Characterization of legacy organic carbon in a culturally eutrophic lake : the role of the historic carbon deposition in the time course and extent of lake recovery

The time course of lake recovery after a reduction in external loading of nutrients is often controlled by conditions in the sediment. Remediation of eutrophication is hindered by the presence of legacy organic carbon deposits, that exert a demand on the terminal electron acceptors of the lake and contribute to problems such as internal nutrient recycling, absence of sediment macrofauna, and flux of toxic metal species into the water column. Being able to quantify the timing of a lake’s response requires determination of the magnitude and lability, i.e., the susceptibility to biodegradation, of the organic carbon within the legacy deposit. This characterization is problematic for organic carbon in sediments because of the presence of different fractions of carbon, which vary from highly labile to refractory. The lability of carbon under varied conditions was tested with a bioassay approach. It was found that the majority of the organic material found in the sediments is conditionally-labile, where mineralization potential is dependent on prevailing conditions. High labilities were noted under oxygenated conditions and a favorable temperature of 30 °C. Lability decreased when oxygen was removed, and was further reduced when the temperature was dropped to the hypolimnetic average of 8° C . These results indicate that reversible preservation mechanisms exist in the sediment, and are able to protect otherwise labile material from being mineralized under in situ conditions. The concept of an active sediment layer, a region in the sediments in which diagenetic reactions occur (with nothing occurring below it), was examined through three lines of evidence. Initially, porewater profiles of oxygen, nitrate, sulfate/total sulfide, ETSA (Electron Transport System Activity- the activity of oxygen, nitrate, iron/manganese, and sulfate), and methane were considered. It was found through examination of the porewater profiles that the edge of diagenesis occurred around 15-20 cm. Secondly, historical and contemporary TOC profiles were compared to find the point at which the profiles were coincident, indicating the depth at which no change has occurred over the (13 year) interval between core collections. This analysis suggested that no diagenesis has occurred in Onondaga Lake sediment below a depth of 15 cm. Finally, the time to 99% mineralization, the t99, was viewed by using a literature estimate of the kinetic rate constant for diagenesis. A t99 of 34 years, or approximately 30 cm of sediment depth, resulted for the slowly decaying carbon fraction. Based on these three lines of evidence , an active sediment layer of 15-20 cm is proposed for Onondaga Lake, corresponding to a time since deposition of 15-20 years. While a large legacy deposit of conditionally-labile organic material remains in the sediments of Onondaga Lake, it becomes clear that preservation, mechanisms that act to shield labile organic carbon from being degraded, protects this material from being mineralized and exerting a demand on the terminal electron acceptors of the lake. This has major implications for management of the lake, as it defines the time course of lake recovery following a reduction in nutrient loading.

DEVELOPMENT OF A CONCEPTUAL MODEL AND PARAMETERIZATION OF A MATHEMATICAL MODEL FOR SEDIMENT PHOSPHORUS DIAGENESIS, SORPTION AND RELEASE FROM ONONDAGA LAKE SYRACUSE, NEW YORK

Eutrophication is a persistent problem in many fresh water lakes. Delay in lake recovery following reductions in external loading of phosphorus, the limiting nutrient in fresh water ecosystems, is often observed. Models have been created to assist with lake remediation efforts, however, the application of management tools to sediment diagenesis is often neglected due to conceptual and mathematical complexity. SED2K (Chapra et al. 2012) is proposed as a "middle way", offering engineering rigor while being accessible to users. An objective of this research is to further support the development and application SED2K for sediment phosphorus diagenesis and release to the water column of Onondaga Lake. Application of SED2K has been made to eutrophic Lake Alice in Minnesota. The more homogenous sediment characteristics of Lake Alice, compared with the industrially polluted sediment layers of Onondaga Lake, allowed for an invariant rate coefficient to be applied to describe first order decay kinetics of phosphorus. When a similar approach was attempted on Onondaga Lake an invariant rate coefficient failed to simulate the sediment phosphorus profile. Therefore, labile P was accounted for by progressive preservation after burial and a rate coefficient which gradual decreased with depth was applied. In this study, profile sediment samples were chemically extracted into five operationally-defined fractions: CaCO3-P, Fe/Al-P, Biogenic-P, Ca Mineral-P and Residual-P. Chemical fractionation data, from this study, showed that preservation is not the only mechanism by which phosphorus may be maintained in a non-reactive state in the profile. Sorption has been shown to contribute substantially to P burial within the profile. A new kinetic approach involving partitioning of P into process based fractions is applied here. Results from this approach indicate that labile P (Ca Mineral and Organic P) is contributing to internal P loading to Onondaga Lake, through diagenesis and diffusion to the water column, while the sorbed P fraction (Fe/Al-P and CaCO3-P) is remaining consistent. Sediment profile concentrations of labile and total phosphorus at time of deposition were also modeled and compared with current labile and total phosphorus, to quantify the extent to which remaining phosphorus which will continue to contribute to internal P loading and influence the trophic status of Onondaga Lake. Results presented here also allowed for estimation of the depth of the active sediment layer and the attendant response time as well as the sediment burden of labile P and associated efflux.

