River-dominated Estuary Research Articles

English

The biological productivity of river-dominated estuaries is affected strongly by variation in freshwater inflow, which affects nursery habitat quality. Previous research has shown this is generally true in the upper San Francisco Estuary, California, USA; however, one endemic species of high management importance, delta smelt (Hypomesus transpacificus), has shown ambiguous population responses to river inflow variation. We hypothesized that population-level associations with abiotic habitat metrics have not been apparent because the effects occur seasonally, and at spatial scales smaller than the entire upper San Francisco Estuary. We tested this hypothesis by applying regression techniques and principal components analysis (PCA) to a long-term data-set (1970–2004) of summertime fish catch, and concurrently measured water quality (specific conductance, Secchi disk depth, and water temperature). We found that all three water quality variables predicted delta smelt occurrence, and we identified three distinct geographic regions that had similar long-term trends in delta smelt capture probabilities. The primary habitat region was centered on the confluence of the Sacramento and San Joaquin rivers; delta smelt relative abundance was typically highest in the Confluence region throughout the study period. There were two marginal habitat regions—including one centered on Suisun Bay—where specific conductance was highest and delta smelt relative abundance varied with specific conductance. The second marginal habitat region was centered on the San Joaquin River and southern Sacramento-San Joaquin Delta. The San Joaquin region had the warmest water temperatures and the highest water clarity, which increased strongly in this region during 1970–2004. In the San Joaquin region, where delta smelt relative abundance was correlated with water clarity, catches declined rapidly to zero from 1970–1978 and remained consistently near zero thereafter. However, when we combined these regional results into estuary-wide means, there were no significant relationships between any of the water quality variables and delta smelt relative abundance. Our findings support the hypothesis that basic water quality parameters are predictors of delta smelt relative abundance, but only at regional spatial scales.

Seasonal Variability and Estuary–Shelf Interactions in Circulation Dynamics of a River-dominated Estuary

The long-term response of circulation processes to external forcing has been quantified for the Columbia River estuary using in situ data from an existing coastal observatory. Circulation patterns were determined from four Acoustic Doppler Profilers (ADP) and several conductivity–temperature sensors placed in the two main channels. Because of the very strong river discharge, baroclinic processes play a crucial role in the circulation dynamics, and the interaction of the tidal and subtidal baroclinic pressure gradients plays a major role in structuring the velocity field. The input of river flow and the resulting low-frequency flow dynamics in the two channels are quite distinct. Current and salinity data were analyzed on two time scales—subtidal (or residual) and tidal (both diurnal and semidiurnal components). The residual currents in both channels usually showed a classical two-layer baroclinic circulation system with inflow at the bottom and outflow near the surface. However, this two-layer system is transient and breaks down under strong discharge and tidal conditions because of enhanced vertical mixing. Influence of shelf winds on estuarine processes was also observed via the interactions with upwelling and downwelling processes and coastal plume transport. The transient nature of residual inflow affects the long-term transport characteristics of the estuary. Effects of vertical mixing could also be seen at the tidal time scale. Tidal velocities were separated into their diurnal and semidiurnal components using continuous wavelet transforms to account for the nonstationary nature of velocity amplitudes. The vertical structure of velocity amplitudes were considerably altered by baroclinic gradients. This was particularly true for the diurnal components, where tidal asymmetry led to stronger tidal velocities near the bottom.

The potential for mitten crab Eriocheir sinensis H. Milne Edwards, 1853 (Crustacea: Brachyura) invasion of Pacific Northwest and Alaskan Estuaries

Eriocheir sinensis H. Milne Edwards, 1853 is on the list of top 100 invaders compiled by the International Union for Conservation of Nature and Natural Resources. The recent establishment of a large Chinese mitten crab population in San Francisco Bay and the potential for introductions from California, Asia and Europe pose a significant invasion potential for estuaries and rivers from California to Alaska. This alien species would place at risk the catchment areas of the Pacific Northwest including the economic and social activities that depend upon intact aquatic systems. An analysis of ecological conditions that define the mitten crab’s native and introduced range suggests that large stable estuaries with long flushing times are necessary to sustain significant populations. Most Pacific Northwest estuaries have limited salinity intrusion, estuarine habitat and short flushing times and face a reduced risk of population establishment. Large, stable estuaries, such as the Puget Sound, may support significant populations. River-dominated estuaries, such as the Columbia River, have flushing times less than the duration of larval development and wouldn’t support populations. An application of a temperature based larval development rate to near-shore and estuary sea surface temperatures suggests that estuaries in Oregon and Washington have sufficient thermal regimes to support larval development. Most estuary systems in Alaska have limited periods where water temperatures are above the mortality threshold for the larval stages and are at a low risk for the establishment of populations. A potential sea temperature rise of two degrees Celsius would permit larval development in Alaskan estuaries, where sufficient estuarine and freshwater habitats exist.

