Spatiotemporal Variation of Bacterial Community Composition and Possible Controlling Factors in Tropical Shallow Lagoons

Bacterial community composition (BCC) has been extensively related to specific environmental conditions. Tropical coastal lagoons present great temporal and spatial variation in their limnological conditions, which, in turn, should influence the BCC. Here, we sought for the limnological factors that influence, in space and time, the BCC in tropical coastal lagoons (Rio de Janeiro State, Brazil). The Visgueiro lagoon was sampled monthly for 1 year and eight lagoons were sampled once for temporal and spatial analysis, respectively. BCC was evaluated by bacteria-specific PCR-DGGE methods. Great variations were observed in limnological conditions and BCC on both temporal and spatial scales. Changes in the BCC of Visgueiro lagoon throughout the year were best related to salinity and concentrations of NO ₃ ^? , dissolved phosphorus and chlorophyll-a, while changes in BCC between lagoons were best related to salinity and dissolved phosphorus concentration. Salinity has a direct impact on the integrity of the bacterial cell, and it was previously observed that phosphorus is the main limiting nutrient to bacterial growth in these lagoons. Therefore, we conclude that great variations in limnological conditions of coastal lagoons throughout time and space resulted in different BCCs and salinity and nutrient concentration, particularly dissolved phosphorus, are the main limnological factors influencing BCC in these tropical coastal lagoons.     

Within- and between-lake variability in the composition of bacterioplankton communities: investigations using multiple spatial scales

This study examined the similarity of epilimnetic bacterial community composition (BCC) across several within- and among-lake spatial scales, and the environmental factors giving rise to similar bacterial communities in different lakes were also explored. Samples were collected from 13 northern and southern Wisconsin lakes representing gradients in lake size, productivity, dissolved organic carbon and humic acid contents, and pH. Hypotheses regarding patchy distribution of bacterial communities in lakes were tested by comparing samples collected from nearby (tens of meters) and distant (hundreds of meters) sampling sites in the same lake. BCC was characterized by using a molecular fingerprinting technique, automated ribosomal intergenic spacer analysis (ARISA). Overall, samples collected at the 10-m, 100-m, and between-lake scales differed by 13, 17, and 75%, respectively. Variation at these last two scales was significant. The development of within-lake variation in BCC appeared to depend on the isolation of water by lake shoreline features such as bays or narrow constrictions. ARISA profiles from northern lakes had fewer peaks and were less similar to each other than were those of the southern lakes, suggesting that regional features do not necessarily lead to the development of similar bacterial communities. Lakes at similar positions on productivity and dissolved organic carbon concentration gradients had similar bacterial communities, and bacterial diversity was positively correlated with lake productivity and water temperature. Factorial studies taking into account these gradients, as well as regional spatial scales, should provide much insight into the nature of aquatic bacterial biogeography.     

Within- and between-Lake Variability in the Composition of Bacterioplankton Communities: Investigations Using Multiple Spatial Scales

This study examined the similarity of epilimnetic bacterial community composition (BCC) across several within- and among-lake spatial scales, and the environmental factors giving rise to similar bacterial communities in different lakes were also explored. Samples were collected from 13 northern and southern Wisconsin lakes representing gradients in lake size, productivity, dissolved organic carbon and humic acid contents, and pH. Hypotheses regarding patchy distribution of bacterial communities in lakes were tested by comparing samples collected from nearby (tens of meters) and distant (hundreds of meters) sampling sites in the same lake. BCC was characterized by using a molecular fingerprinting technique, automated ribosomal intergenic spacer analysis (ARISA). Overall, samples collected at the 10-m, 100-m, and between-lake scales differed by 13, 17, and 75%, respectively. Variation at these last two scales was significant. The development of within-lake variation in BCC appeared to depend on the isolation of water by lake shoreline features such as bays or narrow constrictions. ARISA profiles from northern lakes had fewer peaks and were less similar to each other than were those of the southern lakes, suggesting that regional features do not necessarily lead to the development of similar bacterial communities. Lakes at similar positions on productivity and dissolved organic carbon concentration gradients had similar bacterial communities, and bacterial diversity was positively correlated with lake productivity and water temperature. Factorial studies taking into account these gradients, as well as regional spatial scales, should provide much insight into the nature of aquatic bacterial biogeography.     

