A Cross-System Comparison of Bacterial and Fungal Biomass in Detritus Pools of Headwater Streams

The absolute amount of microbial biomass and relative contribution of fungi and bacteria are expected to vary among types of organic matter (OM) within a stream and will vary among streams because of differences in organic matter quality and quantity. Common types of benthic detritus [leaves, small wood, and fine benthic organic matter (FBOM)] were sampled in 9 small (1st-3rd order) streams selected to represent a range of important controlling factors such as surrounding vegetation, detritus standing stocks, and water chemistry. Direct counts of bacteria and measurements of ergosterol (a fungal sterol) were used to describe variation in bacterial and fungal biomass. There were significant differences in bacterial abundance among types of organic matter with higher densities per unit mass of organic matter on fine particles relative to either leaves or wood surfaces. In contrast, ergosterol concentrations were significantly greater on leaves and wood, confirming the predominance of fungal biomass in these larger size classes. In general, bacterial abundance per unit organic matter was less variable than fungal biomass, suggesting bacteria will be a more predictable component of stream microbial communities. For 7 of the 9 streams, the standing stock of fine benthic organic matter was large enough that habitat-weighted reach-scale bacterial biomass was equal to or greater than fungal biomass. The quantities of leaves and small wood varied among streams such that the relative contribution of reach-scale fungal biomass ranged from 10% to as much as 90% of microbial biomass. Ergosterol concentrations were positively associated with substrate C:N ratio while bacterial abundance was negatively correlated with C:N. Both these relationships are confounded by particle size, i.e., leaves and wood had higher C:N than fine benthic organic matter. There was a weak positive relationship between bacterial abundance and streamwater soluble reactive phosphorus concentration, but no apparent pattern between either bacteria or fungi and streamwater dissolved inorganic nitrogen. The variation in microbial biomass per unit organic matter and the relative abundance of different types of organic matter contributed equally to driving differences in total microbial biomass at the reach scale.     

Inter-annual variability in marine coastal Antarctic bacterioplankton

The dynamics of Antarctic coastal marine bacterioplankton has been studied over a 2-year period. Two field stations were sampled between one and three times a week in 1989 and 1991 in the “Terre Adélie” area. The survey included physicochemical (temperature and particulate organic matter) and bacteriological (total and heterotrophic counts, cell volume and frequency of dividing cells estimation) measurements. The results suggest that a strong interannual variability affects the total bacterial abundance, the mean cell volume, the percentage of free living cells and, to a lesser extent. the culturable saprophytic bacterial communities. The observed variability could be partly explained by a large deficit of solar irradiance during the 2nd year of study that may have affected sea ice and seawater primary production.     

Barophilic Bacteria Associated with Digestive Tracts of Abyssal Holothurians

Abyssal holothurians and sediment samples were collected at depths of 4,430 to 4,850 m in the Demerara abyssal plain. Bacterial concentrations in progressive sections of the holothurian digestive tract, as well as in surrounding surface sediments, were determined by epifluorescence microscopy. Total bacterial counts in sediments recently ingested by the animals were 1.5- to 3-fold higher than in surrounding sediments at the deepest station. Lowest counts were observed consistently in the foregut, where the digestive processes of the holothurian are believed to occur. In most animals, counts increased 3- to 10-fold in the hindgut. Microbial activity at 3°C and in situ and atmospheric pressure were determined for gut and sediment samples by measuring the utilization of [¹⁴C]glutamic acid, the doubling time of the mixed-population of culturable bacteria, and the percentage of the total bacterial count responsive to yeast extract in the presence of nalidixic acid, using epifluorescence microscopy. A barophilic microbial population, showing elevated activity under deep-sea pressure, was detected by all three methods in sediments removed from the hindgut. Transmission electron micrographs revealed intact bacteria directly associated with the intestinal lining only in the hindgut. The bacteria are believed to be carried as an actively metabolizing, commensal gut flora that transforms organic matter present in abyssal sediments ingested by the holothurian. Using data obtained in this study, it was calculated that sediment containing organic matter altered by microbial activity cleared the holothurian gut every 16 h, suggesting that abyssal holothurians and their associated gut flora are important participants in nutrient cycles of the abyssal benthic ocean.     

