Seasonal changes in the abundance and distribution of submerged aquatic vegetation in a highly managed coastal lagoon

The inflow of fresh water into coastal lagoons is a key factor influencing the structure and function of these ecosystems. Biscayne Bay, a coastal lagoon adjacent to the city of Miami, is located downstream of the Everglades ecosystem where the extensive water management system now in place has modified the historical hydrology, replacing groundwater and overland flows with pulsed releases from canals. In areas where canals discharge directly into littoral habitats, an environment with low-mean salinity and high-salinity variability is created. In this study, we characterize the salinity patterns of nearshore habitats (<500 m from shore) and document patterns of seasonal abundance and distribution of submerged aquatic vegetation (SAV) to evaluate the impacts of water management practices. Seagrasses were the principal component of the SAV community during the 2005 dry season (mean cover = 25.5%), while macroalgae dominated during the wet season (mean cover = 33.4%). The distribution and abundance of SAV were directly related to the tolerance of each taxon to salinity patterns. Seagrass species with high tolerance to low and variable salinity such as Halodule wrightii and Ruppia maritima were found only in canal-influenced areas and increased in abundance and spatial distribution in the wet season when freshwater inflow is highest. The dominance of rhizophytic macroalgae during the wet season was determined by the appearance and high abundance of Chara, a taxon commonly associated with freshwater environments. Thalassia testudinum, the most abundant seagrass species, was found throughout the study region, but decreased in abundance in the canal-influenced areas during the wet season when lower, more variable salinity resulted in lowered productivity. The data presented here showed a significant relationship between salinity patterns and the seasonal abundance and distribution of SAV. These findings support the use of SAV as appropriate indicators of changes in water quality resulting from future restoration projects associated with the Everglades Restoration Plan, which will once again modify the delivery of fresh water into littoral habitats with unknown ecological consequences. Handling editor: S. M. Thomaz     

Spatial variability of stable carbon and nitrogen isotope ratios in a Mediterranean coastal lagoon

Exploring the trophic pathway of organic matter within the Mauguio lagoon (southern France, western Mediterranean), we found spatial differences in the isotopic composition (both δ¹³C and δ¹⁵N values) of organic matter sources (primary producers, particulate and sedimentary organic matter), which were mirrored in the upper trophic levels (invertebrates and fish). On average, δ¹³C was heavier by about 1.5–2‰ in the location under marine influence than in the sites influenced by freshwater discharge. The opposite trend was found for δ¹⁵N, which attained maximum values in the north-central zone influenced by freshwater delivery. For both C and N stable isotope ratios, the highest spatial variability was found in organic matter sources (2–3‰), while invertebrates and fish exhibited less variability (\~1–2‰). The differences observed may be related to both anthropogenic (wastewater input) and natural (marine vs. terrestrial inputs) factors. Discharge of wastewater, which affects the innermost location, generally determines an increase in the relative abundance of ¹⁵N. In addition, terrestrially derived nutrients and organic matter, which also affect the innermost location, are known to determine a shift towards ¹³C-depleted values. Our results substantiate the finding that the analysis of carbon and nitrogen stable isotopes can help in elucidating origin and fate of organic matter in coastal lagoons, which are characterised by a great spatial variability and complexity.     

Phytoplankton biomass and species composition in a Mediterranean coastal lagoon

Phytoplankton populations were investigated weekly at a central station in the Fusaro lagoon (Mediterranean Sea) from 27 November 1989 to 18 June 1990 to assess species composition, temporal succession and standing stock of the different species. Chlorophyll concentrations varied from 1.2 to 73.2 µg 1^–1 in surface waters, and from 1.3 to 53.5 µg 1^–1 at the 4.5 m depth. Phytoplankton communities were dominated by Prorocentrum micans Ehrenberg in December and January, and by small-sized diatoms in the rest of the sampling period. In surface waters, a maximum biomass of 9.5 mg C 1^–1 was measured in January, in correspondence with high concentrations (8.1 × 10⁶ cells 1^–1) of P. micans, whereas an abundance peak of 159.9 × 10⁶ cells 1^–1 was registered on the last sampling date due to a massive bloom of a very small diatom, Minutocellus polymorphus (Hargraves & Guillard) Hasle, von Stosch & Syvertsen. On the whole, phytoplankton populations of the Fusaro lagoon showed distinct characters as compared to those of southern Tyrrhenian coastal waters and of other lagoons.     