Terrestrial dissolved organic matter inflow drives temporal dynamics of the bacterial community of a subarctic estuary (northern Baltic Sea)

Climate change is projected to cause increased inflow of terrestrial dissolved organic matter to coastal areas in northerly regions. Estuarine bacterial community will thereby receive larger loads of organic matter and inorganic nutrients available for microbial metabolism. The composition of the bacterial community and its ecological functions may thus be affected. We studied the responses of bacterial community to inflow of terrestrial dissolved organic matter in a subarctic estuary in the northern Baltic Sea, using a 16S rRNA gene metabarcoding approach. Betaproteobacteria dominated during the spring river flush, constituting ~ 60% of the bacterial community. Bacterial diversity increased as the runoff decreased during summer, when Verrucomicrobia, Betaproteobacteria, Bacteroidetes, Gammaproteobacteria and Planctomycetes dominated the community. Network analysis revealed that a larger number of associations between bacterial populations occurred during the summer than in spring. Betaproteobacteria and Bacteroidetes populations appeared to display similar correlations to environmental factors. In spring, freshly discharged organic matter favoured specialists, while in summer a mix of autochthonous and terrestrial organic matter promoted the development of generalists. Our study indicates that increased inflows of terrestrial organic matter-loaded freshwater to coastal areas would promote specialist bacteria, which in turn might enhance the transformation of terrestrial organic matter in estuarine environments.     

Phosphorus and DOC availability influence the partitioning between bacterioplankton production and respiration in tidal marsh ecosystems

The organic carbon consumed by aquatic bacteria (BCC) is partitioned between bacterial production (BP) and respiration (BR), but the factors that determine BCC and its partition into BP and BR are not well understood. We explored the coupling between BR, BR and BCC, and their links to dissolved organic carbon (DOC) and nutrient availability in natural and restored tidal marshes and in the adjoining waters of Delaware Bay estuary. Labile DOC (LDOC) ranged from 3% to 22% of the DOC pool, and explained more of the variance in both BR and BCC than did bulk DOC. Bacterial growth efficiency (BGE) was highly variable (0.09-0.58), and natural Spartina alterniflora marshes had consistently higher BGE than both restoration marshes and tidal floodwaters. BGE was negatively related to the ratio of LDOC to total dissolved phosphorous, which was highest in natural marshes. The enhancement of BP observed in the marshes relative to the estuarine floodwaters had different origins: In natural marshes it was mostly due to increases in BGE, whereas in restored marshes it followed increased BCC. These results highlight the importance of P in regulating microbial metabolism in coastal areas, and the need to understand the pathways that lead to BP in these systems.     

Effects of food on bacterial community composition associated with the copepod Acartia tonsa Dana

The estuarine copepod Acartia tonsa naturally carried diverse strains of bacteria on its body. The bacterial community composition (BCC) remained very conservative even when the copepod was fed different axenic algal species, indicating that the food per se did not much affect BCC associated with the copepod. In xenic algal treatments, however, copepod-associated BCC differed with each alga fed, even though the same bacterial source was used to inoculate the algae. In addition, starved copepods taken at the same location but at different times significantly differed in their BCC. Algal species composition and copepod life history therefore serve to regulate BCC associated with copepods, and spatial and temporal variations in algal species composition and copepod origin would alter bacteria–copepod interactions.     

The effect of temperature and algal biomass on bacterial production and specific growth rate in freshwater and marine habitats

