The effect of agrochemicals on indicator bacteria densities in outdoor mesocosms

Water bodies, which are monitored for microbial water quality by quantification of faecal indicator organisms (IOs), can contain various zoonotic pathogens contributed by livestock waste and other sources. Sediments can serve as reservoirs of IOs and other enteric microorganisms, including pathogens. Agrochemicals may influence the survival of these microorganisms in water bodies impacted by livestock waste by enhancing or reducing their survival. Complex, 1100 l, freshwater mesocosms containing leaf litter, zooplankton, periphyton, phytoplankton, and invertebrate and vertebrate animals were used to investigate the response of Escherichia coli and enterococci to agrochemicals. Replicate tanks were treated with atrazine, malathion, chlorothalonil and inorganic fertilizer, either alone at 1× or 2× their expected environmental concentrations (EECs) or in pair‐wise combinations at their EECs. IOs inoculated in sediment (～10⁴ cfu per 100 ml) were enumerated over 28 days. IOs generally declined over time, but manova revealed that addition of fertilizer and atrazine resulted in significantly greater IO densities. Malathion, chlorothalonil and agrochemical concentration (1× vs 2×) did not significantly affect IO densities and no significant interactions between agrochemicals were noted. The augmentation of IO densities in sediments by fertilizer and atrazine may impact their reliability as accurate predictors of water quality and human health risk, and indicates the need for a better understanding of the fate of IOs and enteric pathogens in sediments exposed to agrochemicals.     

Lack of Direct Effects of Agrochemicals on Zoonotic Pathogens and Fecal Indicator Bacteria

Agrochemicals, fecal indicator bacteria (FIB), and pathogens frequently contaminate water simultaneously. No significant direct effects of fertilizer, atrazine, malathion, and chlorothalonil on the survival of Escherichia coli, Enterococcus faecalis, Salmonella enterica, human polyomaviruses, and adenovirus were detected, supporting the assertion that previously observed effects of agrochemicals on FIB were indirect.     

Short-term influence of recolonisation by the polycheate worm [2pt] Nereis succinea on oxygen and nitrogen fluxes and [2pt] denitrification: a microcosm simulation

The short-term effects of sediment recolonisation by Nereis succinea on sediment-water column fluxes of oxygen and dissolved inorganic nitrogen, and rates of denitrification, were studied in microcosms of homogenised, sieved sediments. The added worms enhanced oxygen uptake by the sediments, due to the increased surface area provided by the burrow walls and the degree of stimulation was stable with time. Similarly, ammonium fluxes to the water column were stimulated by N. succinea, but declined over the 3 day incubation in all microcosms including the controls. Nitrate fluxes were generally greater in the faunated microcosms, but highly variable with time. Denitrification rates were positively stimulated by N. succinea populations, denitrification of water column nitrate was stimulated 10-fold in comparison to denitrification coupled to nitrification in the sediments. Rates of denitrification of water column nitrate were not significantly different from rates in undisturbed sediment cores with similar densities of N. succinea, whereas rates of coupled nitrification–denitrification were 3-fold lower in the experimental set-up. These results may reflect the relative growth rates of nitrifying and denitrifying bacteria, which allow more rapid colonisation of new burrow surfaces by denitrifier compared to nitrifier populations. The data indicate that recolonisation by burrowing macrofauna of the highly reduced sediments of the Sacca di Goro, Lagoon, Italy, following the annual dystrophic crisis, may play a significant role in the reoxidation and detoxification of the sediments. The increased rates of denitrification associated with the worm burrows, may promote nitrogen losses, but due to the low capacity of nitrifying bacteria to colonise the new burrow structures, these losses would be highly dependent upon water column nitrate concentrations.     