A comparative study of the effect of pH and inorganic carbon resources on the photosynthesis of three floating macroalgae species of a Mediterranean coastal lagoon

This study examines the effect of pH changes on photosynthetic characteristics and the role of dissolved inorganic carbon (DIC) in determining the dominance of three species of macroalgae Chaetomorpha linum (O.F. Müller) Kützing, Gracilaria verrucosa (Hudson) Papenfuss and Ulva sp. in a Mediterranean coastal lagoon. Fluctuations of pH were measured in the lagoon in summer. Water column CO(2) and HCO(3)(-) concentrations inside the algal mat showed significant diurnal fluctuations, from a morning peak to an afternoon low, decreasing 96 and 40%, respectively. The response of photosynthesis to increased pH was examined in laboratory conditions in spring (May) and summer (July). The photosynthetic rate declined rapidly at pH above 8.5 and below 6.5. G. verrucosa responded differently in spring and summer showing acclimation to higher pH in summer than in spring. In Ulva sp. incubations, we observed optimum photosynthesis between pH 6 and 7.5. The decrease in photosynthetic rate below pH 6 was lower (12.30%) than above pH 8 (81.03%). This difference may be related to the origin of the macroalgae, suggesting acclimation to the original pH of the environment. Results from instantaneous photosynthesis measurements indicate that low DIC-availability limits the photosynthetic capacity of G. verrucosa, C. linum and Ulva sp. in spring and of C. linum at high irradiances in summer. Our results also show that G. verrucosa has a higher efficiency at low CO(2) concentrations than C. linum and Ulva sp. It is suggested that G. verrucosa may be better adapted to maintain higher photosynthetic rates than other macroalgae in conditions of tissue N sufficiency and low water DIC concentrations which are typical of shallow coastal environments in summer.     

Presence of Gymnodinium catenatum (Dinophyceae) in a coastal Mediterranean lagoon

The occurrence and abundance of the toxic, chain-forming dinoflagellateGymnodinium catenatum in a Tyrrhenian coastal lagoon, the Fusaro,during an annual sampling cycle are reported. Peak abundanceswere observed from late spring until early autumn Although veryhigh cell numbers were recorded, up to 1 5 x 10⁶ cells l^–1,no monospecific bloom of this species occurred. The first observationof G.catenatum in the Mediterranean occurred in the Fusaro andthe appearance of this species in a traditional shellfish farmingarea, where no shellfish intoxication has been reported to date,is discussed in relation to human interventions in the basin.In particular, intensive dredging in recent years with resuspensionof bottom sediments may have seeded the water body with cysts.A Gymnodinium n d species, illustrated using scanning electronmicroscopy, caused a monospecific bloom in concomitance withmaximum abundances of G.catenatum, apparently outcompeting thislatter species     

Seasonal plankton dynamics in a Mediterranean hypersaline coastal lagoon: the Mar Menor

The seasonal distribution of plankton in a Mediterranean hypersalinecoastal lagoon has been studied through a dataset, comprisingthe taxonomic composition and the size–abundance distributionof both phyto- and zooplankton, measured by image analysis techniquesduring a one-year time series of weekly samplings. The studiedorganisms ranged from small nanoplanktonic heterotrophic flagellates(2 µm diameter) to fish larvae (>2 µm). The phytoplanktonannual succession was characterized by a winter period dominatedby Rhodomonas spp. and Cryptomonas spp. with Cyclotella spp.as the main diatom represented, a spring phase where diatoms(mainly Cyclotella) were the dominant group with some monospecificblooms of other diatoms (mainly of Chaetoceros sp.), a summerphase characterized by diatoms with blooms of Niztschia closterium,and a post-summer phase where dinoflagellates increased withpeaks of Ceratium furca. High densities of the microbial foodweb elements, flagellates and ciliates, indicate the importanceof the microbial loop in the ecosystem. Meroplankton contributedwidely to the seasonal character of the zooplankton distribution.Copepods, represented by Oithona nana, Centropages ponticusand Acartia spp. (mainly latisetosa), remained relatively constantthroughout the year, exhibiting a lower density in the warmerwater period (July–September). At the end of the samplingperiod, a massive proliferation of copepods (>1000 ind l^–1), mainly due to O. nana, took place. The autotrophsto heterotrophs biovolume ratio (A:H) remained lower than 1throughout the year except when, occasionally, large phytoplanktoncells bloomed. Persistent very low values of A:H suggest thatadditional sources of energy, such as the microbial loop ordetrital pathways, would be needed to sustain the high heterotrophicbiovolume found in the lagoon.     