Stable isotope response to lake eutrophication: Calibration of a high-resolution lacustrine sequence from Baldeggersee, Switzerland

Stable isotope analyses of discrete seasonal layers from a 108-yr annually laminated freeze-core from Baldeg-gersee, a small, eutrophic lake in central Switzerland, provide information on the climatological and environmental factors, including lake eutrophication, that control oxygen and carbon isotopic composition of epilimnic biologically induced calcite precipitate. During the last 100 yr, Baldeggersee has undergone major increases in productivity and eutrophication in response to nutrient loading from agriculture and industrialization in the lake's watershed. Calibration of the isotopic signal in Baldeggersee to historical limnological data quantitatively links evidence of isotopic depletion in the sedimented calcite to trophic state of the lake. δ18O values from the spring/summer “light” sediment layers steadily diverged to more depleted values in response to historical eutrophication: measured δ18O values were up to 21.5‰ more negative than calculated equilibrium δ18O values. Evidence for 13C depletion in the calcite, relative to equilibrium values, is more difficult to ascertain because of an overall dominance of isotopic enrichment in the dissolved inorganic pool as productivity in Baldeggersee increases. A positive association exists between the degree of oxygen-18 depletion and the calcite crystal size. Thus, large amorphous calcite grains can be used as a proxy for recognizing apparent isotopic nonequilibrium in sediment sequences from highly productive lacustrine environments from all geologic time scales. In contrast to the light layers, the oxygen isotopic composition of the calcite in the late summer/fall “dark” sediment layers is unaffected by the apparent isotope nonequilibrium. Oxygen and carbon isotope values from the dark laminae in the Baldeggersee sediment therefore provide environmental and climatological proxies that can be calibrated with known environmental and regional climate data for the last century.

Ecosystem regimes and responses in a coupled ancient lake system from MIS 5b to present: the diatom record of lakes Ohrid and Prespa

We reconstruct the aquatic ecosystem interactions since the last interglacial period in the oldest, most diverse, hydrologically connected European lake system, by using palaeolimnological diatom and selected geochemistry data from Lake Ohrid “DEEP site” core and equivalent data from Lake Prespa core, Co1215. Driven by climate forcing, the lakes experienced two adaptive cycles during the last 92 ka: "interglacial and interstadial" and "glacial" cycle. The short-term ecosystems reorganizations, e.g. regime shifts within these cycles substantially differ between the lakes, as evident from the inferred amplitudes of variation. The deeper Lake Ohrid shifted between ultra oligo- and oligotrophic regimes in contrast to the much shallower Lake Prespa, which shifted from a deeper, (oligo-) mesotrophic to a shallower, eutrophic lake and vice versa. Due to the high level of ecosystem stability (e.g. trophic state, lake level), Lake Ohrid appears relatively resistant to external forcing, such as climate and environmental change. Recovering in a relatively short time from major climate change, Lake Prespa is a resilient ecosystem. At the DEEP site, the decoupling between the lakes' response to climate change is marked in the prolonged and gradual changes during the MIS 5/4 and 2/1 transitions. These response differences and the lakes' different physical and chemical properties may limit the influence of Lake Prespa on Lake Ohrid. Regime shifts of Lake Ohrid due to potential hydrological change in Lake Prespa are not evident in the data presented here. Moreover, a complete collapse of the ecosystems functionality and loss of their diatom communities did not happen in either lake for the period presented in the study.