Use of microcosms to determine persistence of Escherichia coli in recreational coastal water and sediment and validation with in situ measurements

AIMS: To determine the persistence of the faecal indicator organism Escherichia coli in recreational coastal water and sediment using laboratory-based microcosms and validation with in situ measurements. METHODS AND RESULTS: Intact sediment cores were taken from three distinct coastal sites. Overlying estuarine water was inoculated with known concentrations of E. coli and decay rates from both overlying water and sediment were determined following enumeration by the membrane filtration method at fixed time intervals over a 28-day period. It was demonstrated that E. coli may persist in coastal sediment for >28 days when incubated at 10 degrees C. Escherichia coli survival was found to have an inverse relationship with temperature in both water and sediment. In general the decay rate for E. coli was greater in water than in sediment. Small particle size and high organic carbon content were found to enhance E. coli survival in coastal sediments in the microcosms. CONCLUSIONS: Results of this microcosm study demonstrated the more prolonged survival of E. coli in coastal sediments compared with overlying water, which may imply an increased risk of exposure because of the possible resuspension of pathogenic micro-organisms during natural turbulence or human recreational activity. SIGNIFICANCE AND IMPACT OF THE STUDY: A more accurate estimate of exposure risk has been described which may subsequently be used in a quantitative microbial risk assessment for recreational coastal waters.     

Effects of sediment resuspension on phytoplankton production: teasing apart the influences of light, nutrients and algal entrainment

1. Wind‐induced sediment resuspension can affect planktonic primary productivity by influencing light penetration and nutrient availability, and by contributing meroplankton (algae resuspended from the lake bed) to the water column. We established relationships between sediment resuspension, light and nutrient availability to phytoplankton in a shallow lake on four occasions. 2. The effects of additions of surficial sediments and nutrients on the productivity of phytoplankton communities were measured in 300 mL gas‐tight bottles attached to rotating plankton wheels and exposed to a light gradient, in 24 h incubations at in situ temperatures. 3. While sediment resuspension always increased primary productivity, resuspension released phytoplankton from nutrient limitation in only two of the four experiments because the amount of available nitrogen and phosphorus entrained from the sediments was small compared with typical baseline levels in the water column. In contrast, chlorophyll a entrainment was substantial compared with baseline water column concentrations and the contribution of meroplankton to primary production was important at times, especially when seasonal irradiance in the lake was high. 4. Comparison of the in situ light climate with the threshold of light‐limitation of the phytoplankton indicated that phytoplankton in the lake were only likely to be light‐limited at times of extreme turbidity (e.g. >200 nephelometric turbidity units), particularly when these occur in winter. Therefore, resuspension influenced phytoplankton production mainly via effects on available nutrients and by entraining algae. The importance of each of these varied in time. 5. The partitioning of primary productivity between the water column and sediments in shallow lakes greatly influences the outcome of resuspension events for water column primary productivity.     

Quantifying environmental reservoirs of fecal indicator bacteria associated with sediment and submerged aquatic vegetation

Elevated concentrations of fecal indicator bacteria (FIB) in aquatic sediments and vegetation have prompted concern that environmental reservoirs of FIB disrupt the correlation between indicator organisms, pathogens and human health risks. FIB numbers, however, are typically normalized to volume of water or mass of substrate. Because these reservoirs tend to differ greatly in magnitude within and between water bodies, direct comparison between water column and benthic population sizes can be problematic. Normalization to a set volume of water or mass of substrate, e.g. cfu (100 ml)(-1) or cfu(100 g)(-1), can give a false picture of the relative contributions of various reservoirs to FIB numbers across the ecosystem, and of the potential for FIBs to trigger health advisories as they pass from one reservoir to another. Here, we normalized enterococci concentrations from water, sediment and submerged aquatic vegetation (SAV) to land surface area (m(2) ) to compare their relative importance in the entire system. SAV-associated enterococci comprised only 0-18% of the entire population, even though they displayed the highest concentrations of enterococci per unit mass. The largest proportion of the enterococci population was in the water column (4-77%) or sediments (20-95%), depending on the volume of each substrate available at a site and FIB concentrations within them. Models indicated that large shifts in the relative size of FIB populations in each substrate can result from changes in per cent SAV cover, water depth and depth of sediment colonization. It follows that high concentrations of FIB in sediments or SAV do not necessarily signify large environmental reservoirs of FIB that can affect the water column. Comprehensive analyses that include FIB measurements from water, SAV and sediment normalized to land surface area offer a more balanced perspective on total FIB numbers contained in various matrices of an aquatic system.     