Content and isotopic composition of organic carbon within a flood layer in the Po River prodelta (Adriatic Sea)

Thirty-three surface sediment samples from cross-shelf transects on the northern Adriatic shelf were collected in December 2000, soon after a 100-yr flood of the Po River, in order to determine the distribution of organic carbon (OC) along the main sediment dispersal system. To evaluate the temporal variability, stations were re-occupied eight times at seasonal intervals until June 2003. Downcore sediment profiles from two sites characterized by high flood deposit thicknesses were also examined to assess the OC variability within the flood layer. In December 2000, the highest contents of OC (up to 1.24 wt%) were measured in front of the main distributary mouths (Pila, Tolle and Gnocca-Goro) where the greatest thicknesses of the flood deposit were recorded. However, the influence of the Po di Gnocca-Goro sediment supply on the OC surface distribution declined after ∼1.5 years from the fall-2000 river flood, probably because these mouths are less active when the water discharge is lower. The δ 13C of organic matter was used to trace the dispersal of fluvial OC on the continental shelf. The δ 13C values ranged from −25.9‰ to −23.1‰. The fraction of fluvially derived organic particles decreased with increasing water depth according to a radial dispersal pattern around the Po River delta. This pattern persisted in all cruises. δ 13C values increased progressively until April 2002, suggesting an increasing marine contribution to the OC content but decreased again following a second minor flood event in November 2002. The molar C/N ratio was on average 10.0±1.6, with slightly lower values in southern and central areas. Assuming contributions from three OC end-members (terrestrial, riverine and marine), a mixing model based on δ 13C and the ratio of N to C (statistically more robust than C/N; Goñi, M.A., Teixeir, M.J., Perkley, D.W., 2003. Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coastal and Shelf Science 57, 1023–1048) was applied in order to quantitatively assess the OC sources for Po shelf sediments. δ 13C is significantly and positively correlated with the marine OC fraction. The terrestrial fraction is inversely correlated with N/C, while the riverine fraction is positively correlated with N/C. The terrestrial OC source was the most abundant end-member (>70%) showing only little temporal variability regardless of the Po River water discharge. Temporal and spatial changes in OC composition suggest that: (a) the Po River prodelta is always dominated from terrestrial OC input; (b) the Po della Pila supplies most terrestrial OC, whilst other tributaries (e.g., Po di Gnocca-Goro) are secondary sources. However, these mouths are as important as the Po della Pila in affecting the riverine OC signature; (c) offshore, biological primary production raises the marine OC contribution. At two sites on the Po River prodelta, the 2000-flood deposit shows slight but consistent compositional changes of organic matter (N/C and δ 13C) which can help to recognize other flood events in the sedimentary record. The OC budget for the 2000-flood deposit accounts for a terrestrial+riverine OC supply of 68–162 Gg (10 9 g) against an OC deposition of 106–162 Gg (excluding the marine fraction), which implies a rapid and efficient sedimentation of the flood material, and scarce or negligible export out the study area. Flood events may thus enhance terrestrial carbon burial, whereas marine carbon arrives more slowly and may be largely mineralized at the sediment–water interface.