Seasonal variations in photosynthetic irradiance response curves of macrophytes from a Mediterranean coastal lagoon

The main photosynthesis and respiration parameters (dark respiration rate, light saturated production rate, saturation irradiance, photosynthetic efficiency) were measured on a total of 23 macrophytes of the Thau lagoon (2 Phanerogams, 5 Chlorophyceae, 10 Rhodophyceae and 6 Phaeophyceae). Those measurements were performed in vitro under controlled conditions, close to the natural ones, and at several seasons. Concomitantly, measurements of pigment concentrations, carbon, phosphorous and nitrogen contents in tissues were performed. Seasonal intra-specific variability of photosynthetic parameters was found very high, enlightening an important acclimatation capacity. The highest photosynthetic capacities were found for Chlorophyceae (e.g. Monostroma obscurum thalli at 17 °C, 982 μmol O₂ g⁻¹ dw h⁻¹ and 9.1 μmol O₂ g⁻¹ dw h⁻¹/μmol photons m⁻² s⁻¹, respectively for light saturated net production rate and photosynthetic efficiency) and Phanerogams (e.g. Nanozostera noltii leaves at 25 °C, 583 μmol O₂ g⁻¹ dw h⁻¹ and 2.6 μmol O₂ g⁻¹ dw h⁻¹/μmol photons m⁻² s⁻¹ respectively for light saturated net production rate and photosynthetic efficiency). As expected, species with a high surface/volume ratio were found to be more productive than coarsely branched thalli and thick blades shaped species. Contrary to R_d (ranging 6.7–794 μmol O₂ g⁻¹ dw h⁻¹, respectively for Rytiphlaea tinctoria at 7 °C and for Dasya sessilis at 25 °C) for which a positive relationship with water temperature was found whatever the species studied, the evolution of P/I curves with temperature exhibited different responses amongst the species. The results allowed to show summer nitrogen limitation for some species (Gracilaria bursa-pastoris and Ulva spp.) and to propose temperature preferences based on the photosynthetic parameters for some others (N. noltii, Zostera marina, Chaetomorpha linum).     

The distribution of phosphate in sediments and its relation with eutrophication of a Mediterranean coastal lagoon

A major problem of the Mediterranean coastal lagoons is an excessive input of nutrients (i.e. N and P), causing eutrophic conditions in summer. The sediments of these lagoons can serve as a reservoir by fixing phosphate, or as a source when this phosphate is released under certain conditions. Knowledge of nutrient sources and fluxes is needed if coastal lagoons are to be protected against eutrophication. Therefore, we have evaluated the total pool of phosphate in the lagoon sediments, and the quantity of phosphate which may be released.Sediment profiles have been analysed at two stations of the Lagune de Thau both in and outside the oyster-bank zone. A sequential fractionation, using chelating agents was performed to extract the inorganic (iron and calcium bound phosphate) and the organic phosphate fractions. A statistical analysis of the data set has revealed several significant factors which explain the fluctuations of the concentrations of each phosphate fraction. These factors are: the time of year (seasons), the depth (5 cm layers of sediment), and the site (station).A spatial and a temporal variation of the concentration of Tot-P was found. The largest variation between the two different zones appeared only in the first five cm of sediment. There is only a slight seasonal variation in the amount of phosphate at other depths at the two different zones. Season and station are the factors which control the variations in distribution of phosphate fractions. The spatial and temporal variations of the iron and calcium bound phosphate are explained by the redox potential and pH in the top layer of the sediment.Abbreviations In principle the general abbreviations are used (see page vi). Futhermore Fe(OOH)-P= Ferric hydroxide bound phosphate - CaCO₃-P= Calcium carbonate bound phosphate - ASOP= Acid soluble organic phosphate - ROP= Residual organic phosphate     

Diel variations in Acartia tonsa feeding, respiration and egg production in a Mediterranean coastal lagoon

Short-term changes in abundance, feeding, respiration and spawningby the copepod Acartia tonsa were studied during four 24 h periodsat two stations in a Mediterranean lagoon in May and June of1989 and 1990. Nightly increases of abundance in the upper layer,attributed to nocturnal upward migration, were observed in threeout of the four time series. Nevertheless, occasional dailyincreases were observed too, so that no clear diel patternsin abundance could be defined. Contrarily, diel patterns seemedto appear: (i) for feeding (higher at the beginning of the night),based on gut fluorescence and clearance and ingestion rate measurements;(ii) for respiration (lower at night and based on 8 h incubationperiods); and (iii) for spawning (higher at the end of the night,based on experimental data). The origin of these patterns, aswell as their relationships to environmental data and theirpossible association, are discussed with regard to the availableliterature.     