Wrack promotes the persistence of fecal indicator bacteria in marine sands and seawater

Algae on freshwater beaches can serve as reservoirs for fecal indicator bacteria (FIB). Wrack (especially kelp) at marine beaches might sustain FIB as well. This study examines the relationship between beach wrack, FIB, and surrounding water and sediment at marine beaches along the California coast. Surveys of southern and central California beaches were conducted to observe environmental wrack-associated FIB concentrations. FIB concentrations normalized to dry weight were the highest in stranded dry wrack, followed by stranded wet and suspended 'surf' wrack. Laboratory microcosms were conducted to examine the effect of wrack on FIB persistence in seawater and sediment. Indigenous enterococci and Escherichia coli incubated in a seawater microcosm containing wrack showed increased persistence relative to those incubated in a microcosm without wrack. FIB concentrations in microcosms containing wrack-covered sand were significantly higher than those in uncovered sand after several days. These findings implicate beach wrack as an important FIB reservoir. The presence of wrack may increase water and sediment FIB levels, altering the relationship between FIB levels and actual health risk while possibly leading to beach closures. Further work will need to investigate the possibility of FIB growth on wrack and the potential for pathogen presence.     

Sediment Composition Influences Spatial Variation in the Abundance of Human Pathogen Indicator Bacteria within an Estuarine Environment

Faecal contamination of estuarine and coastal waters can pose a risk to human health, particularly in areas used for shellfish production or recreation. Routine microbiological water quality testing highlights areas of faecal indicator bacteria (FIB) contamination within the water column, but fails to consider the abundance of FIB in sediments, which under certain hydrodynamic conditions can become resuspended. Sediments can enhance the survival of FIB in estuarine environments, but the influence of sediment composition on the ecology and abundance of FIB is poorly understood. To determine the relationship between sediment composition (grain size and organic matter) and the abundance of pathogen indicator bacteria (PIB), sediments were collected from four transverse transects of the Conwy estuary, UK. The abundance of culturable Escherichia coli, total coliforms, enterococci, Campylobacter, Salmonella and Vibrio spp. in sediments was determined in relation to sediment grain size, organic matter content, salinity, depth and temperature. Sediments that contained higher proportions of silt and/or clay and associated organic matter content showed significant positive correlations with the abundance of PIB. Furthermore, the abundance of each bacterial group was positively correlated with the presence of all other groups enumerated. Campylobacter spp. were not isolated from estuarine sediments. Comparisons of the number of culturable E. coli, total coliforms and Vibrio spp. in sediments and the water column revealed that their abundance was 281, 433 and 58-fold greater in sediments (colony forming units (CFU)/100g) when compared with the water column (CFU/100ml), respectively. These data provide important insights into sediment compositions that promote the abundance of PIB in estuarine environments, with important implications for the modelling and prediction of public health risk based on sediment resuspension and transport.     

122 Local to Coastal-Scale Macrophyte Community Structure: Surprizing Patterns and Possible Mechanisms

Diatom blooms may be initiated by cells that have survived in the plankton or germinated resting stages from the sediments. However, it is not well understood how these different inocula contribute to bloom development. We followed diatom community development in twenty-liter microcosms given different inocula. Surface sediment and phytoplankton were collected in Gullmar Fjord, Sweden. Replicate microcosms were then dosed with local sediment and/or plankton and incubated in situ in Gullmar Fjord. We also followed the concurrent development of the phytoplankton community in the fjord. Experiments run in both spring and fall 2002 showed that bloom development in the microcosms was significantly faster when seeded by planktonic cells. However, addition of sediment may have stimulated planktonic growth and also provided additional propagules. The type of inoculum used strongly influenced the diatom composition in the microcosms. Sediment additions, through germination of resting stages, resulted in communities dominated by Detonula confervacea and Thalassiosira minima in spring, and Skeletonema costatum in fall. Planktonic inocula resulted in blooms of T. nordenskioeldii and Chaetoceros debilis in spring, and S. costatum and several Chaetoceros spp. in fall. Microcosms dosed with both plankton and sediment showed a mixed species assemblage. Comparisons between the microcosms and fjord phytoplankton suggest an important role for benthic seeding of diatom blooms.     

