Phytoplankton are free-floating microscopic plants that are mostly unicellular and produce chemical energy from light. This process is called primary production. Phytoplankton have a critical role in primary production, nutrient cycling, and food webs and make up a significant proportion of the primary production in aquatic systems (Dawes 1998). In many coastal systems, primary production is almost entirely a function of the phytoplankton. Even in salt marsh estuaries, where vascular plant biomass can greatly exceed that of algae, phytoplankton can contribute substantially to overall primary production (Sandifer et al. 1980; Lewitus et al. 1998).
Primary productivity is the rate at which solar energy is converted into chemical energy by photosynthetic and chemosynthetic organisms (Dillon and Rodgers 1980), and is usually expressed as grams of carbon fixed per unit area per unit of time (Dawes 1998). Dardeau et al. (1992) reported that annual phytoplankton primary production in southeastern estuaries ranges from 67-375 grams of carbon per square meter per year (gC/m2/year) while Verity et al. (1993) reported values of 600-700 gC/m2/year for Wassaw Sound in Georgia. These primary productivity values are very high compared to other ecosystems. For example, the primary productivity of a rice field has been reported to be 4.0 gC/m2/year (Dawes 1998).
Phytoplankton are classified as microalgae and include species from the following divisions: Cyanobacteria (blue-green algae), Chlorophyta (green algae), Prochlorophyta, Euglenophyta, Pyrrhophyta (dinoflagellates ), Cryptophyta (cryptomonads), Chrysophyta, and Bacillariophyta (includes diatoms). Most phytoplankton are motile, however, movement in the water column is mostly through transport by currents (Dawes 1998; Sandifer et al. 1980). Phytoplankton are usually grouped according to cell size. Picoplankton are the smallest and are identified as phytoplankton <2 micrometers (µm) in diameter. Nanoplankton are intermediate sized microalgae and range in size from 2-20 µm. Microplankton are the largest phytoplankton and include those algae >20 µm in diameter.
Microplankton are made up mostly of diatoms and dinoflagellates. Nanoplankton are dominated by phytoplankton with flagellas (e.g. cryptophytes and chrysophytes, and prymnesiophytes). These plankters can account for 75% of the total primary productivity of a system (Dawes 1998). Picoplankton are mostly prokaryotes such as cyanobacteria and prochlorophytes as well as several eukaryotic alga species. These tiny phytoplankters can account for as much as 50% of the primary production in oceanic waters (Dawes 1998).
Phytoplankton must be in the photic zone to photosynthesize. They rely on many different adaptations to move into or remain there. Non-motile phytoplankton rely on physical factors (water viscosity, convection cells, and wind- induced rotations), morphological features (branching frustules, colony formation, and bladder- like or needles-like cell shape), and physiological adaptations (production of mucilage and accumulation of lighter ions with a reduction of heavier ions or compounds) to reduce sinking rates (Dawes 1998). Cyanobacteria contain gas vacuoles that act like flotation and motile species can swim toward light.
Phytoplankton are the food source for numerous other organisms, especially the zooplankton. Zooplankton grazers can significantly decrease phytoplankton density. For example, at a grazing rate of 20%, zooplankton can decrease phytoplankton populations by approximately 75% (Dawes 1998). As with other factors which affect phytoplankton production, the effect of grazers is seasonal. Because grazers decline in winter, there is a lag in the spring before grazers become effective in controlling the spring bloom. Phytoplankton growth and productivity are affected by several factors which are called limiting factors. These limiting factors include light, temperature, circulation, grazing, and nutrients (Dardeau et al. 1992).
Effect of Light
Phytoplankton experience the greatest productivity when they encounter their optimal light and nutrient conditions. With adequate nutrients, phytoplankton growth and productivity increases with increasing light levels until a certain light level is reached. At this point photosynthesis is at a maximum (Pmax) and further photosynthesis is inhibited with increasing light levels. Phytoplankton can only photosynthesize in the photic zone , which is limited to the maximum depth to which light can penetrate the ocean. Nutrients are often depleted in the photic zone because of utilization by phytoplankton and other organisms. The optimal depth for maximum production is reached when the light limiting effects of vertical mixing are balanced out by the benefits of advection into nutrient-rich bottom waters (Yentsch 1981). Seasonal changes in the length of the day also influence phytoplankton production, especially in mid and high latitudes. Phytoplankton abundance and biomass greatly increase during the summer months in higher latitudes because of the increased amount of light.