Importance of low salinity areas for juvenile blue crabs, Callinectes sapidus Rathbun, in river-dominated estuaries of southeastern United States

Early juveniles represent a critical stage for population dynamics of the blue crab, Callinectes sapidus. Periodic high predation pressure on small juveniles emphasizes the importance of suitable habitat for refuge. Seagrass beds are an important nursery habitat in some areas, providing refuge from predation as well as possible greater growth rates. However, seagrass beds and certain other subtidal structural habitats, such as subtidal oyster beds, are largely absent from many estuaries along the southeast Atlantic coast from southern North Carolina to southern Georgia. Understanding limitation of blue crab populations in these areas requires knowledge of alternative juvenile habitats. On smaller spatial scales, marsh channels and woody debris may provide refuge and foraging habitat. However, low salinity regions of these estuaries may also provide critical nursery habitat over broader spatial scales. We examined juvenile blue crab use of low salinity areas through a combination of monthly monitoring of distribution patterns across a salinity gradient in two estuaries, examination of growth and survival within single-crab enclosures at sites of varying salinity, and examination of predation pressure from oligohaline to river mouth locations using tethering studies and sampling of predator abundances. Juveniles <13 mm CW were initially more common at higher salinity sites in spring, but exhibited a seasonal change and were more common at lower salinity locations in summer and fall. Juveniles 13–24 mm CW were negatively correlated with salinity during spring and summer. Larger crabs (>24 mm CW) were uncorrelated with salinity or were more common at higher salinity sites. Lower salinity sites were associated with greater survivorship, more rapid molting, and greater crab dry weight compared to higher salinity areas. Tethering indicated a trend towards lower predation mortality at lower salinity areas and abundances of larger conspecifics, a potential predator, were positively associated with increasing salinity in these estuaries. These results suggest that lower salinity regions of the small, river-dominated estuaries of southeastern United States may be utilized as important nursery areas, may be associated with increased growth and reduced predation, and may represent a vital habitat for maintenance of local blue crab populations.

The flood-tide ichthyoplanktonic community at the entrance into a Brazilian tropical estuary

The ichthyoplanktonic community of the smallest inlet into a two-entrance Brazilian estuary was studied monthly between January and December 1999. Six samples were collected during the two central hours of each night flood tide. The community was dominated by Engraulidae, various species of Gobiidae, mainly Microgobius meeki, and Gerreidae as a distant third. Non-resident species showed low abundance. Seasonality was well marked, with highest abundance, lowest diversity and equitability between February and April, though individual taxa displayed large differences amongst themselves. Variations in the structure and abundance of the community did not present any obvious correlation with freshwater input, salinity or water temperature, features that appear similar to those detected in the single other South-American tropical estuary similarly studied. At the continental scale, the communities of these two estuaries differ from those of Amazonian river-dominated estuaries and of wind-driven sub-tropical or temperate lagoonal systems. In both, the community is numerically dominated by estuarine-dependent taxa, amongst which numerous Sciaenidae, and seasonal variations appear regulated by climatic conditions: either rainfall or temperature.

Bacterioplankton abundance and growth in a river-dominated estuary: relationships with temperature and resources

We measured bacterial abundance and productivity, along with other biological, chemi- cal, and physical variables, during a 17 mo study in Mobile Bay, USA. Mobile Bay is a shallow, river- dominated estuary, with highly variable freshwater inputs and relatively short freshwater residence time (ca. 2 to 3 wk). In addition to the field measurements, we performed bioassay experiments to evaluate the degree of resource limitation of the bacterioplankton. As in other seasonally variable estuaries, temperature appeared to be an important regulator of bacterioplankton in Mobile Bay, with minima in abundance and growth rates occurring during winter. The bioassay experiments, in which an inoculum of the natural bacterioplankton community was diluted into filtered bay water and incubated for 4 d, always resulted in rapid net growth, indicating the presence of labile sub- strates. The net yield of bacterial cells in these assays was always equal to or greater than the abun- dance of in situ populations, suggesting that an amount of substrate about equal to in situ biomass di- vided by growth efficiency was present year round. During winter, the yields in the bioassays were highest, suggesting an accumulation of resources when bacterial growth was limited by temperature.

The brackishwater hyporheic zone: invertebrate community structure across a novel ecotone