Effect of nutrient pulses on photosynthesis of Chaetomorpha linum from a shallow Mediterranean coastal lagoon

The influence of nitrogen and phosphorus pulses on Chaetomorpha linum (Muller) Kutzing growth and photosynthesis was studied in laboratory experiments. Photosynthesis and growth of C. linum from Tancada lagoon seems limited by both nitrogen and phosphorus, as indicated by the high rate (4.7–11.6 mg O₂ g⁻¹ dry weight h⁻¹) of light-saturated photosynthesis (P_m) and growth rates observed under nitrogen plus phosphorus enrichment in relation to enrichment by nitrogen alone (2.9–7.6 mg O₂ g⁻¹ dry weight h⁻¹). Significant increase in nitrogen and phosphorus content as percentage of dry weight was observed in C. linum fertilized with a single nutrient or with nitrogen plus phosphorus. In Tancada lagoon, when availability of nitrogen to primary producers is by pulses, an increase of nitrate concentration in the water column (from 6 to 100 μM) has a greater effect on growth of C. linum (growth rate: 0.13 day⁻¹) than an increase in ammonium concentration (from 20 to 100 μM and growth rate: 0.11 day⁻¹). For a given thallus nitrogen content (0.6–1.4% N), both P_m and the photosynthetic efficiency (α) normalized to dry weight were correlated (r² = 0.73, p < 0.005) indicating that variations in electron transport were coupled to variations in C-fixation capacity. Optimizing both α and P_m may be a general characteristic of thin-structured opportunistic algae in more variable estuarine environments.     

Features of hypereutrophic Molino lagoon ecosystem dominated by sedentary polychaetes

Hydrobiologia - Ecological study of the Molino lagoon situated in lowland area at the NW Adriatic coast was undertaken to expose the causes of an extreme sulfide contamination of its bottom...     

Colloidal organic carbon and phytoplankton speciation during a coastal bloom

During the 1995 spring bloom in Bedford Basin, Nova Scotia,dissolved organic carbon (DOC) in the photic zone was separatedinto low-molecular-weight and colloidal size fractions by cross-flowultrafiltration. DOC, colloidal organic carbon (COC) and low-molecular-weightorganic carbon (LOC) were then analyzed by high-temperaturecatalytic oxidation. COC (associated with the production ofphytoplankton exudates) did not follow the concentration ofbulk chlorophyll a (chi a) or the total number of phytoplanktoncells. Instead, surface-active COC (that adhered to the ultrafiltrationsystem) was produced early during the bloom when the diatomSkeletonema costatum was at a maximum. Later on, as the bloombegan to decline, less surface-active COC (that remained largelyfree of the ultrafiltration surfaces) was produced and was associatedmore with variations in cell number of Chaetoceros socialis,the predominant diatom for most of the bloom. These resultssuggest that chl a or the total number of phytoplankton cellsmay not be reliable indices of the production of COC. On thecontrary, the results suggest that relatively high COC concentrationswere associated with specific diatom species. In addition, themaximum amount of COC was associated during the early stagesof the bloom with a diatom (S.costatum) that was a small fraction(<5.6%) of total phytoplankton cell number. This finding,that relatively large amounts of colloids were associated withexopolymer production during the onset and development of abloom, does not agree with reports suggesting that the productionof exopolymers by diatoms is primarily an end-of-bloom occurrence.     

Enhanced biodegradation of diesel fuel through the addition of particulate organic carbon and inorganic nutrients in coastal marine waters