Lake metabolism and the diel oxygen technique: State of the science

Significant improvements have been made in estimating gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) from diel, “free-water” changes in dissolved oxygen (DO). Here we evaluate some of the assumptions and uncertainties that are still embedded in the technique and provide guidelines on how to estimate reliable metabolic rates from high-frequency sonde data. True whole-system estimates are often not obtained because measurements reflect an unknown zone of influence which varies over space and time. A minimum logging frequency of 30 min was sufficient to capture metabolism at the daily time scale. Higher sampling frequencies capture additional pattern in the DO data, primarily related to physical mixing. Causes behind the often large daily variability are discussed and evaluated for an oligotrophic and a eutrophic lake. Despite a 3-fold higher day-to-day variability in absolute GPP rates in the eutrophic lake, both lakes required at least 3 sonde days per week for GPP estimates to be within 20% of the weekly average. A sensitivity analysis evaluated uncertainties associated with DO measurements, piston velocity (k), and the assumption that daytime R equals nighttime R. In low productivity lakes, uncertainty in DO measurements and piston velocity strongly impacts R but has no effect on GPP or NEP. Lack of accounting for higher R during the day underestimates R and GPP but has no effect on NEP. We finally provide suggestions for future research to improve the technique.

Geochemistry, chlorin concentrations and diatom abundance in sediments of lake Rotsee, Switzerland

The effects of eutrophication on short term changes in the microbial community were investigated using high resolution lipid biomarker and trace metal data for sediments from the eutrophic Lake Rotsee (Switzerland). The lake has been strongly influenced by sewage input since the 1850s and is an ideal site for studying an anthropogenically altered ecosystem. Historical remediation measures have had direct implications for productivity and microbial biota, leading to community composition changes and abundance shifts. The higher sewage and nutrient input resulted in a productivity increase, which led predominantly to a radiation in diatoms, primary producers and methanogens between about 1918 and 1921, but also affected all microorganism groups and macrophytes between about 1958 and 1972. Bacterial biomass increased in 1933, which may have been related to the construction of a mechanical sewage treatment plant. Biomarkers also allowed tracing of fossil organic matter/biodegraded oil contamination in the lake. Stephanodiscus parvus, Cyclotella radiosa and Asterionella formosa were the dominant sources of specific diatom biomarkers. Since the 1850s, the cell density of methanogenic Archaea (Methanosaeta spp.) ranged within ca. 0.5-1.8 x 10**9 cells/g dry sediment and the average lipid content of Rotsee Archaea was ca. 2.2 fg iGDGTs/cell. An altered BIT index (BITCH), indicating changes in terrestrial organic matter supply to the lake, is proposed.

Talviaikainen happitilanne eräissä Pohjois-Savon järvissä vuosina 1997-2008

Pohjois-Savossa seurattiin talviaikaista happitilannetta vuosina 1997-2008. Seurannan kohteena oli neljä pienehköä järveä (Iso-Valkeinen, Kevätön, Kolmisoppi ja Vehmasjärvi), jotka ovat erityyppisiä syvyydeltään, rehevyystasoltaan ja humuspitoisuudeltaan. Näiden esimerkkijärvien oli tarkoitus antaa yleisemminkin viitteitä happitilanteen kehityksestä talven aikana. Alkutalven tulosten perusteella annettiin vuosittain tiedote, jossa arvioitiin happikatojen mahdollisuutta kevättalven kuluessa. Yleisöllä oli myös mahdollisuus seurata happi- ja lämpötilatuloksia Pohjois-Savon ympäristökeskuksen verkkopalvelun kautta. Hapenkulutusnopeus oli rehevimmässä seurantajärvessä kaksinkertainen verrattuna karumpiin ja syvyyden myötä ero vain korostui. Pohjanläheisessä vesikerroksessa 1 mg/l:n happipitoisuus kului karuissa järvissä noin kahdessa viikossa ja rehevässä noin kolmessa päivässä. Vuosien välinen vaihtelu oli kuitenkin hyvin suurta. Vaihtelu oli suurta myös karuissa järvissä. Veden jäätymisajankohdalla ja veden lämpötilalla on merkittävä vaikutus siihen, millainen kevättalven happitilanteesta muodostui. Seurantajärvien aineiston perusteella voidaan karkeasti arvioida, että kuukauden viivästyminen jäätymisessä tai vesipatsaan viilentyminen ennen jäätymistä asteen verran kylmemmäksi merkitsevät noin kolmanneksen korkeampaa happipitoisuutta kevättalvella. Vesipatsaan happitilanteen heikentymisen sekä ravinne- ja rautapitoisuuksien välillä todettiin vahvat yhteydet. Kaikkien järvien aineistossa happitilanteen heikkeneminen johti voimakkaimmin alusveden ammoniumtyppi-, kokonaisfosfori- ja rautapitoisuuksien kasvuun. Pitoisuusmuutokset olivat talven aikana suurimmat rehevimmässä kohdejärvessä, Kevättömässä, jossa kokonaisfosforipitoisuudet keskimäärin kymmenkertaistuivat, fosfaattifosforipitoisuudet kasvoivat keskimäärin 20-kertaisiksi ja rautapitoisuudet yli seitsenkertaisiksi.