This study describes a hyporheic zone that exists beneath a river-dominated estuary in North Wales, and which spans the freshwater/saltwater boundary. A series of 72 cores was taken from the Aber Estuary, in September, at depths from 10–60 cm below the bed surface. Site 1 was above the extreme high water mark and therefore was never inundated by salt water. Site 5 was the furthest downstream and was inundated twice daily by incoming tides. Substrates contained more gravel at the upstream sites and closer to the bed surface, with more sand and silt at downstream sites and at depth. Salinity of interstitial water increased both with distance away from Site 1 and with increasing depth into the zone, reaching a maximum of 15‰ at 60 cm at Site 5, as measured at low tide. Longitudinal and vertical zonations of the invertebrate taxa were evident, and densities were greater in the top 40 cm than below. Nematodes were especially abundant from 10–60 cm, and genera differed along the estuary. Oligochaetes were well represented at most sites, with the less saline sites yielding Naididae, Enchytraeidae, Aeolosomatidae and Tubificidae; Site 5 was dominated by Tubifex costatus. Chironomid larvae were most abundant to 40 cm at Sites 1 and 2 (primarily Brillia modesta, Corynoneura sp., Tanytarsus sp., and unidentified Tanypodinae), but they were also found at Site 3 (to 60 cm), and at 10 cm at Site 5 (Orthocladius sp.). Nymphs of mayflies and caddisfly larvae were found primarily at Site 1, but small stonefly nymphs and elmid beetle larvae were taken at Site 2 depths where the interstitial water was brackish. The most saline sediments were populated by the triclad Uteriporus vulgaris, the polychaete Nereis sp., and snails (Hydrobia sp.). Site, salinity, pH, and sand and silt contents appeared to influence species distribution most; together, these explained 37.8% of the variance (CANOCO). There was an overall negative relationship between silt content of the sediments and total invertebrate density. The hyporheic zone of the Aber Estuary is thus not only influenced by surface water and, presumably, estuarine groundwater exchanges, but is further complicated, biologically, by having a salinity gradient running through it. The term `brackishwater hyporheic zone' (BHZ) is proposed to describe this and similar systems.

Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA)

The sources and distribution of organic matter (OM) in surface waters and sediments from Winyah Bay (South Carolina, USA) were investigated using a variety of analytical techniques, including elemental, stable isotope and organic biomarker analyses. Several locations along the estuary salinity gradient were sampled during four different periods of contrasting river discharge and tidal range. The dissolved organic carbon (DOC) concentrations of surface waters ranged from 7 mg l −1 in the lower bay stations closest to the ocean to 20 mg l −1 in the river and upper bay samples. There was a general linear relationship between DOC concentrations and salinity in three of the four sampling periods. In contrast, particulate organic carbon (POC) concentrations were significantly lower (0.1–3 mg l −1) and showed no relationship with salinity. The high molecular weight dissolved OM (HMW DOM) isolated from selected water samples collected along the bay displayed atomic carbon:nitrogen ratios ([C/N]a) and stable carbon isotopic compositions of organic carbon ( δ 13C OC) that ranged from 10 to 30 and from −28 to −25‰, respectively. Combined, such compositions indicate that in most HMW DOM samples, the majority of the OM originates from terrigenous sources, with smaller contributions from riverine and estuarine phytoplankton. In contrast, the [C/N]a ratios of particulate OM (POM) samples varied significantly among the collection periods, ranging from low values of ∼5 to high values of >20. Overall, the trends in [C/N]a ratios indicated that algal sources of POM were most important during the early and late summer, whereas terrigenous sources dominated in the winter and early spring. In Winyah Bay bottom sediments, the concentrations of the mineral-associated OM were positively correlated with sediment surface area. The [C/N]a ratios and δ 13C OC compositions of the bulk sedimentary OM ranged from 5 to 45 and from −28 to −23‰, respectively. These compositions were consistent with predominant contributions of terrigenous sources and lesser (but significant) inputs of freshwater, estuarine and marine phytoplankton. The highest terrigenous contents were found in sediments from the river and upper bay sites, with smaller contributions to the lower parts of the estuary. The yields of lignin-derived CuO oxidation products from Winyah Bay sediments indicated that the terrigenous OM in these samples was composed of variable mixtures of relatively fresh vascular plant detritus and moderately altered soil OM. Based on the lignin phenol compositions, most of this material appeared to be derived from angiosperm and gymnosperm vascular plant sources similar to those found in the upland coastal forests in this region. A few samples displayed lignin compositions that suggested a more significant contribution from marsh C 3 grasses. However, there was no evidence of inputs of Spartina alterniflora (a C 4 grass) remains from the salt marshes that surround the lower sections of Winyah Bay.

