). Lastly, microscale horizontal gradients in nutrient, irradiance, water content and salinity are often overlaid upon the vertical gradients, in effect combinatorial shaping the growth of MPB communities (Wolff, 1979). In general it is traditionally accepted that the growth of benthic microalgae is most probably not limited by nutrients, since nutrient concentrations in the interface water are usually high (Cadée & Hegemann, 1974; Admiraal, 1984). However, the unusually high concentration of diatoms in the upper layer of sediment may lead to temporary and heterogeneous nutrient depletion (Admiraal, 1977). Nutrient concentrations can differ as much as 10-times between the sediment-water interface and subsurface sediments (Sakamaki et al. 2006, Leynaert et al. 2009). Additionally, tidal oscillation in concentrations of key nitrogenous nutrients, particularly ammonium and nitrate in intertidal sediments are well known (Kuwae et al. 2003, Sakamaki et al. 2006). Therefore, in spite of having an overall nutrient-rich habitat, MPB could be effectively subjected to significant fluctuations in nutrient availability (Ni Longphuirt, 2009).
Seasonal variations in MPB biomass are well documented (Sullivan & Moncreiff, 1988; Cahoon & Cooke, 1992). Usually a single peak occurring in late winter or early spring characterizes their annual biomass dynamics. This peak is believed to be triggered by high nutrient concentration, increasing temperature and increasing day length coincident with the phytoplankton spring bloom in the water column. On the other hand, a decrease in biomass in late spring is generally attributed to increased grazing pressure as opposed to decreased production . In summer microphytobenthic biomass not only decreases, the MPB community also undergoes changes in composition such that the dominance of the diatoms is eroded and Cyanobacteria and Euglenophytes coexist. The shift in the community composition is most probably linked to the decrease in silicon concentration in the overlying water (Barranguet, 1997).
Primary production of MPB:
MPB production is crucial and particularly relevant in shallow water systems including intertidal and subtidal marine ecosystems (MacIntyre, 1996; Underwood, 1999). The characteristic of benthic productivity is its direct coupling with the pelagic system – this renders benthic production susceptible to disturbances such as wind forcing and evaporation (Molen, 2011). Strong wind can resuspend MPB in the water column, generating a scenario in which MPB contributes to pelagic production (MacIntyre, 2007). The major factors affecting MPB production are parameters such as salinity, irradiance, temperature, DIN/DIP ratio, etc (Longphuirt, 2007). Globally MPB may contribute to (8.9 – 14.4 Gt C m-2 yr-1) 20% of ocean’s production and specifically subtidal MPB on continental shelves account for about 42% of total benthic primary production (Nelson, 1999; Cahoon, 1999). It is well known that in several shallow water strong-current systems, phytoplankton production is lower than that of benthic microalgae (Underwood, 1999). For example, on the pacific coast of the USA, Puget Sound’s coarse sandy sediments have annual net benthic and pelagic primary production of 676 and 649 g C m-2 yr-1 respectively (Thom and Albright, 1990). Studies in the temperate zone demonstrated values for annual primary productivity as high as 892 g C m-2 yr-1 and hourly production rates matching 0.8 g C m-2 hr-1 (Hargrave, Prouse, Phillips, & Neame, 1983). Generally in temperate coastal regions as a rule of thumb, up to 20% of primary production can stem from the MPB. Examples are the Bay of Brest of Franceor where benthic primary production is estimated to be 57-111 mg C m-2 day-1, which is 12-20% of total productivity (Longphuirt, 2007) or, in Weeks Bay, Alabama, USA, where benthic production was estimated at 90 g C m-2 yr-1 which is roughly 21% of total system production (Schreiber & Pennock; 1995). Production rate estimations for benthic microalgae in tropical waters are generally even higher – with annual values topping 3760 g C m-2 hr-1 (Hawkins & Lewis, 1982). As an example, MPB production in a Gulf of Mexico seagrass-dominated coastal ecosystem was estimated as 339 g C m-2 hr-1, which is ten times higher than that of the in this study investigated temperate Bay of Brest (Daehnick, Sullivan, & Moncreiff, 1992; Moncreiff, Sullivan, & Daehnick, 1992).
Table of contents :
Chapter 1 General introduction
A. Ecological importance coastal ecosystem
B. Phytoplankton
C. Microphytobenthos
D. Bay of Brest
E. Objectives
Chapter 2 Comparative dynamics of pelagic and benthic micro-algae in a coastal ecosystem
Abstract
Introduction
Materials and methods
Results
Discussion
Conclusion
Acknowledgements
References
Chapter 3 Primary production and photosynthetic performance of subtidal benthic microalgae in a North Atlantic coastal ecosystem
Abstract
Introduction
Materials and methods
Results
Discussion
Conclusion
Acknowledgements
References
Chapter 4 Temporal distribution of pelagic and benthic microalgae in a temperate coastal ecosystem
Abstract
Introduction
Materials and methods
Results
Discussion
Conclusion
Acknowledgements
References
Chapter 5 General conclusion
Bibliography