The use of diatom assemblages to determine paleotrophic changes in lakes

The purpose of this study was to develop a classifi cation scheme for l ake trophic status based on the relative abundance of l ake sediment diatom trophic indicator species. A total of 600 diatom frustules were counted from the surface sediments of e a ch of 30 lakes selected to repr e seni~ a continuum from u.lt ra-oligotrophic t,o fairly eutrophic but not hype r-' eutrophic conditions. Published autecological information was used to determine the trophic indicator status of each of the s pecies. A quotieht was derived by dividing the s um of all the e utrophic indicator species by the sum of all oligotrophic indicai.-:.or species. Oligo'- mesotrophic. mesotrophic and meso-eutrophic species were added to both the numerator and denomina tor. Five categories of diatom i.nferred trophic status were recognized : ultra-oligotrophic - 0'-0.2:3, oligotrophic::: 0.24-0 . 70, mesotrophic :: 0.'71 -0.99, meso-elxtrophic :: 1. 00-1. '78 and eutrophic:: 1.. 79-2. 43. But only three of these (oligotrophic:: 0-0.69, mesotrophic ::: 0 . 70'-1.69 j and eutrophic:: 1.70-2.50) proved usef ul. The present study of the relationship between diatom inferred trophic status and the literature-derived trophic status of SO lake s (which were purposely chosen to represent a broad spectrum of lakes in Canada) indicated that: 1) Based on diatom species (assemblages ) it is possible to segregate the lakes from which. th",)se diatoms were taken into three basic categories : o ligotrophic, mesotrophic and eutrophic lake types. ~~) It was not possible t,o separate meso-eutrophic and o l igo-mesotrophic lakes f rom mesotrophic l akes as the the degree of overlap betwee n the diat,om species in these lake types was extremely high. 3) Ha d mo r e ul tra-oligotroph,ic lakes been sampled it might have been possible to more a ccurately s eparate them f rom oligotrophic Jakes. 4 ) Had. more humic lakes been sampled in this s tudy I f eel it would have been possible to identify a unique diatom a ssemblage which would h a ve chara cterized t his lake type . Re gression analyses were performed using the aforementioned diatom inferred trophic index as a f unction of 1) log Sec chi transparency (r = - 0.70) 2) total phosphorus (r = 0. 77 ) and 3) chlorophyll-a (r = 0.74). Once e ach of these rel ationships had been established , it was possible to infer paleotrophic (downcore) changes in an oligotrophic lake (Barbara Lake) and in a eutrophic lake (Chemung Lake) . Barbara Llake was dominated by oligotrophic s pecies and remained oligotrophic throughout the 200-·year history r epresented by i t s 32 em long sediment core. Chemung Lake is currently dominated. by eutrophic species but went through a mesotrophic st,age which was associated with a rise in the water level of the lake followi n g dam construction in its watershed in the early 1.900 J ::;. This was followed by its reversion to it,s present day eutrophic stage (dominated by eutrophic species) possibly as a r esult of shallowing process which can be attributed to " silting' up" of the reservoir and the invasion of the l ake by Myriophyllum spjcatum (Eurasian milfoil) i n the 1970's . In addition, nutrient .:r':l.ch run"'offwhich resulted from increased human a.ctivities associated with cottage development along its shores has contribut ed to its eutrophication. There is some evidence that the rat,e o :f its prog ressive eutrophication has declined during the last decade. This was correlated with legislation enacted in the 60's and 70's in Ontario which was aimed at reducing nutrient loading from cottages.

Storlommens (Gavia arctica) häckningsframgång relaterad till vattenkvalitet

This work investigates the breeding success of the Black-throated Diver, Gavia arctica, related to the water quality in three lakes located in the Vallentuna area, north of Stockholm, Sweden.For more than 30 years, I have studied the bird life around these three and other nearby lakes, and noticed that the Black-throated Diver breeds in some lakes, but not in others.The results confirm that the deep and clear lakes Tärnan and Stora Harsjön have good water quality regarding pH, alkalinity, aluminium and Secchi disk values. As expected, the Black-throated Diver shows breeding success in those two lakes, but does not breed in the shallow and eutrophic lake Mysslingen.