The role of extreme floods in estuary-coastal behaviour: contrasts between river- and tide-dominated microtidal estuaries

Abstract Contrasting modes of sedimentation and facies arrangement in tide- and river-dominated microtidal estuaries arise from the degree to which river or tidal discharge and sediment supply influences an estuary. A distinct facies gradation exists in tide-dominated systems from sandy, barrier/tidal delta-associated environments at the coast through deep mud-dominated middle reaches to fluvial sediment in the upper reaches. In river-dominated systems, fluvial sediment extends to the barrier and flood-tidal deltas are poorly developed or absent from the estuary. A number of independent observations during extreme floods on the South African coast indicate that these types of estuary respond differently to extreme river floods and that the mode of response corresponds to estuary type. Tide-dominated systems exhibit preferential erosion of noncohesive barrier and tidal delta sediments during river floods while the middle reaches remain little modified. River-dominated systems experience consistent erosion throughout their channel length during extreme floods. The increased cohesion of riverine sediments and stabilisation of bars by vegetation in river-dominated channels means that higher magnitude floods are necessary to effect significant morphological change. Barrier erosion, including the tidal delta, results in deposition of an ephemeral delta composed almost entirely of sands from these deposits in tide-dominated estuaries. In river-dominated systems, eroded channel sediments and material from the river catchment may augment barrier sediments in the ephemeral delta deposit. Post-flood, wave-reworking of ephemeral delta sediments acts to restore barriers to pre-flood morphology within a few years; however, in river-dominated systems, the additional sediment volume may produce significant coastal progradation that requires several years or decades to redistribute. These different modes of flood response mediated by the nature of the estuary have implications for coastal behaviour at the time scale of months to several decades. Estuary-coastal behaviour at river-dominated estuaries may be influenced for several decades by post-flood morphological adjustment. Tide-dominated estuaries, however, respond more rapidly in reworking flood-eroded sediment and are typically fully adjusted to modal wave and tidal conditions within a few months to a few years. In addition, the facies arrangement within the two estuary types renders tide-dominated estuaries more responsive to minor floods, while river-dominated estuaries, by virtue of more cohesive channel sediments, require greater discharges to effect significant morphological change.

Geomorphological variability among microtidal estuaries from the wave-dominated South African coast

Over 300 independent river outlets exist around the microtidal, wave-dominated South African coast. Most of these are located in drowned river valley settings and have acquired their present morphology during the Holocene marine transgression. A variety of geomorphological forms are identified in these estuaries that result from variation in antecedent topography, fluvial sediment supply and marine sediment supply. Five distinctive estuary types are identified on the basis of contemporary morphodynamics. These are categorised into three types of normally open estuary (that maintain a semi-permanent connection with the open sea) and two types of normally closed estuary, which are separated from the sea for long periods by a continuous supratidal barrier. The contemporary morphodynamics and sedimentary environments of each estuary type are discussed. Open estuaries include barrier–inlet systems maintained by fluvial discharge (termed river-dominated estuaries) and tidal discharge (termed tide-dominated estuaries). A third category of open estuary lacks a supratidal barrier due to inadequate marine sediment availability. Closed estuaries receive marine influence via barrier overwash and occasional breaching but are typically enclosed behind a continuous supratidal barrier. Two categories of closed estuary are identified: perched and non-perched. Perched estuaries develop behind high berms and maintain a water level above high tide level in the open sea. They are often dominated by fresh to brackish water and breach and drain periodically. Non-perched estuaries are developed behind low-elevation barriers fronted by wide dissipative beach profiles. High overwash frequency introduces marine water into such systems. Breaching of such estuaries does not produce dramatic draining as the water level may only drop according to the stage of the open sea tidal cycle. The variety of estuary types suggests several potential pathways may exist in estuarine development and that the progressive infilling model is an over simplification.

Control of phytoplankton production and biomass in a river-dominated estuary:Apalachicola Bay, Florida, USA

