The Ecology of Tidal Freshwater Marshes of the - USGS National ...

govern the seasonal blooms and successionsof phytoplankton populations. In undisturbedtidal freshwater locales this successionalperiodicity is fairly constantfroin year to year; however, biotic transitionsmay be muted in southern Atlanticcoastal regions where cl iniatic changes areless drastic (Sandifer et al. 1980).General izations concerni ng seasonalabundances and periodi cities of phytoplanktersin fresh water are difficult tonake, especial ly if unnatural nutrientloading occurs in the estuary (Wetzel1975). Dne of the feu existing quantitativeassessments of tidal freshwater phytoplanktoncommunities was coinpiled forthe Potomac River in Virginia and rlaryland(Lippson et al. 1979). These algal populationswere largely characterized by (1)species of green a1 gae (Chlorophytes)which are moderate to high in abundanceyear around, (2) diatoms (8acill ariophytes), which are extremely prevalent inall seasons except nidsuminer and earlyfall, and (3) moderate numbers of bluegreenphytoplankters (Cyanophytes) predortiinatingin the summer and fa1 1 vonths.Chlorophyta account for as much asone-third of the total tidal freshwaterphytoplankton communi ty in the Potomac.Over 100 species have been recorded fromthis area with no single species dorninating.J. Fourqurean and D. Childers(Depa rtnient of Environmental Sciences3niversi ty of Virginia, Cliarlottesville;pers. comm.) found that deslnids and filainentousChlorophytes comprised over 50percent of a Virginia tidal freshwaterpiiytopl ankton community in late fa1 1.Species conimonlv found in both of thesestudies inclu-de ilicractinius spp.,Pedl astrum spp., Scenedesmus spp., Spi rogyraspp., and Microspora spp.The rnost ubiquitous and abundant ofa1 1 phytopl ankters are the Saci11 ariop!iytes.Many of these species are actiiallyepibenthic algae which becomeentrained in the water coluinn via tidalcurrents. Peak diatom biomass oftenexceeds one mill ion cells per literjiippson et al. 1979); however, no informationis available concerning the generamost consistently encountered in tidalfresh waters.The remaining phytopl anktonic componentsinclude various Cyanophytes, euglenoids,and di nof 1 age1 1 ates. Freshwaterspecies of bl ue-green a1 gae are stronglyinhibited by salinities greater than a fewparts per thousand and generally do notexceed densities over one-hundred thousandcells per liter. Conmon aenera"includeAnabaena, Anacys ti s and Osci 11 i toria.Populations of eual enoids are transient.and occur only d;ring midsuminer in thePotomac. nensities do not exceed 10,000cells per liter. Durins late suminer themost ?revalent genera are Euglena and z-chelmonas. The dinoflagellate, Peridini-- urn, was found to be an abundant consti tuentof phytoplankton communities in theJames River in Virginia (Fourqurean andChilders, pers. coam.).Benthic/Mud A1 qaeEpibenthic alqae grow wholly or Dartiallysubmerged on a variety of surfaces.They occur as microscopic unicells orlarger colonial forms, with many speciesresiding only temporarily on the benthos.Planktonic forms commonly settle onto Senthicor marsh surface substrates to completethe reproductive or resting stagesof their life cycles. The benthic algalcommunities of tidal freshwater marshesare not well studied, and like phytoplankton,are sub.ject to complex blooms andsuccessions which make it difficult togive general demographic descriptions.The limi ted information available suqgeststhat Cyanophytes, Bacil lari ophytes, . andChl orophytes dominate epibenthic algalcommunities' tidal freshwater habitats.Many of these communi ties are comprised ofriverine species which are intolerant ofsal ine conditions.Summer communities in tidal freshwaterportions of the Potomac River aretypified by the bl ue-green a1 gaeShizothri x spp. and Chromul ina paschreiand by green algae such as Cosmarium sop.and Clostreium spp. (Lippson et al. 1979).In contrast, 1 ate fa1 1 benthic algalcounts in a tidal freshwater tributary ofthe James River showed centric (Cycl otell asp., Ste hanodiscus sp., Coscinodiscus---I- pennatesp.) andNavicul a1 es sp. ) diatomsto be the dominant community constituents(Fourqurean and Childers, pers. comm.).