We analyzed heterotrophic, pelagic bacterial production and specific growth rate data from 57 studies conducted in fresh, marine and estuarine/coastal waters. Strong positive relationships were identified between 1) bacterial production and bacterial abundance and 2) bacterial production and algal biomass. The relationship between bacterial production and bacterial abundance was improved by also considering water temperature. The analysis of covariance model revealed consistent differences between fresh, marine and estuarine/coastal waters, with production consistently high in estuarine/coastal environments. The log-linear regression coefficient of abundance was not significantly different from 1.00, and this linear relationship permitted the use of specific growth rate (SGR in day⁻¹) as a dependent variable. A strong relationship was identified between specific growth rate and temperature. This relationship differed slightly across the three habitats. A substantial portion of the residual variation from this relationship was accounted for by algal biomass, including the difference between marine and estuarine/coastal habitats. A small but significant difference between the fresh- and saltwater habitats remained. No significant difference between the chlorophyll effect in different habitats was identified. The model of SGR against temperature and chlorophyll was much weaker for freshwater than for marine environments. For a small subset of the data set, mean cell volume accounted for some of the residual variation in SGR. Pronounced seasonality, fluctuations in nutrient quality, and variation of the grazing environment may contribute to the unexplained variation in specific growth.     

Iron and fluorescent dissolved organic matter in an estuarine and coastal system in Japan

Terrestrially derived dissolved organic matter (DOM) plays an important role in providing ligands that regulate iron transport to coastal ecosystems. Fluorescent DOM (FDOM) is widely used to evaluate DOM behavior, but the relationship between iron and FDOM distributions in estuarine and coastal systems is poorly understood. In this study, we investigated the distributions of dissolved iron (DFe < 0.7-µm fraction) and FDOM as measured using fluorescence excitation-emission matrices in the Yura River–Tango Bay system. The DFe distribution was not closely correlated with that of the humic-like FDOM, indicating that humic-like FDOM is not the main regulator of the DFe concentration in this system. The relatively low fluorescence intensity of humic-like FDOM demonstrates that colloidal inorganic iron is the major component of riverine DFe. A large proportion of the riverine DFe (> 94%) was removed in the mixing process, but humic-like FDOM likely supports the transport of DFe to the coastal zone.     

Impact of environmental factors on couplings between bacterial community composition and ectoenzymatic activities in a lacustrine ecosystem

This study examined the effects of temporal changes in bacterial community composition (BCC) and environmental factors on potential ectoenzymatic activities (α-glucosidase, β-glucosidase, alkaline phosphatase and leucine aminopeptidase) in a lacustrine ecosystem (Sep reservoir, France). BCC was assessed by terminal restriction fragment length polymorphism. Physical parameters, and inorganic and organic nutrient concentrations (dissolved carbohydrates and proteins) were measured in lakes and tributaries. According to the multivariate statistics (redundancy analysis), physical and chemical factors explained the largest part of leucine aminopeptidase activity, whereas the temporal changes of other ectoenzymatic activities were partly dependent on the variations in the BCC. In particular, the occurrence of occasional bacterial populations seemed to explain a lot of the variation in rates and patterns of polymer hydrolysis. The relation observed in this study between the bacterial structure and activity is discussed within the framework of biodiversity–ecosystem functioning.     

Response of bacterial communities to cyanobacterial harmful algal blooms in Lake Taihu,China

Cyanobacterial harmful algal blooms are prevalent around the world, influencing aquatic organisms and altering the physico-chemical properties in freshwater systems. However, the response of bacterial communities to toxic cyanobacterial blooms and associated microcystins (MC) remain poorly understood even though global concentrations of MC have increased dramatically in the past few decades. To address this issue, the dynamics of bacterial community composition (BCC) in the water column and how BCC is influenced by both harmful cyanobacterial blooms and environmental factors were investigated on a monthly basis from August 2013 to July 2014 in Lake Taihu, China. Non-metric multidimensional scaling (NMDS) revealed that seasonal variation in BCC was significant, and that the succession of BCC greatly depends on changes in environmental conditions. Redundancy analysis (RDA) results showed that the overall variation of BCC was explained mainly by dissolved oxygen (DO), nitrate nitrogen (NO₃⁻-N), and Microcystis. The alpha biodiversity of the bacterial community was different among months with the highest diversity in February and the lowest diversity in October. Furthermore, significant negative relationships were found between alpha biodiversity indices and Microcystis abundance as well as with intracellular MC concentrations, indicating that Microcystis and associated MC may influence the bacterial community structure by reducing its biodiversity. This study shows that potential associations exist between toxic cyanobacterial blooms and bacterial communities but more investigations are needed to obtain a mechanistic understanding of their complex relationships.     