Physical, chemical, and biological processes interact in complex patterns through time to control estuarine phytoplankton productivity and biomass. Hydrodynamic model results, together with biological, nutrient, and physical data acquired from Apalachicola Bay from May 1993 through May 1996, were used to elucidate factors that control phytoplankton productivity and chlorophyll (chl a). The estuary receives freshwater, which originates in the Apalachicola, Chattahoochee, and Flint River watersheds, with maximum river flow occurring in late winter and early spring. Maximum chl a (mean ± 1 SE: 5.38 ± 0.40 μg chl a l -1 ) values occurred during winter, while primary productivity maxima occurred in late spring and summer months when temperature and photosynthetically active radiation (PAR) reached their annual maxima. Approximately 75% of annual primary production (255 ± 78 g C m -2 yr -1 ) occurred from May through November of each year. During this period, however, river dissolved inorganic nitrogen and soluble reactive phosphorus input accounted for 40% of the annual inputs (30.9 ± 5.1 g N m -2 yr -1 and 0.60 ± 0.15 g P m -2 yr -1 , respectively). Approximately 25 % of annual primary production occurred from December to April, concurrent with low PAR values and low water temperature. Low chl a concentrations in summer months were concurrent with high phytoplankton productivity, high zooplankton abundance, low river flow, and low nutrient input to the estuary. In 2 of 3 years, export from Apalachicola Bay provided a significant control for phytoplankton biomass magnitude during winter. However, on an annual basis grazing accounted for 80% of the chl a loss from the estuary.

Phosphorus budget of Apalachicola Bay:a river-dominated estuary in the northeastern Gulf of Mexico

Estuarine nutrient budgets are used to understand the transformation of river-born nutrients delivered to the sea, and to gain insights into the impact of terrestrially-derived nutrients on coastal ecosystems. To construct a phosphorus budget for Apalachicola Bay, we measured soluble reactive, dissolved organic, and particulate phosphorus in the Apalachicola River and estuary on a monthly basis over a 2 yr period. The results of the chemical time-series measurements were coupled with river flow data and results of a 3-dimensional hydrodynamic model of the estuary to estimate phosphorus input to the estuary and phosphorus exchange with the Gulf of Mexico (GOM). On average, the Apalachicola River accounted for 70 and 78 % of the water and total phosphorus (TP) input to the estuary, respectively, while exchange with the GOM accounted for the remainder. TP input to the estuary was 4.57 ± 0.21 (± 1SD) g P m -2 yr -1 , the majority of which was in the particulate form (2.69 ± 0.19 g P m -2 yr -1 ). Dissolved organic phosphorus (DOP) represented 24% of the total input, while soluble reactive phosphorus (SRP) represented the smallest fraction (17%) of the total input. Particulate phosphorus export to the GOM (2.75 ± 0.19 g P m -2 yr -1 ) was similar to particulate phosphorus input to the estuary. However, only 64 and 58 % of the DOP and SRP input to the estuary, respectively, were exported to the GOM. Phosphorus accumulation in the sediments accounted for 25 % (1.10 ± 0.37 g P m -2 yr -1 ) of the TP input to the estuary. Phytoplankton phosphorus demand for the study period was 6.75 g P m -2 yr -1 , of which 11% could be supplied by net SRP and DOP input to the estuary. The phosphorus budget for Apalachicola Bay was balanced to within 9% of TP input.

Plankton and Dissolved Inorganic Carbon Isotopic Composition in a River-Dominated Estuary: Apalachicola Bay, Florida

To characterize the isotopic composition of organisms at the base of the food web and the controls on their variability, the concentration and δ13C isotopic composition of dissolved inorganic carbon (DIC) and plankton δ13C, δ15N, and δ34S were measured. The measurements were made during periods of high and low river flow in Apalachicola Bay, Florida, United States, over 3 yr. DIC concentration and δ13C values were related to salinity, indicating that conservative mixing of riverine and marine waters was responsible for the overall distributions. The usefulness of DIC δ13C data for characterizing the trophic processes within the estuary was dependent upon the residence time of water within the season. Plankton δ13C values varied from −22‰ to −30‰ and were directly related to estuarine DIC δ13C, offset by a factor of roughly −20‰. This offset factor varied with salinity. Values of δ34S in estuarine plankton (station means ranged from 11.4‰ to 13.1‰) were depleted relative to marine plankton (17.7±0.4‰) possibly due to the admixture of34S-depleted sedimentary sulfide with estuarine samples. Values of δ34S in plankton were not related to δ13C values of plankton and were only weakly correlated to the salinity of the water from which the plankton were collected, indicating that marine sulfate was the primary source of planktonic sulfur. Values of δ15N in plankton varied from 5.5‰ to 10.7‰ and appeared related to dominance of the sample by phytoplankton or zooplankton. Estuarine plankton was15N enriched relative to offshore plankton and estuarine sediment.

