Delaware Inland Bays Research Articles

Heterosigma akashiwo does not serve as prey and chloroplast donor for the toxic dinoflagellate, Dinophysis acuminata

In laboratory culture, the toxic dinoflagellate Dinophysis acuminata acquires plastids from the ciliate, Mesodinium rubrum, which, in turn, acquires plastids from the cryptophyte, Teleaulax amphioxeia. Reports of D. acuminata from field samples found plastids of the raphidophyte, Heterosigma akashiwo within D. acuminata cells, suggesting a broader range of prey. Dinophysis blooms often co-occur with H. akashiwo in Delaware's inland bays. In the study presented here, predation on H. akashiwo by D. acuminata was investigated. Growth rates of D. acuminata were measured when cultured with H. akashiwo either alone or with its known prey, M. rubrum. M. rubrum was also cultured with H. akashiwo to examine predation by the ciliate as a vector for Heterosigma plastids. Ingestion rates by D. acuminata were measured when presented with H. akashiwo as prey, and retention of plastids from H. akashiwo was investigated by measuring chlorophyll a fluorescence intensities in D. acuminata cells presented with H. akashiwo as prey compared to M. rubrum. Additionally, a fluorescence-based method was developed to identify the presence of the accessory pigment fucoxanthin from H. akashiwo plastids in cells of D. acuminata. Results showed that the growth rate of D. acuminata was significantly lower when offered H. akashiwo as prey compared the growth rate when offered M. rubrum as prey. Likewise, no predation was observed when D. acuminata was offered H. akashiwo as prey. Intensity of chlorophyll a fluorescence was lower when H. akashiwo was offered as prey compared to M. rubrum, and fucoxanthin was not detected in any of the Dinophysis cells examined after incubation with H. akashiwo. Results of this investigation do not support the hypothesis that D. acuminata preys on H. akashiwo and highlight the need for further research on factors that stimulate the growth of Dinophysis in field populations.

Functional trait thermal acclimation differs across three species of mid-Atlantic harmful algae

Characterizing the thermal niche of harmful algae is crucial for understanding and projecting the effects of future climate change on harmful algal blooms. The effects of 6 different temperatures (18–32 °C) on the growth, photophysiology, and toxicity were examined in the dinoflagellate Karlodinium veneficum, and the raphidophytes, Heterosigma akashiwo and Chattonella subsalsa from the Delaware Inland Bays (DIB). K. veneficum and H. akashiwo had skewed unimodal growth patterns, with temperature optima (Topt) at 28.6 and 27.3 °C respectively and an upper thermal niche limit of 32 °C. In contrast, C. subsalsa growth increased linearly with temperature, suggesting Topt and upper thermal boundaries >32 °C. K. veneficum photosystem II (PSII) photochemical efficiency remained stable across all temperatures, while H. akashiwo PSII efficiency declined at higher temperature and C. subsalsa was susceptible to low temperature (~18 °C) photoinactivation. Cell toxicity thermal response was species-specific such that K. veneficum toxicity increased with temperature above Topt. Raphidophyte toxicity peaked at 25–28 °C and was in close agreement with Topt for growth in H. akashiwo but below C. subsalsa maximal growth. The mode of toxicity was markedly different between the dinoflagellate and the raphidophytes such that K. veneficum had greater hemolytic activity while the raphidophytes had pronounced fish gill cell toxicity. These results and patterns of natural abundance for these algae in the DIB suggest that continued ocean warming may contribute to C. subsalsa bloom formation while possibly promoting highly toxic blooms of K. veneficum.

Analysis and prediction of storm water levels in Delaware inland bays

A simple approach based on an analytical model and available tide gauge data is proposed for the analysis of storm tide damping inside inland bays with complex bathymetry and for the prediction of peak water levels at gauge locations during storms. The approach was applied to eight tide gauges in the vicinity of inland bays in Delaware. Peak water levels at the gauge locations were analyzed for 34 storms during 2005-2017. A damping parameter in the analytical model was calibrated for each bay gauge. The calibrated model predicted the peak water levels within errors of about 0.2 m except for Hurricane Sandy in 2012. The analytical model including wave overtopping was used to estimate the peak wave overtopping rate over the barrier beach from the measured peak water level in the adjacent bay.

Delaware Inland Bays and Market Oyster (Crassostrea virginica) Quality for Consumption

Consumption of raw oysters is known to cause serious health conditions due to bioaccumulation of contaminants. As filter feeders, oysters ingest bacteria along with phytoplankton from their surrounding habitats. Ensuring seafood safety for human consumption is always a concern. Since oysters are consumed raw, disease causing organisms, environmental contaminants, toxins, chemicals, and even physical hazards such as soils and metals retained in the oysters can enter through feeding. The objective of this study was to determine the quality of oysters collected from Delaware Inland Bays (DIB) and compare them with market oysters. Environmental parameters were monitored from local waters of DIB classified as closed versus open for shellfish harvesting. Total aerobic bacteria and vibrio were higher in market oysters during the warmer months, with open water having the least microbial loads. There were no significant differences in total vibrio counts between the study sites (p=0.396), but significant differences were recorded over time (p=0.004). Water temperature and turbidity were directly proportional to total vibrio in oysters, and salinity was inversely related. Research findings in this study may help bring awareness of changes in bacterial loads due to seasonal changes and additional handling and storage.