Differential decomposition of humic acids by marine and estuarine bacterial communities at varying salinities

Humic substances (HS) constitute 50–80% of total dissolved organic matter (DOM) in freshwaters but in the open ocean <3%, indicating that large fractions of DOM and HS are removed in the estuarine and coastal zone. In order to assess the role of bacteria in this removal, we conducted experiments in flow-through cultures to examine the decomposition of peat bog-born humic acids (HA) by marine (salinity 30, Exp1) and estuarine (salinity 10, Exp2) bacterial communities. After ~70 days 40–>60% of the HA were decomposed. Highest fractions were decomposed in treatments in which the bacterial communities were fed by HA media of a foreign salinity, e.g., of 14 in Exp1 and of 28 in Exp2. Some of the HA material was not decomposed but broken down to smaller moieties entering the fulvic acid (FA) fraction. The analysis of the HA media and their residuals after bacterial decomposition by pyrolysis GC/MS showed that individual organic compounds were decomposed. In Exp1, 32 aliphatic and aromatic compounds, including lignin biomarkers, were detected in the HA medium of which mainly aliphatic compounds were decomposed. In Exp2, 49 compounds were detected of which ~40–60% were not detected any more after bacterial decomposition in the HA fraction but still in the FA fraction. The results show that estuarine and marine bacterial communities can decompose large amounts of HA and that this process is important in reducing the amount of terrestrial HS and DOM entering the estuarine and coastal region.     

Increased freshwater discharge shifts the trophic balance in the coastal zone of the northern Baltic Sea

Increased precipitation is one projected outcome of climate change that may enhance the discharge of freshwater to the coastal zone. The resulting lower salinity, and associated discharge of both nutrients and dissolved organic carbon, may influence food web functioning. The scope of this study was to determine the net outcome of increased freshwater discharge on the balance between auto‐ and heterotrophic processes in the coastal zone. By using long‐term ecological time series data covering 13 years, we show that increased river discharge suppresses phytoplankton biomass production and shifts the carbon flow towards microbial heterotrophy. A 76% increase in freshwater discharge resulted in a 2.2 times higher ratio of bacterio‐ to phytoplankton production (P_b:P_p). The level of P_b:P_p is a function of riverine total organic carbon supply to the coastal zone. This is mainly due to the negative effect of freshwater and total organic carbon discharge on phytoplankton growth, despite a concomitant increase in discharge of nitrogen and phosphorus. With a time lag of 2 years the bacterial production recovered after an initial decline, further synergistically elevating the microbial heterotrophy. Current climate change projections suggesting increased precipitation may therefore lead to increased microbial heterotrophy, thereby decreasing the transfer efficiency of biomass to higher trophic levels. This prognosis would suggest reduced fish production and lower sedimentation rates of phytoplankton, a factor of detriment to benthic fauna. Our findings show that discharge of freshwater and total organic carbon significantly contributes to the balance of coastal processes at large spatial and temporal scales, and that model's would be greatly augmented by the inclusion of these environmental drivers as regulators of coastal productivity.     

Microbial Biogeography along an Estuarine Salinity Gradient: Combined Influences of Bacterial Growth and Residence Time

Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([¹⁴C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.     

Microbiome mediating methane and nitrogen transformations in a subterranean estuary

Subterranean estuaries (STEs) are important coastal biogeochemical reactors facilitating unique niches for microbial communities. A common approach in determining STE greenhouse gas and nutrient fluxes is to use terrestrial endmembers, not accounting for microbially mediated transformations throughout the STE. As such, the microbial ecology and spatial distribution of specialists that cycle compounds in STEs remain largely underexplored. In this study, we applied 16S rRNA amplicon sequencing with paired biogeochemical characterisations to spatially evaluate microbial communities transforming greenhouse gases and nutrients in an STE. We show that methanogens are most prevalent at the terrestrial end (up to 2.81% relative abundance) concomitant to the highest porewater methane, carbon dioxide and dissolved organic carbon concentrations (0.41 ± 0.02 μM, 273.31 ± 6.05 μM and 0.51 ± 0.02 mM, respectively). Lower ammonium concentrations corresponded with abundant nitrifying and ammonia-oxidising prokaryotes in the mixing zone (up to 11.65% relative abundance). Methane, ammonium and dissolved organic carbon concentrations all decreased by >50% from the terrestrial to the oceanic end of the 15 m transect. This study highlights the STE's hidden microbiome zonation, as well as the importance of accounting for microbial transformations mitigating nutrient and greenhouse gas fluxes to the coastal ecosystems.     