121 Influence of Benthic vs. Pelagic Seeding on Coastal Diatom Bloom Development

Diatom blooms may be initiated by cells that have survived in the plankton or germinated resting stages from the sediments. However, it is not well understood how these different inocula contribute to bloom development. We followed diatom community development in twenty‐liter microcosms given different inocula. Surface sediment and phytoplankton were collected in Gullmar Fjord, Sweden. Replicate microcosms were then dosed with local sediment and/or plankton and incubated in situ in Gullmar Fjord. We also followed the concurrent development of the phytoplankton community in the fjord. Experiments run in both spring and fall 2002 showed that bloom development in the microcosms was significantly faster when seeded by planktonic cells. However, addition of sediment may have stimulated planktonic growth and also provided additional propagules. The type of inoculum used strongly influenced the diatom composition in the microcosms. Sediment additions, through germination of resting stages, resulted in communities dominated by Detonula confervacea and Thalassiosira minima in spring, and Skeletonema costatum in fall. Planktonic inocula resulted in blooms of T. nordenskioeldii and Chaetoceros debilis in spring, and S. costatum and several Chaetoceros spp. in fall. Microcosms dosed with both plankton and sediment showed a mixed species assemblage. Comparisons between the microcosms and fjord phytoplankton suggest an important role for benthic seeding of diatom blooms.     

A comparison of natural and human determinants of phytoplankton communities in the Kentucky River basin,USA

Physical, chemical, and biological characteristics of the Kentucky River and its tributaries were assessed for one year to compare effects of seasonal, spatial, and human environmental factors on phytoplankton. Phytoplankton cell densities were highest in the fall and summer and lowest in the winter. Cell densities averaged 1162 (± 289 SE) cells m1^–1. Cell densities were positively correlated to water temperature and negatively correlated to dissolved oxygen concentration and to factors associated with high-flow conditions (such as, suspended sediment concentrations). Chrysophytes, diatoms, and blue-green algae dominated winter, spring, and summer assemblages, respectively. Ordination analyses (DCCA) indicated that variation in taxonomic composition of assemblages was associated with stream size as well as season.Spatial variation in phytoplankton assemblages and effects of humans was investigated by sampling 55 sites in low flow conditions during August. Phytoplankton density increased with stream size. Assemblages shifted in composition from those dominated by benthic diatoms upstream to downstream communities dominated by blue-green algae and small flagellates. Human impacts were assumed to cause higher algal densities in stream basins with high proportions of agricultural or urban land use than in basins with forested/mined land use. While density and composition of phytoplankton were positively correlated to agricultural land use, they were poorly correlated to nutrient concentrations. Phytoplankton diversity changed with water quality: decreasing with nutrient enrichment and increasing with conditions that probably changed species composition or inhibited algal growth. Human impacts on phytoplankton in running water ecosystems were as great or greater than effects by natural seasonal and spatial factors. Our results indicated that phytoplankton could be useful indicators of water quality and ecosystem integrity in large river systems.     