Distribution and speciation of phosphorus along a salinity gradient in intertidal marsh sediments

We examined forms of solid phosphorus fractions in intertidal marsh sediments along a salinity (0–22‰) gradient in a river-dominated estuary and in a marine-dominated salt marsh with insignificant freshwater input. Freshwater marsh sediments had the highest ratio of organic N:P of between 28:1 and 47:1 mol:mol, compared to 21:1 to 31:1 mol:mol in the saltmarshes, which is consistent with a trend toward P-limitation of primary production in freshwater and N-limitation in salt marshes. However, total P concentration, 24.7 ± 11.1 µmol P g dw-1 (±1 SD) averaged over the upper meter of sediment, was greatest in the freshwater marsh where bioavailablity of P is apparently limited. In the freshwater marsh the greatest fraction of total P (24–51%) was associated with humic acids, while the importance of humic-P decreased with increasing salinity to 1–23% in the salt marshes. Inorganic P contributed considerably less to total sediment P in the freshwater marsh (15–40%) than in the salt marshes (33–85%). In reduced sediments at all sites, phosphate bound to aluminum oxides and clays was an important inorganic P pool irrespective of salinity. Inorganic P associated with ferric iron [Fe(III)] phases was most abundant in surface sediments of freshwater and brackish marshes, while Ca-bound P dominated inorganic P pools in the salt marshes. Thus, our results showed that particle-bound P in marsh sediments exhibited changes in chemical association along the salinity gradient of an estuarine system, which is a likely consequence of changes in ionic strength and the availability of iron and calcium.

Microzooplankton Grazing and Nitrogen Excretion across a Surface Estuarine-Coastal Interface

The role of the microzooplankton community in regulating phytoplankton biomass was examined across a gradient from a river-dominated estuary to an oceanic-influenced coastal zone. Three stations located along a salinity gradient from the central region of Mobile Bay to 10 km off the coast were sampled from May 1994 to August 1995. Microzooplankton herbivory rates on phytoplankton and microzooplankton excretion of nitrogen derived from phytoplankton were estimated using the dilution technique. Microzooplankton grazing rates (range of station means=0.57–1.10 d−1) and phytoplankton growth rates (0.70–1.62 d−1) both increased across the salinity gradient from the bay station to the offshore station. However, the percent of primary production grazed per day was highest at the bay station (mean=83%) and decreased to a low at the offshore station (mean=64%). Excretion of phytoplankton-derived nitrogen by the microzooplankton was greatest at the bay and bay mouth stations. Excreted nitrogen could potentially supply 39%, 29%, and 20% of phytoplankton nitrogen demand at the bay, bay mouth, and offshore stations, respectively. These results support the idea that herbivorous microzooplankton are important in mediating nitrogen flow to both lower and higher trophic levels. *** DIRECT SUPPORT *** A01BY085 00012

Seasonal and fine-scale spatial variations in egg production and triacylglycerol content of the copepod Acartia tonsa in a river-dominated estuary and its coastal plume

We measured egg production rates of the estuarine calanoid copepod Acartia tonsa in Mobile Bay, an estuary in the northern Gulf of Mexico. Two stations were sampled approximately monthly, one at the mouth of the bay and the other just beyond the mouth in the salinity front between bay and coastal waters. Over the whole year, temperature was the most important environmental variable controlling egg production. Rates increased with temperature up to 30°C and 140 eggs female1 day-'. We found no evidence of food limitation. There was no correlation between egg production and phytoplankton abundance, nor increased egg production in response to supplements of phytoplankton added to natural food, suggesting that non-phytoplankton food was important in the diet. At the highest egg production rates, the amount of the storage lipid triacylglycerol (TAG) in adult females was greatly diminished, to <50 ng female1. This suggests that lipids in the diet can be very tightly coupled to egg production. Both egg production and TAG content of females showed significant variability on spatial scales of 3-15 km, especially in relation to the salinity front separat- ing water outwelling from the bay from open coastal water. For organisms that are using copepods and their eggs as food, this variability would result in a heterogeneous food environment, both in terms of the abundance of food and its nutritional content.

