Measurements ofSeasonal RatesandAnnualBudgets of Organic CarbonFluxes inan Antarctic Coastal Environment atSignyIsland, SouthOrkneyIslands, Suggest a Broad Balance between Production andDecomposition

We report herethefirst comprehensive seasonal studyofbenthic microbial activity inan Antarctic coastal environment. Measurements were madefromDecember 1990toFebruary 1992ofoxygen uptake andsulfate reduction byinshore coastal sediments atSignyIsland, SouthOrkneyIslands, Antarctica. Fromthese measurements therateofbenthic mineralization oforganic matterwas calculated. Inaddition, boththe deposition rateoforganic matter tothebottom sediment andtheorganic carbon content ofthebottom sediment were measured during thesame period. Organic matterinput tothesediment was small underwinter ice cover, andthebenthic respiratory activity andtheorganic content ofthesurface sediment declined during this period as available organic matterwas depleted. On an annualbasis, about32% ofbenthic organic matter mineralization was anoxic, buttheproportion ofanoxic compared withoxicmineralization increased during thewinter asorganic matter was increasingly buried bytheamphipod infauna. Freshorganic input occurred astheseaicemelted andicealgae biomass sedimented ontothebottom, andinput was sustained during the spring after icebreakup bycontinued primary production inthewatercolumn. Thebenthic respiratory rate andbenthic organic mattercontent correspondingly increased towards theendofwinter withtheinput ofthis fresh organic matter. Therates ofoxygenuptake during thesouthern summer (80to90mmol of02m-2day-') were ashigh asthose reported forother sediments atmuchhigher environmental temperatures, andtheannual mineralization oforganic matter was equally high(12molofC m-2year-'). Seasonal variations ofbenthic activity inthis antarctic coastal sediment wereregulated bytheinput andavailability oforganic matterandnot byseasonal watertemperature, which was relatively constant atbetween -1.8and0.5°C. We conclude that despite thelowenvironmental temperature, organic matterdegradation broadly balanced organic matter production, although there may besignificant interrannual variations inthesourcesoftheorganic matterinputs. TheAntarctic region traditionally hasbeenconsidered a region ofhighmarineproductivity, although that conception may havebeendistorted bymeasurementsofhighproductivity ratesduring theshort southern summers. Mostworkersconsider theproductivity oftheSouthern Oceantobein theregion of16g ofC m- year-' (6,11), buttheproductivity ofinshore watersmay bemuchgreater thanthis. The phytoplankton primary production ofinshore watersat SignyIsland, SouthOrkneyIslands, hasbeenshowntovaxy between86and289g ofC m-2 year-1(12,30)inthree different years.Thesevalues arecomparable toproduction inrichupwelling areas,butthereare largeinterannual variations (2).In a stable system,production must be balanced byequivalent organic matterremoval, either by degradation or byexport. Antarctic marineandfreshwater environments contain large andactive populations ofmicroflora whichcontain allthefunctional groupsofmicroorganismsthatoccur inmore temperate regions (5,9,10),but there iscomparatively little information abouttheir dynamicsor thebudgetary significance oftheir activities (3,4). Intheinshore marineenvironment, bottomsediments are important sites oforganic matterdegradation andrecycling ofnutrients backtothewatercolumn(16,20).Organotrophicactivity inmarinebottomsediments islargely driven by oxicrespiratory processesandbyanoxicsulfate respiration