Effects of sediment type on macrobenthic infaunal colonization of laboratory microcosms

We tested the effects of four different sediment types collected from northern Gulf of Mexico estuarine systems on macroinfaunal colonization and community development in our laboratory flow-through microcosm system. Four sediments, types included , a beach sand, two fine-grained muds, but from different locations, and a 50:50 mixture of one of the mud sediments and beach sand. Our hypothesis was that the pattern of colonization would differ among sediment types based on empirical field data and theory but the differences would be expressed most strongly at sediment type extremes (e.g., mud versus sand). A total of 49 taxa colonized the four sediments. Unidentified Actiniaria (sea anemones) numerically dominated densities among all four sediments with densities ranging between 46.5 to 60.5 per microcosm (20 cm side^–1). Average taxa richness per microcosm (N: 10 replicates per sediment treatment) ranged from 10.4 in one of the mud treatments to 14.9 taxa in the sand. These were the only significant differences among sediment types (P0.05) in taxa richness and we detected no significant effects of sediment type on animal densities. Differences in community metrics, although statistically significant, were generally of a relatively small magnitude. Five of 10 microcosms per treatment were randomly selected to test for effects of sediment depth (e.g., top, mid, and bottom). In vertically sectioned microcosms, average taxa richness in sand treatments was significantly greater than those of the other three sediments. A non-parametric multivariate analysis (Primer) revealed that community structure in the vertically sectioned microcosms differed significantly between sand and one of the mud treatments. Mean taxa richness of top sections differed significantly from mid and bottom sections. We detected significantly higher animal densities and taxa richness (p0.05) in vertically sectioned versus non-sectioned microcosms. However, these differences were unexplained based on experimental protocols.     

Influence of soil on fecal indicator organisms in a tidally influenced subtropical environment

The potential regrowth of fecal indicator bacteria released into coastal environments in recreational water bodies has been of concern, especially in tropical and subtropical areas where the number of these bacteria can be artificially elevated beyond that from fecal impacts alone. The task of determining the factors that influence indicator bacterial regrowth was addressed though a series of field sampling and laboratory experiments using in situ densities of Escherichia coli, enterococci, and Clostridium perfringens in river water, sediment, and soil. Field sampling efforts included the collection of surface sediments along the cross section of a riverbank, a 20-cm-deep soil core, and additional surface soils from remote locations. In addition to field sampling, two types of laboratory experiments were conducted. The first experiment investigated the survival of bacteria already present in river water with the addition of sterile and unsterile sediment. The second experiment was designed to simulate the wetting and drying effects due to tidal cycles. The results from the sampling study found elevated numbers of E. coli and C. perfringens in surficial sediments along the riverbank near the edge of the water. C. perfringens was found in high numbers in the subsurface samples obtained from the soil core. Results from laboratory experiments revealed a significant amount of regrowth for enterococci and E. coli with the simulation of tides and addition of sterile sediment. Regrowth was not observed for C. perfringens. This study demonstrates the need to further evaluate the characteristics of indicator microbes within tropical and subtropical water systems where natural vegetation, soil embankments, and long-term sediment accumulation are present. In such areas, the use of traditional indicator microbes to regulate recreational uses of a water body may not be appropriate.     

Cattle water troughs as reservoirs of Escherichia coli O157.

Environmental survival of Escherichia coli O157 may play an important role in the persistence and dissemination of this organism on farms. The survival of culturable and infectious E. coli O157 was studied using microcosms simulating cattle water troughs. Culturable E. coli O157 survived for at least 245 days in the microcosm sediments. Furthermore, E. coli O157 strains surviving more than 6 months in contaminated microcosms were infectious to a group of 10-week-old calves. Fecal excretion of E. coli O157 by these calves persisted for 87 days after challenge. Water trough sediments contaminated with feces from cattle excreting E. coli O157 may serve as a long-term reservoir of this organism on farms and a source of infection for cattle.     