Sink or swim? Copepod population maintenance in the Columbia River estuarine turbidity-maxima region

Maintenance of estuarine zooplankton populations in large river-dominated estuaries with short residence times has been an intriguing subject of investigation. During three different hydrological seasons, autumn 1990, summer 1991, and spring 1992, we intensively sampled zooplankton populations in the estuarine turbidity maxima (ETM) region of the Columbia River estuary of Oregon and Washington, USA. One of the principal objectives was to investigate retention mechanisms of the predominant zooplankton species, the harpacticoid copepod Coullana canadensis and the epibenthic calanoid copepod Eurytemora affinis. In the ETM, C. canadensis densities mirrored those of turbidity gradients and were almost always greater at the river bed, while E. affinis densities were greater higher in the water column during the flood and lower in the water column during the ebb. Cross-correlation and time-series analyses determined that C. canadensis densities were highly positively correlated with turbidity and that most of the variability was explained by the lunisolar diurnal (K1) and principal lunar (M2) tidal components occurring once every 23.93 h and once every 12.42 h, respectively. This indicates that C. canadensis populations are most probably maintained in the estuary through the same near-bottom circulation features that trap and concentrate particles in the ETM. In contrast, densities of the more motile species E. affinis were highly correlated with negative velocities, or ebb tide, and most of the variability in population densities could be explained by the principal lunar tidal component; therefore, we hypothesize that this species is probably vertically migrating on a tidal cycle into different flow layers to avoid population losses out of the estuary.

Tidal current shear in a broad, shallow, river-dominated estuary

Approximately one year of current, temperature, salinity and pressure data were collected at a site near the mouth of the Mobile Bay estuary in Alabama. This estuary is broad, shallow (3m) and relatively flat. Diurnal tides account for 85% of the current variance in the lower estuary. Tidal currents have significant vertical shear in both amplitude and direction. The principal diurnal constituents in the lower bay, O 1 and K 1, have amplitudes that range from 12 to 25 cm/s in the near-surface layer, 9 to 12 cm/s near the bed. The principal axis of the near-surface diurnal ellipse is rotated 30° clockwise relative to the near-bottom ellipse. This strong tidal current shear can exist because the estuary has a strong, persistent, haline stratification maintained by a continuous river input. The cause and strength of the tidal shear depends on a combination of factors, including differential topographic forcing through separate entrances into the bay and spatial and temporal variations in water column stability caused by changes in the magnitude of river flow and wind stress.

FRESHWATER INPUT TO A GULF ESTUARY: LONG-TERM CONTROL OF TROPHIC ORGANIZATION

A long-term (9.5 yr) study addressed the relationship of the trophic orga- nization of a river-dominated Gulf of Mexico estuary with interannual trends of freshwater input and biological controlling features. Alluvial river flow characteristics were evaluated with respect to seasonal and interannual changes in physical, chemical, and biological trends in the receiving estuary. Infaunal and epifaunal macroinvertebrates and fishes taken over the period of sampling in the Apalachicola Bay system were transformed into their trophic equivalents. The long-term trophic organization of the bay was then related to observed changes in the physical and chemical conditions in the receiving estuary with particular attention to long-term response to a 2-yr drought. Within limited natural bounds of fresh- water flow from the Apalachicola River, there was little change in the trophic organization of the receiving estuary over prolonged periods. The physical instability of the estuary was actually a major component in the continuation of a biologically stable estuarine system. However, when a specific threshold of freshwater reduction was reached during a prolonged natural drought, we suggest that the clarification of the normally turbid and highly colored river-estuarine system led to rapid changes in the pattern of primary production, which, in turn, were associated with major changes in the trophic structure of the system. Increased light penetration due to the cessation of river flow was an important factor in the temporal response of bay productivity and herbivore/omnivore abundance. There was a dichotomous response of the estuarine trophic organization, with herbivores and omnivores responsive to river-dominated physicochemical factors whereas the carnivores responded to biological factors. Trophic response time could be measured in months to years from the point of the initiation of low-flow conditions. The reduction of nutrient loading during the drought period was postulated as a major cause of the loss of productivity of the river-dominated estuary during and after the drought period. Recovery of such productivity with resumption of increased river flows was likewise a long-term event. Based on the observed trends in the bay, postulated permanent reductions of freshwater flows due to anthropogenous ac- tivities could lead to major reductions of biological productivity in the Apalachicola Bay system. The long-term data indicated that, with reduction of freshwater flow below a level specific for the receiving system, the physically controlled, highly productive river-estu- arine system would become a species-rich, biologically controlled bay with substantially reduced productivity.