Community-Level and Species-Specific Associations between Phytoplankton and Particle-Associated Vibrio Species in Delaware's Inland Bays.

Vibrio species are an abundant and diverse group of bacteria that form associations with phytoplankton. Correlations between Vibrio and phytoplankton abundance have been noted, suggesting that growth is enhanced during algal blooms or that association with phytoplankton provides a refuge from predation. Here, we investigated relationships between particle-associated Vibrio spp. and phytoplankton in Delaware's inland bays (DIB). The relative abundances of particle-associated Vibrio spp. and algal classes that form blooms in DIB (dinoflagellates, diatoms, and raphidophytes) were determined using quantitative PCR. The results demonstrated a significant correlation between particle-associated Vibrio abundance and phytoplankton, with higher correlations to diatoms and raphidophytes than to dinoflagellates. Species-specific associations were examined during a mixed bloom of Heterosigma akashiwo and Fibrocapsa japonica (Raphidophyceae) and indicated a significant positive correlation for particle-associated Vibrio abundance with H. akashiwo but a negative correlation with F. japonica. Changes in Vibrio assemblages during the bloom were evaluated using automated ribosomal intergenic spacer analysis (ARISA), which revealed significant differences between each size fraction but no significant change in Vibrio assemblages over the course of the bloom. Microzooplankton grazing experiments showed that losses of particle-associated Vibrio spp. may be offset by increased growth in the Vibrio population. Moreover, analysis of Vibrio assemblages by ARISA also indicated an increase in the relative abundance for specific members of the Vibrio community despite higher grazing pressure on the particle-associated population as a whole. The results of this investigation demonstrate links between phytoplankton and Vibrio that may lead to predictions of potential health risks and inform future management practices in this region.

Beach response to a fixed sand bypassing system

Indian River Inlet is located at roughly the mid-point of the Atlantic coast of Delaware and connects the ocean to two Delaware inland bays. Jetties constructed in 1940 have maintained the inlet for navigation purposes but have also acted as a barrier to net northerly alongshore sediment transport causing downdrift erosion. A mobile, land-based bypassing system was initiated in 1990 in an effort to counteract this erosion. Beach profile data from 1985 (pre-bypassing) until 2008 are used to investigate the effect of the sand bypassing system on beaches adjacent to the inlet. The downdrift beach experienced horizontal shoreline erosion between 10 and 60m during the pre-bypassing period but accreted 10–20m during the bypassing period. The mean shoreline location on the updrift beach during bypassing is 10–20m landward (erosion) of its position during the pre-bypassing period. Empirical orthogonal function (EOF) amplitudes from analyses performed on mean-removed elevation surfaces during the periods of highest bypassed volume (average of 83% of design rate) showed that the influence of the bypassing system on the downdrift beach extends to about 1500m of the inlet. An EOF analysis showed that different morphologic responses were evident following the initiation of bypass operations. Temporal variations of shoreline and beach morphology were correlated to the temporal variations in bypassing rates on the downdrift beach only. The downdrift beach response was greatest near the inlet for larger bypassing volumes. Correlation in these instances occurred with a roughly 1-year time lag suggesting that the beach quickly redistributes the bypassed sand. EOF amplitude and shoreline response are weakly correlated to bypassed volumes when the system bypassed smaller volumes (average of 56% of design rate) of sand suggesting that there is a minimum bypassing rate, regardless of yearly variability, below which the effect on the downdrift beach is obscured.

DESCRIPTION OF VIRIDILOBUS MARINUS (GEN. ET SP. NOV.), A NEW RAPHIDOPHYTE FROM DELAWARE'S INLAND BAYS.

Delaware's Inland Bays (DIB), USA, are subject to blooms of potentially harmful raphidophytes, including Heterosigma akashiwo. In 2004, a dense bloom was observed in a low salinity tributary of the DIB. Light microscopy initially suggested that the species was H. akashiwo; however, the cells were smaller than anticipated. 18S rDNA sequences of isolated cultures differed substantially from all raphidophyte sequences in GenBank. Phylogenetic analysis placed it approximately equidistant from Chattonella and Heterosigma with only ~96% sequence homology with either group. Here, we describe this marine raphidophyte as a novel genus and species, Viridilobus marinus (gen. et sp. nov.). We also compared this species with H. akashiwo, because both species are superficially similar with respect to morphology and their ecological niches overlap. V. marinus cells are ovoid to spherical (11.4×9.4μm), and the average number of chloroplasts (4 per cell) is lower than in H. akashiwo (15 per cell). Pigment analysis of V. marinus revealed the presence of fucoxanthin, violaxanthin, and zeaxanthin, which are characteristic of marine raphidophytes within the family Chattonellaceae of the Raphidophyceae. TEM and confocal microscopy, however, revealed diagnostic microscopic and ultrastructural characteristics that distinguish it from other raphidophytes. Chloroplasts were in close association with the nucleus and thylakoids were arranged either parallel or perpendicular to the cell surface. Putative mucocysts were identified, but trichocysts were not observed. These features, along with DNA sequence data, distinguish this species from all other raphidophyte genera within the family Chattonellaceae of the Raphidophyceae.