Effect of Temperature
Effect of Nutrients
Of the three other inorganic compounds (silicon, nitrogen, and phosphorous), nitrogenous compounds have the lowest concentrations in oceanic waters and are generally thought to limit phytoplankton growth in marine waters. Phosphate is the most biologically available form of phosphorous for most phytoplankton. This nutrient is found in low concentrations in marine systems, and in some cases, may be a significant limiting factor in phytoplankton growth. Finally, silicon, which is used in the cell structure of diatoms and silicoflagellates, usually has concentrations higher than nitrogen and phosphorous. During diatom blooms, however, the bioavailability of silicon may be severely lowered (Dawes 1998).
Eutrophication is an increase in the nutrient level in an aquatic system. As nutrient levels rise, growth of phytoplankton is no longer nutrient-limited and algal blooms occur. If the blooming algae produce toxic chemicals, fish kills and adverse human health effects can occur. If the algae dont produce toxins, the ecosystem can still be affected because phytoplankton respiration, zooplankton grazing, and bacterial decomposition can act to deplete dissolved oxygen to undesirable levels.
Phytoplankton composition changes with nutrient fluxes because individual taxa have different requirements (Tilman 1977; Kilham and Kilham 1984). Enclosure experiments have confirmed that nutrient enrichment alters the community composition of phytoplankton (Goldman and Stanley 1974; Sanders et al. 1987). Field studies report similar occurrences. In Moriches and Great South Bays (NY), for example, extremely dense populations of chlorophytes and cyanobacteria developed in waters fertilized by effluent from adjacent duck farms. These blooms coincided with collapse of the oyster fishery (Ryther 1954). Similar occurrences of dense concentrations of phytoflagellates in other eutrophic waters (e.g., Mahoney and McLaughlin 1977) imply that elevated nutrient concentrations alter phytoplankton community composition toward prevalence of smaller "less desirable" species (Smayda 1983; Verity 1998).
However, dominance by flagellates is not exclusively predicated on nutrient loading. The availability of dissolved silicate may be equally or more important, because diatoms are dependent upon it while the vast majority of non-diatoms are not. Introduced by Schelske and Stoermer (1971, 1972), the hypothesis that silicate availability regulated phytoplankton community composition was formalized by Officer and Ryther (1980) and Ryther and Officer (1981). These studies describe two types of planktonic ecosystems, one dominated by diatoms, and another by flagellates and other small non-motile cells. They proposed that the diatom food web was associated with extensive fisheries (and therefore was preferable), while the flagellate food web was undesirable because it coincided with higher trophic levels which were not economically useful, or it was associated with hypoxia. While details are still being resolved, e.g. all diatoms are not apparently of equal value as food resources for zooplankton (Ianora et al. 1995) or necessarily better than equivalent-sized flagellates or ciliates (Verity and Paffenhfer 1996), mesocosim studies support the relationship between silicate availability, diatoms, and fish (Doering et al. 1989). (Verity 1998).
Silicate concentrations in major southeastern rivers, in contrast to nitrogen, have been constant or declining during the past 25 years (Windom et al. 1993). If land use patterns result in nitrogen and phosphorus loading while silicate delivery to southeastern estuaries declines, diatom- dominated ecosystems may be replaced by flagellate-dominated ecosystems. This same pattern of changing phytoplankton community composition is also hypothesized to occur in the presence of pollutants (Greve and Parsons 1977), and was observed in mesocosim experiments with additions of trace metals (Cu, Hg) and hydrocarbons (Dunstan et al. 1975; Gray 1982). Thus, increased land use can be expected to produce three broad impacts: eutrophication, shifting nutrient ratios, and introduction of contaminants. These changes have been associated with shifts from diatom- to flagellate-dominated ecosystems. If such land use impacts occur in South Carolina and Georgia watersheds, a concurrent shift in ecosystems food chain dynamics may occur (Greve and Parsons 1977). Increasing occurrence and spread of novel phytoplankton blooms with nutrient loading and shifts in N:Si ratios may also occur (Smayda 1989). Blooms of toxic dinoflagellates that can cause major fish kills have already been reported in estuarine waters of North Carolina and Florida (Burkholder et al. 1992, 1995; Lewitus et al. 1995). Thus, phytoplankton community composition can both directly and indirectly affect ecosystem structure and its harvestable resources (Verity 1998).