Diesel fuel pollution in coastal waters, resulting from recreational boating and commercial shipping operations, is common and can adversely affect marine biota. The purpose of this study was to examine the effect of additions of particulate organic carbon (POC) in the form of naturally-occurring marsh grass (Spartina alterniflora), inorganic nutrients (nitrogen and phosphorus), inert particles, and dissolved organic carbon (DOC) on diesel fuel biodegradation and to attempt to formulate an effective bioremedial treatment for small diesel fuel spills in marine waters. Various combinations of treatments were added to water samples from a coastal marina to stimulate diesel fuel biodegradation. Diesel fuel was added in concentrations approximating those found in a spill and biodegradation of straight chain aliphatic constituents was estimated by measuring mineralization of ¹⁴C hexadecane added to diesel fuel. All treatments that included POC showed stimulation of biodegradation. However, the addition of inert particles (glass fiber filters and nylon screening) caused no stimulation of biodegradation. The addition of nitrogen and phosphorus alone did not result in stimulation of biodegradation, but nitrogen and Spartina (although not phosphorus and Spartina) did result in stimulation above that of Spartina alone. Maximum biodegradation rates were obtained by the addition of the Spartina POC, ammonium, and phosphate. The addition of mannitol, a labile DOC source with POC and phosphate resulted in a decrease in diesel fuel biodegradation as compared to POC and phosphate alone. The seasonal pattern of diesel fuel biodegradation showed a maximum in the summer and a minimum in the winter. Therefore, of the treatments tested, the most effective for bioremediation of diesel fuel in marine waters is the addition of POC, nitrogen, and phosphorus.     

Enhanced biodegradation of diesel fuel through the addition of particulate organic carbon and inorganic nutrients in coastal marine waters

Diesel fuel pollution in coastal waters, resulting from recreational boating and commercial shipping operations, is common and can adversely affect marine biota. The purpose of this study was to examine the effect of additions of particulate organic carbon (POC) in the form of naturally-occurring marsh grass (Spartina alterniflora), inorganic nutrients (nitrogen and phosphorus), inert particles, and dissolved organic carbon (DOC) on diesel fuel biodegradation and to attempt to formulate an effective bioremedial treatment for small diesel fuel spills in marine waters. Various combinations of treatments were added to water samples from a coastal marina to stimulate diesel fuel biodegradation. Diesel fuel was added in concentrations approximating those found in a spill and biodegradation of straight chain aliphatic constituents was estimated by measuring mineralization of ¹⁴C hexadecane added to diesel fuel. All treatments that included POC showed stimulation of biodegradation. However, the addition of inert particles (glass fiber filters and nylon screening) caused no stimulation of biodegradation. The addition of nitrogen and phosphorus alone did not result in stimulation of biodegradation, but nitrogen and Spartina (although not phosphorus and Spartina) did result in stimulation above that of Spartina alone. Maximum biodegradation rates were obtained by the addition of the Spartina POC, ammonium, and phosphate. The addition of mannitol, a labile DOC source with POC and phosphate resulted in a decrease in diesel fuel biodegradation as compared to POC and phosphate alone. The seasonal pattern of diesel fuel biodegradation showed a maximum in the summer and a minimum in the winter. Therefore, of the treatments tested, the most effective for bioremediation of diesel fuel in marine waters is the addition of POC, nitrogen, and phosphorus.     

Limitation of lowland riverine bacterioplankton by dissolved organic carbon and inorganic nutrients

Flow regulation in lowland rivers has reduced the amount of allochthonous dissolved organic carbon (DOC) entering main channels through less frequent wetting of benches, flood runners and floodplains. The hypothesis tested was that lowland riverine bacterioplankton are DOC limited when flow events are absent and simulating an increase in assimilable DOC similar to that expected during an environmental flow will lead to heterotrophic dominance. Experiments took place in the Namoi River, a highly regulated lowland river in Australia. Specifically, in situ microcosms were used to examine the responses of bacterioplankton and phytoplankton to various additions of DOC as glucose or leaf leachate, with and without additions of inorganic nutrients. The results indicated that ambient DOC availability limited the bacterioplankton for the three seasons over which we conducted the experiments. When DOC was added alone, dissolved oxygen concentrations decreased primarily because of increased bacterial respiration and bacterioplankton growth generally increased relative to controls. Additions of DOC alone led to a pattern of decreased chlorophyll a concentration relative to controls, except for willow leachate. Additions of inorganic nutrients alone increased chlorophyll a concentrations above controls, indicating limitation of phytoplankton. These findings support our hypothesis. Based on the present results, environmental flows should increase the duration of allochthonously driven heterotrophic dominance, thus shifting regulated lowland rivers to more natural (pre-regulation) conditions for greater periods.     