Primary production under hypertrophic conditions and its relationship with bacterial production

In shallow hypertrophic lakes where light availability restricts the growth of macrophytes and benthic phytoplankton, pelagic phytoplankton modulates importantly ecosystem production and the energy transfer to heterotrophic bacteria. Diel and seasonal variations in primary production (PP) were studied in the hypertrophic Albufera de Valencia (Spain). Additionally, the relationship between PP and heterotrophic bacterial production (BP) was assessed. PP was extremely high, exceeding most values reported for hypertrophic lakes to date. PP displayed marked diurnal variations defined by the solar radiation curve. Likewise, PP changed importantly across seasons. Minimum PP coincided with maximum water transparency and short water residence times in winter, whereas maximum PP was observed in late spring associated with high chlorophyll a. The spring PP maximum contrasted with the summer maximum often observed in hypertrophic lakes. When compared to spring PP values, summer PP values were lower as a result of strong nitrogen limitation. In contrast to PP, BP remained fairly constant across seasons. Nonetheless, there was a joint diminution during increased water transparency followed by an increase in early spring. Phytoplankton was always the most relevant input to particulate carbon production, but the BP/PP ratio showed clear seasonal variations. The BP/PP ratio was minimum in spring, low in summer and highest in winter. The extracellular dissolved organic carbon released by phytoplankton was sufficient to meet bacterial carbon demand in all experimental dates, suggesting that allochthonous carbon sources play a minor role in sustaining BP, though they cannot be excluded. However, we hypothesize that high availability of dissolved organic carbon might explain the lack of coupling observed between BP and PP.     

Effects of the emergent macrophyte Juncus effusus L. on the chemical composition of interstitial water and bacterial productivity

Release of oxygen from the roots ofaquatic macrophytes into anaerobic sediments canaffect the quantity of interstitial dissolved organicmatter and nutrients that are available to bacteria. Nutrient and dissolved organic carbon (DOC)concentrations were compared between subsurface(interstitial) waters of unvegetated sediments andsediments among stands of the emergent herbaceousmacrophyte Juncus effusus L. in a lotic wetlandecosystem. Concentrations of inorganic nitrogen(NH₄ ⁺, NO₃ ^-, and NO₂ ^-)were greater from sediments of the unvegetatedcompared to the vegetated zone. DOC concentrations ofinterstitial waters were greater in sediments of theunvegetated zone both in the winter and springcompared to those from the vegetated zone. AlthoughDOC concentrations in hydrosoils collected from bothzones increased from winter to spring, bacterialproductivity per mg DOC in spring decreased comparedto winter. Greater initial bacterial productivityoccurred on DOM collected from the vegetated comparedto the unvegetated zone in winter samples (days 1 and4), with increased bacterial productivity on samplescollected from the unvegetated zone at the end of thestudy (day 20). Bacterial productivity wassignificantly greater on all sampling days on DOM fromvegetated samples compared to unvegetated samples. In nutrient enrichment experiments, bacterialproductivity was significantly increased (p < 0.05)with phosphorus but not nitrogen only amendments.     

Nutrient stoichiometry,freshwater residence time,and nutrient retention in a river-dominated estuary in the Mississippi Delta