Microbial leaching of zinc concentrate in fresh‐water microcosms: Comparison between aerobic and oxygen‐limited conditions

Microbially mediated leaching and solubilization of zinc ore concentrate by native aquatic microbial communities incubated under aerobic and oxygen‐limited conditions were examined in static microcosms consisting of stream sediment and water. Sterile controls provided information on abiotic sulfide oxidation and leaching of zinc, lead, cadmium, and copper. The flux of these heavy metals from the sediments to the water column was greatest under biotic oxygen‐limited conditions. When calculated as the percentage of total metal available in zinc concentrate‐amended microcosms, the order of metal solubilization under oxygen‐limited conditions was lead, copper, zinc, and cadmium. Under biotic aerobic conditions, the order of solubilization was zinc, lead, cadmium, and copper. This study indicates that aquatic heterotrophs are capable of leaching and solubilizing metallic sulfides under conditions of neutral to slightly acidic pH and are effective in releasing heavy metals to the water column under oxygen‐limited conditions.     

THE EFFECT OF A HORN SNAIL ON ULVA EXPANSA (CHLOROPHYTA): CONSUMER OR FACILITATOR OF GROWTH?1

To determine whether California horn snails are more likely to be consumers or facilitators of Ulva expansa (Setch) S. & G. growth in estuaries, we conducted manipulative experiments that evaluated algal growth and the movement of N between the water column, algal tissue, and, in the second experiment, sediments. Algae grew poorly in the absence of sediments, drawing on their own sequestered N supplies (3.5% of dry weight reduced to <2%) and N released by snails and by depleting inorganic N in the water column. There was no evidence of consumption when snail densities ranged from 0 to 900.m^?2 (0, 3, 6, and 9 per aquarium), as algal growth was similar for all snail densities, and snail lengths did not increase during the 21–d experiment. when sediment was provided, N was depleted in the sediment and enhanced in the algal tissue. As in the first experiment, the water column was depleted of inorganic N and enriched with organic N, mostly in the dissolved form. Because both snails and macroalgae often dominate the shallow waters of southern California's lagoons and estuaries, our evidence that the snails are primarily facilitators of algal growth (via transfer of N from sediments to the water column) suggests that snails may play an important role in both food web and N dynamics.     

Seasonality of phytoplankton in relation to silicon cycling and interstitial water circulation in large,shallow lakes of central Canada

The main basins of Lake Winnipeg (52°N 97°E) and Southern Indian Lake (57°N 99°W) had similar phytoplankton cycles during their open water seasons. A brief spring algal maximum was followed by an early summer minimum and, subsequently, an extended autumnal increase when highest biomasses were observed. The maxima were dominated by Melosira spp. The seasonal cycle of Melosira followed closely the seasonal cycle of dissolved Si. These basins exhibited a typical phytoplankton cycle for dimictic lakes even though they did not form a significant thermocline (1°C per meter).The lakes were well-mixed because they were shallow and had large wind fetches. Although thermal stability of the water column was always low, it was positive until maximum heat content was achieved at which time it became nil or negative. These lakes heated and cooled rapidly, and sediment heat storage was a substantial fraction of the total heat budget. Because heating and cooling of water and of sediments were out of phase, heat exchange at the sediment surface could control vertical circulation of interstitial water, nutrient exchange across the sediment-water interface and the seasonality of phytoplankton. Thermal gradients in the sediments during the heating season would be quite pronounced (4°C per meter).It is proposed that positive stability in interstitial waters during the heating season would impose molecular diffusive transport on the sediment column. When the lakes begin to cool, the upper interstitial water column would become thermally unstable and circulation would occur within the sediments. This would result in the observed net flux of dissolved Si, and other nutrients, out of the sediments into the overlying waters. As a consequence, in Lake Winnipeg and Southern Indian Lake the highest phytoplankton biomasses and productivity occurred in the late summer and autumn.     