Few studies on marine phytoplankton have been conducted in the regions in or surrounding the ACE Basin study area. The National Oceanic and Atmospheric Administrations (NOAA) Estuarine Eutrophication Survey assessed the status and trends of phytoplankton in twelve estuarine systems in Georgia and South Carolina (Verity 1998). This survey was based on qualitative responses from survey participants and are not always based on detailed data collection. Of the twelve estuaries examined, the Stono/North Edisto River system and the St. Helena Sound system are located within the ACE Basin study area (NOAA 1996). This study reported that the phytoplankton community for both estuaries was dominated by diatoms. Chlorophyll a concentrations, which is a measure of phytoplankton concentrations, were unknown in the Stono/North Edisto rivers, but in St. Helena Sound, they were reported to be < 5 microgram per liter (µg/L) for areas with salinity >0.5 parts per thousand (ppt) (NOAA 1996).
Although no other studies are known from the ACE Basin study area, studies in other regions of the southeast can be used to estimate the phytoplankton community in the ACE Basin region. Verity et al. (1993) examined the species composition and productivity of phytoplankton in Wassaw Sound, Georgia. They reported that, during the summer, flagellated nanoplankton (2-4 µm) were the numerical dominants and diatoms dominated the biomass (Dominant phytoplankton in Wassaw Sound, GA ). During the winter, the flagellated nanoplankton Katodinium rotundatum dominated both biomass and abundance. However, a diatom bloom developed each January during the three-year study. Verity et al. (1993) also reported that chlorophyll a concentrations ranged from 3.2-6.3 µg/L over the three-year study with the highest concentrations occurring inshore and during the summer.
Davis and Van Dolah (1992) examined the phytoplankton community in Charleston Harbor, South Carolina. This study identified 451 species of phytoplankton including 170 diatoms, 152 chlorophytes, 48 dinoflagellates, 36 cyanobacteria, 29 euglenophytes, 10 chrysophytes, and 6 cryptomonads in the harbor. They found that the diatom, Skeletonema costatum, along with three other diatoms, and one cyanobacterium were the dominant species (Phytoplankton species in Charleston Harbor ). Davis and Van Dolah also reported that seasonal trends occurred in species composition and abundance. Diatoms dominated the spring and early fall periods while cyanobacteria and flagellates dominated the summer and winter periods. Highest phytoplankton abundances (~ 7,000 cell/ml) occurred in May-June with a smaller peak in abundance in February.
The importance of nano- and picoplankton to primary production in southeastern estuaries was emphasized by Lewitus et al. (1998), who examined the phytoplankton community of North Inlet, South Carolina. They found that picoplankton were the numerically dominant phytoplankton throughout the entire year, and that nanoflagellates contributed substantially to phytoplankton abundance and biomass during the summer bloom. The cyanobacteria Synechococcus spp. composed 95-100% of the phototrophic picoplankton. Other dominant species included the diatoms Cylindrotheca closterium (microplankton), Nitzschia spp. (nanoplankton), and Thalassiosira spp. (nanoplankton). Chlorophyll a concentrations were highest in July at 15 µg/L.
Just like many other aquatic organisms, phytoplankton species distributions are controlled by salinity. So far, this discussion has centered on marine phytoplankton. Freshwater primary producers include members from the same divisions and are affected by the same limiting factors as marine phytoplankton (i.e. light, temperature, circulation, and nutrients). Primary producers in the static environment of freshwater lakes and ponds are mostly phytoplankton, however, in the more dynamic flowing water conditions of freshwater creeks and rivers, attached algae (i.e. periphyton ) are the dominant organisms. To our knowledge, no studies exist which characterize freshwater phytoplankton or periphyton in the ACE Basin study area.
Grant (1974) found that diatoms were the dominant organisms in the upper reaches of the Cooper River-Tailrace Canal system in South Carolina. Molley et al. (1976) examined the freshwater riverine regions surrounding Ocala, Florida. They reported that centric diatoms (i.e. having surface markings radially arranged) were the dominant organisms with blue-green algae such as Chroococcus spp., Microcystis spp., Spirulina spp., and Anabeana spp. also abundant. Camburn et al. (1978) examined the benthic diatom community of Long Branch Creek, South Carolina. They reported 268 diatom taxa with Eunotia spp., Achnanthes spp., Navicula spp., Pinnularia spp., Gomphonema spp., and Nitzschia spp. being the numerical dominants.