Organic matter availability during pre- and post-drought periods in a Mediterranean stream

Mediterranean streams are characterized by water flow changes caused by floods and droughts. When intermittency occurs in river ecosystems, hydrologic connectivity is interrupted and this affects benthic, hyporheic and flowing water compartments. Organic matter use and transport can be particularly affected during the transition from wet to dry and dry to wet conditions. In order to characterize the changes in benthic organic matter quantity and quality throughout a drying and rewetting process, organic matter, and enzyme activities were analyzed in the benthic accumulated material (biofilms growing on rocks and cobbles, leaves, and sand) and in flowing water (dissolved and particulate fractions). The total polysaccharide, amino acid, and lipid content in the benthic organic matter were on average higher in the drying period than in the rewetting period. However, during the drying period, peptide availability decreased, as indicated by decreases in leucine aminopeptidase activity, as well as amino acid content in the water and benthic material, except leaves; while polysaccharides were actively used, as indicated by an increase in β-glucosidase activity in the benthic substrata and an increase in polysaccharide content of the particulate water fraction and in leaf material. During this process, microbial heterotrophs were constrained to use the organic matter source of the lowest quality (polysaccharides, providing only C), since peptides (providing N and C) were no longer available. During the flow recovery phase, the microbial community rapidly recovered, suggesting the use of refuges and/or adaptation to desiccation during the previous drought period. The scouring during rewetting was responsible for the mobilization of the streambed and loss of benthic material, and the increase in high quality organic matter in transport (at that moment, polysaccharides and amino acids accounted for 30% of the total DOC). The dynamics of progressive and gradual drought effects, as well as the fast recovery after rewetting, might be affected by the interaction of the individual dynamics of each benthic substratum: sand sediments and leaves providing refuge for microorganisms and organic matter storage, while on cobbles, an active bacterial community is developed in the rewetting. Since global climate change may favor a higher intensity and frequency of droughts in streams, understanding the effects of these disturbances on the materials and biota could contribute to reliable resource management. The maintenance of benthic substrata heterogeneity within the stream may be important for stream recovery after droughts.     

Contribution of the water-sediment interface to the transformation of biogenic substances: application to nitrogen compounds

Up to now the water-sediment interface (WSI) has not been considered by limnologists and oceanographers as a theoretical frontier between two phases, but as a variable layer comprising the overlying bottom water, the superficial sediment, and the bioturbed sediment. This heterogeneous benthic boundary layer forms a non-equilibrated assemblage of an aqueous phase, a mineral phase influencing the exchanges, and a dissolved gaseous phase regulating biological activity, detrital organic matter present as dissolved or adsorbed compounds and particles, and a living benthic community dominated by microorganisms. Biological activity and the equilibrium between these components are linked to depth, bottom currents, primary production and allochtonous inputs of organic matter. The settling rate of particulate matter in the water column, which depends on both the size and density of particles, is also an important factor. Settled organic matter is broken down mainly by heterotrophic microorganisms. In the first stage, dissolved or particulate matter is depolymerized by exoenzymes linked to the bacterial membrane. Other heterotrophic organisms, like flagellates and ciliates, contribute to the activity of bacterial populations or to a modification of the physical and chemical characteristics of organic matter like macrofauna. Bacterial activity in the WSI is one to two orders of magnitude higher than in overlying bottom water, depending on the density of the bacterial population, on the amount of available organic matter, and on the presence of predators grazing on bacteria and macrofauna and increasing oxygen and dissolved substrates. The WSI is now considered as a sink for particulate matter and biopolymers, and/or as a sink or source for organic monomers and NH₄ ⁺ depending on their concentrations. As a result of its contribution to mineral and organic cycles, the WSI is a crossroad for exchanges between aqueous and sediment layer. During the last decade, intensive work has been performed in this field, but progress has been slow due to the heterogeneity of the layer and the adsorptive properties of the mineral fraction.     

Bacteriological quality of crops irrigated with wastewater in the Xochimilco plots, Mexico City, Mexico

Xochimilco county plots (Mexico City), one of the most fertile agricultural areas in the Valley of Mexico, produce a large portion of the fresh vegetables consumed in the city. These plots are generally irrigated with domestic wastewater, and for this reason, it was deemed important to examine and evaluate the bacteriological quality of the water, soil, and vegetables from these plots that are harvested and marketed. The soils were also examined for the classical parameters such as nitrates, ammonia, etc., and organic matter and texture. The crops selected for this study were radishes, spinach, lettuce, parsley, and celery because they are usually consumed raw. The highest bacterial counts were encountered in leafy vegetables, i.e., spinach (8,700 for total coliform and 2,400 for fecal coliform) and lettuce (37,000 for total coliform and 3,600 for fecal coliform). Statistically significant differences in bacterial counts between rinsed and unrinsed edible portions of the crops were observed even in rinsed vegetables, and high densities of fecal coliform were detected, indicating that their consumption represents a potential health hazard. The total coliform values found in irrigation water ranged from 4 X 10(4) to 29 X 10(4), and for fecal coliform the values ranged from 5 X 10(2) to 30 X 10(2).     