LA35 Poultry Fecal Marker Persistence Is Correlated with That of Indicators and Pathogens in Environmental Waters

Disposal of fecally contaminated poultry litter by land application can deliver pathogens and fecal indicator bacteria (FIB) into receiving waters via runoff. While water quality is regulated by FIB enumeration, FIB testing provides inadequate information about contamination source and health risk. This microbial source tracking (MST) study compared the persistence of the Brevibacterium sp. strain LA35 16S rRNA gene (marker) for poultry litter with that of pathogens and FIB under outdoor, environmentally relevant conditions in freshwater, marine water, and sediments over 7 days. Salmonella enterica, Campylobacter jejuni, Campylobacter coli, Bacteroidales, and LA35 were enumerated by quantitative PCR (qPCR), and Enterococcus spp. and E. coli were quantified by culture and qPCR. Unlike the other bacteria, C. jejuni was not detectable after 48 h. Bacterial levels in the water column consistently declined over time and were highly correlated among species. Survival in sediments ranged from a slow decrease over time to growth, particularly in marine microcosms and for Bacteroidales. S. enterica also grew in marine sediments. Linear decay rates in water (k) ranged from −0.17 day⁻¹ for LA35 to −3.12 day⁻¹ for C. coli. LA35 levels correlated well with those of other bacteria in the water column but not in sediments. These observations suggest that, particularly in the water column, the fate of LA35 in aquatic environments is similar to that of FIB, C. coli, and Salmonella, supporting the hypothesis that the LA35 marker gene can be a useful tool for evaluating the impact of poultry litter on water quality and human health risk.     

Genetically Engineered Erwinia carotovora in Aquatic Microcosms: Survival and Effects on Functional Groups of Indigenous Bacteria

The survival of genetically engineered Erwinia carotovora L-864, with a kanamycin resistance gene inserted in its chromosome, was monitored in the water and sediment of aquatic microcosms. The density of genetically engineered and wild-type E. carotovora strains declined at the same rate, falling in 32 days below the level of detection by viable counts. We examined the impact of the addition of genetically engineered and wild-type strains on indigenous bacteria belonging to specific functional groups important in nutrient cycling. For up to 16 days, the densities of total and proteolytic bacteria were significantly higher (P < 0.05) in microcosms inoculated with genetically engineered or wild-type E. carotovora, but by 32 days after inoculation, they had decreased to densities similar to those in control microcosms. Inoculation of genetically engineered or wild-type E. carotovora had no apparent effect on the density of amylolytic and pectolytic bacteria in water and sediment. Genetically engineered and wild-type E. carotovora did not have significantly different effects on the densities of specific functional groups of indigenous bacteria (P > 0.05).     

The relative importance of sediment and water column supplies of nutrients to the growth and tissue nutrient content of the green macroalga Enteromorpha intestinalis along an estuarine resource gradient

Large blooms of opportunistic green macroalgae such as Enteromorpha intestinalis are of ecological concern in estuaries worldwide. Macroalgae derive their nutrients from the water column but estuarine sediments may also be an important nutrient source. We hypothesized that the importance of these nutrient sources to E. intestinalis varies along a nutrient-resource gradient within an estuary. We tested this in experimental units constructed with water and sediments collected from 3 sites in Upper Newport Bay estuary, California, US, that varied greatly in water column nutrient concentrations. For each site there were three treatments: sediments + water; sediments + water + Enteromorpha intestinalis (algae); inert sand + water + algae. Water in units was exchanged weekly simulating low turnover characteristic of poorly flushed estuaries. The importance of the water column versus sediments as a source of nutrients to E. intestinalis varied with the magnitude of the different sources. When initial water column levels of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) were low, estuarine sediments increased E. intestinalis growth and tissue nutrient content. In experimental units from sites where initial water column DIN was high, there was no effect of estuarine sediments on E. intestinalis growth or tissue N content. Salinity, however, was low in these units and may have inhibited growth. E. intestinalis growth and tissue P content were highest in units from the site with highest initial sediment nutrient content. Water column DIN was depleted each week of the experiment. Thus, the water column was a primary source of nutrients to the algae when water column nutrient supply was high, and the sediments supplemented nutrient supply to the algae when water column nutrient sources were low. Depletion of water column DIN in sediment + water units indicated that the sediments may have acted as a nutrient sink in the absence of macroalgae. Our data provide direct experimental evidence that macroalgae utilize and ecologically benefit from nutrients stored in estuarine sediments.