Zingmark (1975) examined the phytoplankton community in freshwater ponds on Kiawah Island, South Carolina . He found that the cyanobacteria, Oscillatoria spp., made up approximately 50% of the cells counted. Other common phytoplankton species included Crucigenia irregularis, Microcoleus spp., Anabaena spp., Anacystis cyanea, Tricodesmium spp., and Merismopedia spp. In all, Zingmark reported 27 species as being common in these ponds (Phytoplankton in freshwater pond samples from Kiawah Island,SC ). Goldstein and Manzi (1976) examined the freshwater phytoplankton in two ponds in South Carolina. This study identified 259 phytoplankton and periphyton species. Of these, 146 were Chlorophyta, 11 were Pyrrhophyta, 46 were Cyanobacteria, and 45 were Chyrsophyta.
Most phytoplankton have the potential to bloom. Usually environmental (e.g. nutrient depletion) and biological (e.g. grazing) factors are sufficient to limit phytoplankton populations from attaining exceptionally high densities. If, however, these controlling factors are eliminated, blooming phytoplankton can have environmentally detrimental effects either by causing oxygen depletion or toxic poisoning. Oxygen depletion effects occur when respiration by blooming phytoplankton (usually non-toxic species) and by other organisms feeding on the phytoplankton decrease oxygen to low enough levels to cause animal mortalities. Toxic poisoning effects are caused by only a few dozen phytoplankton species and most of these are dinoflagellates, prymnesiophtyes, chloromonads, or diatoms.
The best known toxic algal blooms are those produced by dinoflagellates. Some of these blooms are called red tides because of the red or rust color of the water caused by high concentrations of phytoplankton cells. Some red tides are believed to begin with the germination of dormant cells that settled onto the seafloor during times of nutrient scarcity (Anderson 1994). When conditions improve these cells germinate and bloom. Toxic red tides are often grouped into the following categories: (1) those that kill primarily fish (e.g. Gymnodinium sp.); (2) those that kill invertebrates (e.g. Gonyaulax sp.); and (3) those that dont kill but produce toxins that are sequestered by bivalves (e.g. Protogonyaulax sp.) or fish (e.g. Gambierdiscus toxicus).
Toxins can kill fish and invertebrates directly. For example, in the Gulf of Mexico, the dinoflagellate, Gymnodinium breve, causes massive fish kills by releasing neurotoxins into the gills of nearby fish causing asphyxiation (Anderson 1994). Algal toxins can also cause mortality as they are accumulated in the food web. For example, thousands of herring in the Bay of Fundy died after eating planktonic snails which had consumed the toxic dinoflagellate Alexandrium sp. Toxins bioaccumulated by bivalves can cause paralytic shellfish poisoning (PSP), diarrhetic shellfish poisoning (DSP), neurotoxic shellfish poisoning (NSP), or amnesic shellfish poisoning (ASP) when these contaminated bivalves are consumed by humans. Humans can develop ciguatera fish poisoning (CFP) from eating fish which have consumed the toxic dinoflagellate, Gambierdiscus toxicus.
Recently, a heterotrophic dinoflagellate, Pfiesteria piscicida, has been identified as the causative agent in numerous major fish kills in the southeastern United States (Burkholder et al. 1995). These fish kills occur when dormant benthic cysts of the dinoflagellate encounter chemical cues from nearby fish. The cysts release zoospores which produce an exotoxin that anaesthetizes the fish, causes shedding of the fish epidermis, and produces open ulcerative lesions. The dinoflagellates consume the fish epidermis. The fish eventually dies and the dinoflagellates once again become non-toxic (Burkholder et al. 1995). The presence of Pfiesteria or Pfiesteria-like species has been confirmed at selected, sudden-death fish-kill sites from Delaware to Alabama (Burkholder et al. 1995).
Outbreaks of toxic algal blooms have been reported in many regions of the United States. They are, however, either unknown or thought to have no impacts on resources in any Georgia or South Carolina estuaries. It is generally thought that the rigorous vertical mixing imposed by high tidal amplitudes minimizes the potential development of blooms or high concentrations of the flagellated phytoplankton species generally associated with nuisance or toxic occurrences. In contrast, blooms of the toxic heterotrophic dinoflagellate Pfiesteria are reported to occur in North Carolina and Florida estuaries, where tidal amplitudes are considerably smaller (Verity 1998).
L. Zimmerman, SCDNR Marine Resources Research Institute
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