Seasonal changes in the abundance and composition of marine heterotrophic bacterial communities in an Antarctic coastal area

During a 1-year period, systematic observations of the Antarctic coastal marine bacterioplankton were recorded. Three field stations were sampled weekly in 1989 in Terre Adélie area. The survey included physicochemical (temperature and particulate organic matter) and bacteriological (total and heterotrophic counts, estimation of bacterial production) measurements. The bacterial community structure was investigated by carrying out 27 morphological and biochemical tests on 254 strains isolated during each season. Gram-negative non-fermentative rods were always dominant. However, an obvious difference exists between the communities inhabiting ice-free and ice-covered seawater. The potential metabolic abilities which were relatively significant in the summer community were severely reduced in the winter community. A general increase in bacterial biomass and production was observed in surface water after sea ice formation. The results suggest a close coupling between heterotrophic bacterioplankton and the input of allochthonous organic carbon, for example from the overlying sea-ice communities or from nearby penguin rookeries.     

Bioremediation of a polycyclic aromatic hydrocarbon‐contaminated soil by using different aerobic batch bioreactor systems

The intrinsic depuration capability of a soil contaminated by polycyclic aromatic hydrocarbons (PAHs) originating from a contaminated industrial site was evaluated in this study by using different aerobic batch bioreactors: a slurry‐phase bioreactor, a blade‐agitated bioreactor, and a rotary vessel bioreactor. For each bioreactor, the disappearance of 14 target PAHs and of the total extractable organic matter was monitored. The three treatments exhibited rapid and extensive removal of the PAHs, which disappeared at different degradation rates according to their molecular weight and aromaticity degree. PAHs with two, three, and four aromatic rings were degraded in sequence, with average rates that generally decreased as the number of molecule rings increased. A slight increase in the bacterial biomass concentration and significant CO₂ production were also observed during the time course of the treatments. Among the three treatments, the slurry‐phase system provides the most effective and fastest removal of the PAHs and the organic extractable matter. However, the semisolid‐phase systems exhibited PAH depletion, capabilities higher than those reported in the literature for soils with similar particle size distribution in solid‐phase conditions.     

Spatio-temporal variability in benthic mineralization processes in the eastern English Channel

The effect of phytodetritus derived from Phaeocystis sp. bloom on benthic mineralization processes has been determined at four intertidal stations along the French coast of the eastern English Channel. Sites were chosen to offer a diversity of sediment types, from permeable sandy beach to estuarine mudflats. Sediment Oxygen Demand (SOD) as well as total fluxes of Dissolved Inorganic Nitrogen (DIN) at the sediment–water interface were determined by using whole core incubation technique and diffusive fluxes were predicted from interstitial water concentrations. In the absence of phytodetritus deposits, a marked gradient of granulometric characteristics and organic matter contents were observed, and resulted in more intensive mineralization processes in muddy sediments. Highly significant correlations (P < 0.05) were evidenced between SOD and porosity, bacterial biomass, Organic Carbon and Organic Nitrogen, evidencing the direct link between sediment texture, organic matter accumulation and microbial activity. The spring bloom led to a massive input of organic matter in surficial sediments and mineralization rates significantly increased while higher DIN release towards the water column was observed. A modification of the mineralization pathways was evidenced but clearly depended on the sediment type. With a global view, benthic mineralization processes in the intertidal zone provided significant a part of DIN inputs in the coastal zone while water column was depleted in nutrients.     