Influência das mudanças climáticas na estrutura funcional da comunidade fitoplanctônica em um reservatório da região do semiárido

The Brazilian Northeast is the most vulnerable region to climatic variability risks. For the Brazilian semi-arid is expected a reduction in the overall rates of precipitation and an increase in the number of dry days. These changes predicted by the IPCC (2007) will intensify the rainfall and droughts period that could promote the dominance of cyanobacteria, thus affecting the water quality of reservoirs, that are most used for water supply, in the semi-arid. The aim of this study was to evaluate the effects of increasing temperature combined with nutrient enrichment on the functional structure of the phytoplankton community of a mesotrophic reservoir in the semi-arid, in the worst case scenario of climate change predicted by the IPCC (2007). Two experiments were performed, one in a rainy season and another in the dry season. In the water sampled, nutrients (nitrate and orthophosphate) were added in different concentrations. The microcosms were submitted to two different temperatures, five-year average of air temperature in the reservoir (control) and 4°C above the control temperature (warming). The results of this study showed that warming and nutrient enrichment benefited mainly the functional groups of cyanobacteria. During the rainy season it was verified the increasing biomass of small functional groups of unicellular and opportunists algae such as F (colonial green algae with mucilage) and X1 (nanoplanktonic algae of eutrophic lake systems). It was also observed an increasing in total biomass, in the richness and diversity of the community. In the dry season experiment there was a greater contribution in the relative biomass of filamentous algae, with a replacement of the group S1 (non-filamentous cyanobacteria with heterocytes) for H1 (filamentous cyanobacteria with heterocytes) in nutrient- enriched treatments. Moreover, there was also loss in total biomass, species richness and diversity of the community. The effects of temperature and nutrients manipulation on phytoplankton community of reservoir Ministro João Alves provoked changes in species richness, the diversity of the community and its functional composition, being the dry period which showed the highest susceptibility to the increase in the contribution of potentially toxic cyanobacteria with heterocytes

(Table 1) Biovolume variations of cyanobacteria and total phytoplankton in the Frederiksborg Slotsso, Denmark

Several studies have shown that submerged macrophytes provide a refuge for zooplankton against fish predation, whereas the role of emergent and floating-leaved species, which are often dominant in eutrophic turbid lakes, is far less investigated. Zooplankton density in open water and amongst emergent and floating-leaved vegetation was monitored in a small, eutrophic lake (Frederiksborg Slotsso) in Denmark during July-October 2006. Emergent and floating-leaved macrophytes harboured significantly higher densities of pelagic as well as plant-associated zooplankton species, compared to the open water, even during periods where the predation pressure was presumably high (during the recruitment of 0+ fish fry). Zooplankton abundance in open water and among vegetation exhibited low values in July and peaked in August. Bosmina and Ceriodaphnia dominated the zooplankton community in the littoral vegetated areas (up to 4,400 ind/l among Phragmites australis and 11,000 ind/l between Polygonum amphibium stands), whereas the dominant species in the pelagic were Daphnia (up to 67 ind/l) and Cyclops (41 ind/l). The zooplankton density pattern observed was probably a consequence of concomitant modifications in the predation pressure, refuge availability and concentration of cyanobacteria in the lake. It is suggested that emergent and floating-leaved macrophytes may play an important role in enhancing water clarity due to increased grazing pressure by zooplankton migrating into the plant stands. As a consequence, especially in turbid lakes, the ecological role of these functional types of vegetation, and not merely that of submerged macrophyte species, should be taken into consideration.

Invertebrados aquáticos no detrito de Salvinia herzogii de la Sota em lagos rasos do sul do Brasil