A 3-month field study was conducted to examine the effects of Atchafalaya River discharge on nitrogen and phosphorus concentrations in the Fourleague Bay system, to document patterns with salinity variation, to evaluate stoichiometric nutrient ratios of nitrogen and phosphorus in the river and bay, and to examine the relationship between estuarine freshwater residence time and export of total nitrogen (TN) and total phosphorus (TP) to the Gulf of Mexico. During spring peak discharge of the Atchafalaya River, nutrient ratios in lower Fourleague Bay indicate potential phosphorus limitation with an average dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) ratio of 32:1, primarily a result of high concentrations of nitrogen entering the northern bay from the Atchafalaya River and of fairly stable phosphorus concentrations. Ratios of DIN to phosphorus in the river were much higher (54:1), indicating a significant loss of nitrogen within the Fourleague Bay system. Freshwater residence time averaged approximately 7 days during the study and ranged from 2 to 100 days. TN export averaged 57% over the study and ranged from less than 3% at long residence times to greater than 80% at short residence times. TN export to the coastal ocean with respect to residence time is considerably less than has been shown in other studies. Nitrate + nitrite export averaged 49% for the 3-month study. Percentages of TP export were greater than TN, averaging 82% for the study period. By examining the Atchafalaya River delta as a natural analog for controlled river diversions, which are currently being used as coastal restoration tools, this study shows that discharging river water into highly productive shallow coastal estuarine and wetland systems can significantly reduce the amount of nitrogen exported to the Gulf of Mexico.     

Nutrient biogeochemistry in the water column (N,P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands)

The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients. Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary. Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10–15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing. The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7–18.1 mmol m^?2 d^?1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILSet al., 1993).     

Temporal variation in habitat use by nekton in a subtropical estuarine system

Utilisation by fish of different estuarine habitats is known to vary at many different temporal scales, however no study to date has examined how utilisation varies at all the relevant times scales simultaneously. Here, we compare the utilisation by fish of sandy, intertidal foreshore habitats in a subtropical estuary at four temporal scales: between major spawning periods (spring/summer and winter), among months within spawning periods, between the full and new moon each month, and between night and day within those lunar phases. Comparisons of assemblage composition, abundance of individuals and of fish in seven different ‘ecological guilds’ were used to identify the temporal scales at which fish varied their use of unvegetated sandy habitats in the lower Noosa Estuary, Queensland, Australia. Fish assemblages were sampled with a seine net at three different regions. The most numerically dominant species caught were southern herring (Herklotsichthys castelnaui: Clupeidae), sand whiting (Sillago ciliata: Sillaginidae), weeping toadfish (Torquigener pleurogramma: Tetraodontidae), and silver biddy (Gerres subfasciatus: Gerreidae). Considerable variation at a range of temporal scales from short term (day versus night) to longer term (spawning periods) was detected for all but one of the variables examined. The clearest patterns were observed for diurnal effects, where generally abundance was greater at night than during the day. There were also strong lunar effects, although there were no consistent patterns between full moon and new moon periods. Significant differences among months within spawning periods were more common than differences between the actual spawning periods. The results clearly indicate that utilisation of sandy, unvegetated estuarine habitats is very dynamic and highly variable in space and time.     

Seasonal and Diel Variability in Dissolved DNA and in Microbial Biomass and Activity in a Subtropical Estuary

Dissolved DNA and microbial biomass and activity parameters were measured over a 15-month period at three stations along a salinity gradient in Tampa Bay, Fla. Dissolved DNA showed seasonal variation, with minimal values in December and January and maximal values in summer months (July and August). This pattern of seasonal variation followed that of particulate DNA and water temperature and did not correlate with bacterioplankton (direct counts and [³H]thymidine incorporation) or phytoplankton (chlorophyll a and ¹⁴CO₂ fixation) biomass and activity. Microautotrophic populations showed maxima in the spring and fall, whereas microheterotrophic activity was greatest in late summer (September). Both autotrophic and heterotrophic microbial activity was greatest at the high estuarine (low salinity) station and lowest at the mouth of the bay (high salinity station), irrespective of season. Dissolved DNA carbon and phosphorus constituted 0.11 ± 0.05% of the dissolved organic carbon and 6.6 ± 6.5% of the dissolved organic phosphorus, respectively. Strong diel periodicity was noted in dissolved DNA and in microbial activity in Bayboro Harbor during the dry season. A noon maximum in primary productivity was followed by an 8 p.m. maximum in heterotrophic activity and a midnight maximum in dissolved DNA. This diel periodicity was less pronounced in the wet season, when microbial parameters were strongly influenced by episodic inputs of freshwater. These results suggest that seasonal and diel production of dissolved DNA is driven by primary production, either through direct DNA release by phytoplankton, or more likely, through growth of bacterioplankton on phytoplankton exudates, followed by excretion and lysis.