Grazing and the distribution of sediment particle sizes in artificial stream systems

Recent research has shown that grazing by the algivorous minnows of the genus Campostoma can have a significant effect on both structural and functional parameters in stream ecosystems, influencing algal height and type, primary productivity, carbon dynamics, bacterial biomass and the size fractionation of benthic organic matter. This study was undertaken to evaluate the effect of minnow grazing on benthic particulate organic matter (BPOM) under the controlled conditions available in experimental streams. For this study, four grazed and four ungrazed artificial streams were sampled for BPOM on two dates. The samples were partitioned into large, medium, fine and ultrafine fractions via wet filtration. Fish grazed systems had significantly larger percentages of the fine fraction and significantly smaller percentages of the ultrafine fraction. The ability of grazers to alter particle size distribution is important to overall stream organic matter dynamics because recent studies have shown the importance of particle size in determining bacterial numbers and activity, which in turn influence fundamental stream process like respiration and organic carbon dynamics.     

Short-Term Wavelike Dynamics of Bacterial Populations in Response to Nutrient Input from Fresh Plant Residues

The objectives of the research were to investigate short-term dynamics of bacterial populations in soil after a disturbance in the form of fresh organic matter incorporation and to investigate how these dynamics are linked to those of some environmental parameters. To reach these objectives, soil bacterial populations, mineral nitrogen, pH, and redox potential (ROP) were monitored daily for 1 month after incorporation of clover-grass (CG) plant material in microcosm experiments. Colony-forming units (CFUs) and direct microscopic counts of FDA-stained and FTTC-stained bacteria increased immediately after incorporation of the plant material, dropped within 2 days, and fluctuated thereafter. Harmonics analysis demonstrated that there were significant wavelike fluctuations with three or four significant peaks within 1 month after incorporation of clover-grass material. Peaks in CFUs were 1–2 days ahead of those in direct counts. Ammonium (NH₄) concentrations increased from the start of the experiments until nitrification commenced. Nitrate (NO₃) concentrations dropped immediately after plant incorporation, and then rose monotonically until the end of the experiments. There were no wavelike fluctuations in NH₄ and NO₃ concentrations, so that bacterial fluctuations could not be attributed to alternating mineral N shortages and sufficiencies. pH levels rose and declined with NH₄ levels. ROP dropped shortly before NH₄ concentrations rose, and increased before NH₄ concentrations decreased; there were no regular fluctuations in ROP, so that temporary oxygen shortages may not have been responsible for the observed fluctuations in bacterial populations. Thus, for the first time, regular wavelike dynamics were demonstrated for bacterial populations after perturbation by addition of fresh organic matter to soil, and several potential reasons for the death phase of the fluctuations could be excluded from further consideration.     

Measurements of Seasonal Rates and Annual Budgets of Organic Carbon Fluxes in an Antarctic Coastal Environment at Signy Island, South Orkney Islands, Suggest a Broad Balance between Production and Decomposition

We report here the first comprehensive seasonal study of benthic microbial activity in an Antarctic coastal environment. Measurements were made from December 1990 to February 1992 of oxygen uptake and sulfate reduction by inshore coastal sediments at Signy Island, South Orkney Islands, Antarctica. From these measurements the rate of benthic mineralization of organic matter was calculated. In addition, both the deposition rate of organic matter to the bottom sediment and the organic carbon content of the bottom sediment were measured during the same period. Organic matter input to the sediment was small under winter ice cover, and the benthic respiratory activity and the organic content of the surface sediment declined during this period as available organic matter was depleted. On an annual basis, about 32% of benthic organic matter mineralization was anoxic, but the proportion of anoxic compared with oxic mineralization increased during the winter as organic matter was increasingly buried by the amphipod infauna. Fresh organic input occurred as the sea ice melted and ice algae biomass sedimented onto the bottom, and input was sustained during the spring after ice breakup by continued primary production in the water column. The benthic respiratory rate and benthic organic matter content correspondingly increased towards the end of winter with the input of this fresh organic matter. The rates of oxygen uptake during the southern summer (80 to 90 mmol of O₂ m^-2 day^-1) were as high as those reported for other sediments at much higher environmental temperatures, and the annual mineralization of organic matter was equally high (12 mol of C m^-2 year^-1). Seasonal variations of benthic activity in this antarctic coastal sediment were regulated by the input and availability of organic matter and not by seasonal water temperature, which was relatively constant at between -1.8 and 0.5°C. We conclude that despite the low environmental temperature, organic matter degradation broadly balanced organic matter production, although there may be significant interrannual variations in the sources of the organic matter inputs.     