Sources of organic and inorganic carbon in a headwater stream: Evidence from carbon isotope studies

A combination of stable isotope studies and ¹⁴Cdating were used to identify the main sources andprocesses controlling streamwater DOC and TIC in atemperate non-forested watershed. ¹³Cvalues for terrestrial (–24.9 to –29.1) and aquatic(–30.5 to –33.5) plants were similar to valuesreported in the literature for similar ecosystems.¹³C values for DOC in soil solution andstreamwater were consistent with soil and terrestrialvegetation, indicating that the terrestrial ecosystemis the dominant source of aquatic DOC in thiswatershed. ¹³C values of soil atmosphereCO₂ (–17.2 to –25.2) were slightly lessnegative than would be expected for production viaaerobic soil microbial decomposition and rootrespiration. There was a close correspondence between¹³C values (–15.5 to –21.5) forstreamwater TIC and soil atmospheric CO₂ in thecentral part of the catchment where the stream drainsCO₂-rich peats. ¹⁴C dating showed thatalthough peat has been accumulating in the watershedfor at least 2700 years, DOC in soil pore water andstreamwater contains carbon of predominantly recentorigin (post-AD 1955).     

Phosphate and calcium carbonate saturation in a stratified coastal lagoon

Seasonal variation of phosphate concentration and saturation index for calcite in water of a small stratified coastal lagoon have been studied. In surface waters, where salinity was low and pH high, the saturation index increased to values near 20, whereas in bottom water, with high salinity and low pH, they were usually lower. The ionic product for H₃PO₄ was strongly correlated with the ionic product of Ca(OH)₂ in surface and bottom waters, and with the ionic product of CaCO₃ in bottom, which suggested that chemical composition was mainly controlled by a calcium-phosphate solid phase.The low concentrations of phosphate in surface were due to chemical precipitation and organic sedimentation, whereas in bottom, calcium phosphate redissolved and organic matter was mineralized producing high concentrations of soluble phosphate (> 60 µmol l^–1).Decrease of calcium-bound phosphate in the upper layers of sediment was in agreement with a diminution of calcium-phosphate precipitation, probably due to a lower influence of seawater in the past.     

Diurnal changes in dissolved organic and inorganic carbon and nitrogen in a hardwater stream

Organic and inorganic carbon and nitrogen parameters were sampled simultaneously at 6 h intervals over a diurnal period at seven stations in a small hardwater stream in southern Michigan. Concentrations and budget values (kg/day) varied up to 10-fold at individual stations and between stations. The most constant parameters were total dissolved organic carbon and nitrogen. The most variable parameter was particulate organic carbon. Significant changes were noted in all parameters as the stream passed through different stream-side habitats. Budget values facilitated interpretation of these changes over time between stations. High variability in the concentration values was introduced by large allochthonous inputs of dissolved organic matter (DOM) and water alternately adding to or diluting parameters. Distance from upstream terrestrial DOM sources, DOM residence time, the pulsed nature of DOM inputs and biological utilization of DOM have important effects on the quality and quantity of DOM that enters recipient lakes and rivers. The importance of ground-water flux and DOM content is stressed. The DOM input and turnover within hardwater streams appears t o be in rapid, dynamic equilibrium.     

Transporter genes expressed by coastal bacterioplankton in response to dissolved organic carbon

Coastal ocean bacterioplankton control the flow of dissolved organic carbon (DOC) from terrestrial and oceanic sources into the marine food web, and regulate the release of inorganic carbon to atmospheric and offshore reservoirs. While the fate of the chemically complex coastal DOC reservoir has long been recognized as a critical feature of the global carbon budget, it has been problematic to identify both the compounds that serve as major conduits for carbon flux and the roles of individual bacterioplankton taxa in mediating that flux. Here we analyse random libraries of expressed genes from a coastal bacterial community to identify sequences representing DOC‐transporting proteins. Predicted substrates of expressed transporter genes indicated that carboxylic acids, compatible solutes, polyamines and lipids may be key components of the biologically labile DOC pool in coastal waters, in addition to canonical bacterial substrates such as amino acids, oligopeptides and carbohydrates. Half of the expressed DOC transporter sequences in this coastal ocean appeared to originate from just eight taxa: Roseobacter, SAR11, Flavobacteriales and five orders of γ‐Proteobacteria. While all major taxa expressed transporter genes for some DOC components (e.g. amino acids), there were indications of specialization within the bacterioplankton community for others (e.g. carbohydrates, carboxylic acids and polyamines). Experimental manipulations of the natural DOC pool that increased the concentration of phytoplankton‐ or vascular plant‐derived compounds invoked a readily measured response in bacterial transporter gene expression. This highly resolved view of the potential for carbon flux into heterotrophic bacterioplankton cells identifies possible bioreactive components of the coastal DOC pool and highlights differing ecological roles in carbon turnover for the resident bacterial taxa.