A colonização por invertebrados aquáticos no detrito de Salvinia herzogii foi estudada em dois lagos rasos subtropicais de diferentes estados tróficos (eutrófico e oligotrófico) localizados no campus Carreiros da Universidade Federal do Rio Grande, extremo sul do Brasil. Aproximadamente 6g (peso úmido) de S. herzogii foram incubados em experimentos com litter bags (30 X 20 cm com 10 mm de malha) e a decomposição do detrito acompanhada durante 110 dias (entre setembro de 2008 e janeiro de 2009), período no qual o detrito decompôs-se cerca de 95%. Quatro réplicas foram retiradas após os intervalos de 1, 30, 60, 90 e 110 dias de incubação e o detrito foi limpo em água corrente sob peneira de 250µm de malha para que fossem retidos os organismos e depois fixados em álcool 80%. As plantas depois de limpas foram secas à 60ºC por 72 horas para obtenção do peso seco, depois trituradas, para as análises de nitrogênio e fósforo total e para obtenção do peso seco livre de cinzas, equivalente a porcentagem de massa remanescente (%R). Os invertebrados foram identificados até o menor nível taxonômico possível e depositados na coleção de Invertebrados Límnicos – ICB - FURG. Para cada amostra foram calculadas a riqueza e a densidade de táxons, os índices de diversidade de Shannon - Wiener (H’), homogeneidade de Pielou (J). Para determinar qual grupo de invertebrados foi mais importante para a estruturação da comunidade em cada lago, foi realizada a análise de espécies indicadoras (indicator especies analysis). Um total de 32.399 organismos distribuídos em 38 táxons foram registrados para os lagos estudados. Dezoito táxons foram comuns entre os lagos em cada experimento. Um total de 1.449.612 ind. 100g PS no ambiente eutrófico e 372.380 ind. 100g PS para o ambiente oligotrófico foi registrado (p=0,4771). O táxon mais abundante encontrado para o lago eutrófico foi Goeldichironomus (Chironomidae; 81% de todos os organismos coletados), enquanto queno lago oligotrófico, o táxon mais abundante foi Caenis (Caenidae; 29,1% de todos os organismos coletados). A colonização do detrito foi rápida, nas primeiras 24 horas foram registrados 10 táxons nos lagos eutrófico e oligotrófico, sendo os predadores o grupo dominante (52 e 70%, respectivamente) neste período. Durante o período do experimento,no lago eutrófico, os coletores-catadores foram os mais abundantes (91,2 % do número total de organismos), seguido de coletores-filtradores (4,7 %) e predadores (3,5 %). No lago oligotrófico, os coletores-catadores foram os mais abundantes (34,1 % do número total de organismos), seguido de predadores (26,8 %), raspadores (24,9 %) e fragmentadores (10,5 %). O Índice de diversidade acompanhou os aumentos de densidade durante os experimentos, sendo que o valor máximo observado foi no lago oligotrófico(H’=2,31) no 60º dia de incubação, enquanto que o menor valor foi registrado para o lago eutrófico (H’=0,19) no 110º dia de incubação. A taxa de decomposição de S. herzogii foi diferente no lago eutrófico (k = 0,019; R2 = 0,88) e oligotrófico (k = 0,021; R2 = 0,81),porém essa diferença não foi significativa (ANCOVA; F = 6,38 e p = 0,6755).

Influence of the bacterioplankton community of a tropical eutrophic lagoon on the bacterial community of its neighbouring ocean

The Rodrigo de Freitas lagoon (RFL) is a tropical eutrophic coastal ecosystem located in the urban area of Rio de Janeiro, Brazil. This environment consists of freshwater but has communication with the ocean through a channel (Jardim de Alah`s Channel). The aim of this study was to evaluate the influence of lagoon water on the nearby ocean using molecular and traditional microbiological methods. We hypothesised that due to the eutrophic low-salinity environment, the bacterioplankton community from the RFL would have a native ""brackish"" composition influenced by both freshwater and marine phylotypes, and that bacterial phylotypes of this community would be detected in oceanic samples closer to the channel between the lagoon and the ocean. The cultivation and microscopy experiments clearly showed this influence. Bacterial cell counts revealed that the greater amounts of bacterial cells present in the lagoon increased the observed values seen at oceanic stations near the channel. The Denaturing gradient gel eletrophoresis community profiles also showed a clear influence of Rodrigo de Freitas lagoon waters on the adjacent beaches. The band patterns found for the stations near the channel showed that these communities were mixtures of the communities of the lagoon and sea, and as the distance from the channel increased, the samples became more similar to ocean bacterial communities. A 16S rRNA gene clone library was constructed using a sample acquired from the connection point between the lagoon and the ocean. Around 52% of the sequences in the library showed similarity to the genus Proteobacteria (1% Alpha, 21% Beta, 19% Gamma and 29% unclassified Proteobacteria), and the second most abundant genus was Bacteroidetes, with 15% of the total clones. The results showed that the structure of the bacterial community had both freshwater and marine characteristics.