Impact of bioavailable heavy metals on bacterial activities in coastal marine sediments

Total heavy metal concentrations in marine sediments are not sufficient to reliably predict detrimental biological effects. Here we provide evidence that only bioavailable heavy metals have a significant impact on benthic microbial loop functioning. Sediment samples collected along 250 km of the Apulian coast (Mediterranean Sea) were analysed for total and bioavailable heavy metals (Cr, Cd, Pb and Cu), organic matter content, bacterial abundance, biomass and carbon production and -glucosidase activity. Sampling strategy was specifically designed to cover a wide range of environmental conditions and types of anthropogenic influences. Total heavy metal concentrations in the sediments were tightly coupled with organic matter content, whereas bioavailable heavy metal concentrations displayed an opposite pattern. Bioavailable Cr concentrations were up to 10-fold higher than values observed for the other bioavailable metals and significantly inhibited benthic bacterial metabolism and turnover. Results from this study suggest that functional microbial variables are highly sensitive to heavy metal contamination and could be used as bioindicators of stress conditions in coastal sediments.     

Benthic-Pelagic Coupling: Effects on Nematode Communities along Southern European Continental Margins

Along a west-to-east axis spanning the Galicia Bank region (Iberian margin) and the Mediterranean basin, a reduction in surface primary productivity and in seafloor flux of particulate organic carbon was mirrored in the in situ organic matter quantity and quality within the underlying deep-sea sediments at different water depths (1200, 1900 and 3000 m). Nematode standing stock (abundance and biomass) and genus and trophic composition were investigated to evaluate downward benthic-pelagic coupling. The longitudinal decline in seafloor particulate organic carbon flux was reflected by a reduction in benthic phytopigment concentrations and nematode standing stock. An exception was the station sampled at the Galicia Bank seamount, where despite the maximal particulate organic carbon flux estimate, we observed reduced pigment levels and nematode standing stock. The strong hydrodynamic forcing at this station was believed to be the main cause of the local decoupling between pelagic and benthic processes. Besides a longitudinal cline in nematode standing stock, we noticed a west-to-east gradient in nematode genus and feeding type composition (owing to an increasing importance of predatory/scavenging nematodes with longitude) governed by potential proxies for food availability (percentage of nitrogen, organic carbon, and total organic matter). Within-station variability in generic composition was elevated in sediments with lower phytopigment concentrations. Standing stock appeared to be regulated by sedimentation rates and benthic environmental variables, whereas genus composition covaried only with benthic environmental variables. The coupling between deep-sea nematode assemblages and surface water processes evidenced in the present study suggests that it is likely that climate change will affect the composition and function of deep-sea nematodes.     

Decomposition process of organic matter derived from freshwater phytoplankton

We investigated the biodegradation process of freshwater phytoplanktonic organic matter using incubation experiments, with special reference to changes in three major biomolecules: neutral aldoses, amino acids and fatty acids. The concentration of neutral aldoses decreased drastically relative to amino acids and fatty acids during the early decomposition phase (days 0–7), owing largely to the rapid decomposition of storage carbohydrate. This resulted in a temporary decrease in the C/N ratio of organic matter. During the late phase (days 7–60), however, the rate of decrease in neutral aldoses slowed, while the considerable decrease in amino acids and fatty acids continued. The inconspicuous change in amino acid composition was probably due to the fact that no protein and/or peptide is composed of a limited species of amino acid. Although the compositional variety of organic matter among the phytoplankton was clearly observed at the start of decomposition, it became obscure in the course of 60 days. This indicates that while the organic composition of the labile fraction of phytoplanktonic organic matter varies depending on the phytoplankton groups, the refractory fraction has similar composition. The addition of bacterial organic matter is likely another reason for the similar composition of the remaining organic matter at day 60.     

A study of dissolved oxygen flux across the water-bottom interface in the northeastern Black Sea (in the region of Dzhubga Village)

Oxygen exchange at the water-bottom interface in the northeastern Black Sea was studied using bottom tanks (fluxes and oxygen consumption for organic matter mineralization and for respiration of soil and water organisms). The relationship of biogenic fluxes and patterns of biochemical (enzyme) destruction of organic matter by the components of the bacterial and microproducer community was established. The prevalence of microbial oxidation of organic matter correlated with high proteolytic activity in near-bottom water. The principal significance of organic matter oxidation in near-bottom water for the phytoplankton and its respiratory expenditures was demonstrated both in the open system of the near-bottom layer and in the closed tank system. A similar trend was demonstrated for benthic organisms.     

Comparison of anaerobic and aerobic biodegradation of mineralized skeletal structures in marine and estuarine conditions

The knowledge of the biodegradation rates is essential to studies of the biogeochemistry and ecology of aquatic systems. It helps us to quantify the production and uptake rates of chemical components and their recycling, and to understand the mechanisms and rates of organic matter accumulation in sediments. Experimental studies of biodegradation processes in six types of mineralized skeletons were performed in shallow-marine waters of Calvi Bay, Corsica and in estuarine waters of Roscoff, Brittany. Three types of mollusk shells, sea urchin skeletal plates, crab cuticle and fish vertebrae were exposed to oxic and anoxic conditions over periods of 15 days to 30 months. After recovery of the substrates, protein assays, bacterial counts and organic carbon analyses were performed.Quantitative protein assays and bacterial counts indicate that biodegradation of mineralized skeletal structures occurs at a slower rate in anoxic conditions than in oxic conditions. Bacterial analysis showed that in anoxic environment, less than 0.5% of the consumed organic matter is converted into bacterial biomass. The aerobic biodegradation rate was positively correlated with the organic content of the skeletons.Anoxic biodegradation of skeletons occurred at much slower rates in estuarine sediments than in shallow marine sediments. Preservation of skeletal structures in estuarine conditions appears to be correlated with the abundance of dissolved organic matter rather than with high sedimentation rates.     

Storage and decomposition of particulate organic matter along the longitudinal gradient of an agriculturally-impacted stream

An agriculturally-impacted stream in northern Idaho was examined over a two-year period to determine seasonal and longitudinal patterns of the storage and decomposition of particulate organic matter. Biomass of benthic organic matter (BOM) was considerably less than values reported in the literature for comparable, undisturbed streams. Coarse, fine, and total benthic particulate organic matter were not correlated with parameters pertaining to stream size (e.g., stream order), but were correlated with sample site and amount of litterfall. The association of BOM with site and litterfall suggests that storage of particulate organic matter is a function of local characteristics rather than stream size. Low biomass of stored organic matter is a response to the low input of terrestrially-derived organic matter resulting from removal of climax vegetation.Leaf packs of alder, Alnus sp., were placed in the stream seasonally for 30 and 60 d. While there were significant differences for months, there was no significant difference among sites for leaf packs exposed for 30 d. Significant differences were observed among both sites and months for leaf packs exposed for 60 d; however, differences among sites accounted for only 5% of the variance. The absence of differences in decomposition of organic matter along the gradient of Lapwai Creek, despite heterogeneity of the drainage basin and availability of organic matter, may be in response to the overall low biomass of stored benthic organic matter. This study demonstrates that agricultural activity can substantially influence instream heterotrophic processes through reduced availability of organic matter and can shape community structure and ecosystem dynamics of streams flowing through agricultural drainage basins.     

Carbon and nitrogen stable isotopic inventory of the most abundant demersal fish captured by benthic gears in southwestern Iceland (North Atlantic)

Stable isotopes (δ¹³C and δ¹⁵N) were used to examine the origin of organic matter for the most representative demersal species of the SW Icelandic fishery, accounting for over 70% of landings of those species in the North Atlantic. Samples were collected during a 2-week period in early September 2004 from landings and directly during fishing cruises. Stable isotopes showed that particulate organic matter and sedimentary organic matter were at the base of the food web and appeared to fill two different compartments: the pelagic and the benthic. The pelagic realm was composed of only capelin and sandeel; krill and redfish occupied an intermediate position between pelagic and benthic realms; while anglerfish, haddock, cod and ling resulted as the true demersal species while tusk, rays and plaice were strongly linked to the benthic habitat.     

The effect of terrestrial effluents on the incidence of Aeromonas spp. in coastal waters

The numbers of Aeromonas species were monitored for one year at 18 stations on the Barcelona coast. Their concentrations exceed those of faecal coliforms and faecal streptococci in all samples, both on the shoreline and at 500 m from the coast. The mean values of the three bacterial counts reflected the proximity of the terrestrial effluents. There was a positive correlation between aeromonads and faecal indicators on the shoreline but not at 500 m offshore. This reflected their common origin and different survival rates in seawater. When sterile sea water was inoculated with A. hydrophila ATCC 7966, the numbers decreased initially by three orders of magnitude before they began to multiply. The final count was related to the concentration of organic matter in the water.