Using quantitative real-time PCR to study competition and community dynamics among Delaware Inland Bays harmful algae in field and laboratory studies

The Delaware Inland Bays (DIB) have experienced harmful algal blooms of dinoflagellates and raphidophytes in recent years. We used quantitative polymerase chain reaction (QPCR) techniques to investigate the community dynamics of three DIB dinoflagellates ( Karlodinium veneficum, Gyrodinium instriatum, and Prorocentrum minimum) and one raphidophyte ( Heterosigma akashiwo) at a single site in the DIB (IR-32) in summer 2006 relative to salinity, temperature and nutrient concentrations. We also carried out complementary laboratory culture studies. New primers and probes were developed and validated for the 18S rRNA genes in the three dinoflagellates. K. veneficum, H. akashiwo, and G. instriatum were present in almost all samples throughout the summer of 2006. In contrast, P. minimum was undetectable in late June through September, when temperatures ranged from 20 to 30 °C (average 25.7 °C). Dissolved nutrients ranged from 0.1 to 2.8 μM PO 4 3− (median = 0.3 μM), 0.7–30.2 μM NO x (median = 12.9 μM), and 0–19.4 μM NH 4 + (median = 0.7 μM). Dissolved N:P ratios covered a wide range from 2.6 to 177, with a median of 40. There was considerable variability in occurrence of the four species versus nutrients, but in general P. minimum and H. akashiwo were most abundant at higher (>40) N:P ratios and dissolved nitrogen concentrations, while K. veneficum and G. instriatum were most abundant at low dissolved N:P ratios (<20) and dissolved nitrogen concentrations < 10 μM. The semi-continuous laboratory competition experiment used mixed cultures of K. veneficum, P. minimum, and H. akashiwo grown at dissolved N:P ratios of 5, 16, and 25. At an N:P of 16 and 25 P. minimum was the dominant alga at the end of the experiment, even at a temperature that was much higher than that at which this alga was found to bloom in the field (27 °C). P. minimum and H. akashiwo had highest densities in the N:P of 25. K. veneficum grew equally well at all three N:P ratios, and was co-dominant at times at an N:P of 5. H. akashiwo had the lowest densities of the three algae in the laboratory experiment. Laboratory and field results showed both interesting similarities and significant differences in the influences of important environmental factors on competition between these harmful algal species, suggesting the need for more work to fully understand HAB dynamics in the DIB.

Prevalence of Perkinsus marinus (Dermo), Haplosporidium nelsoni (MSX), and QPX in Bivalves of Delaware's Inland Bays and Quantitative, High‐Throughput Diagnosis of Dermo by QPCR

Restoration of oyster reef habitat in the Inland Bays of Delaware was accompanied by an effort to detect and determine relative abundance of the bivalve pathogens Perkinsus marinus, Haplosporidium nelsoni, and QPX. Both the oyster Crassostrea virginica and the clam Mercenaria mercenaria were sampled from the bays. In addition, oysters were deployed at eight sites around the bays as sentinels for the three parasites. Perkinsus marinus prevalence was measured with a real-time, quantitative polymerase chain reaction (PCR) methodology that enabled high-throughput detection of as few as 31 copies of the ribosomal non-transcribed spacer region in 500 ng oyster DNA. The other pathogens were assayed using PCR with species-specific primers. Perkinsus marinus was identified in Indian River Bay at moderate prevalence ( approximately 40%) in both an artificial reef and a wild oyster population whereas sentinel oysters were PCR-negative after 3-months exposure during summer and early fall. Haplosporidium nelsoni was restricted to one oyster deployed in Little Assawoman Bay. QPX and P. marinus were not detected among wild clams. While oysters in these bays have historically been under the greatest threat by MSX, it is apparent that P. marinus currently poses a greater threat to recovery of oyster aquaculture in Delaware's Inland Bays.

Assessment of Microzooplankton Grazing on Heterosigma akashiwo Using a Species- Specific Approach Combining Quantitative Real-Time PCR (QPCR) and Dilution Methods

Delaware's Inland Bays (DIB) are subject to numerous mixed blooms of harmful raphidophytes each year, and Heterosigma akashiwo is one of the consistently occurring species. Often, Chattonella subsalsa, C. cf. verruculosa, and Fibrocapsa japonica co-occur with H. akashiwo, indicating a dynamic consortium of raphidophyte species. In this study, microzooplankton grazing pressure was assessed as a top-down control mechanism on H. akashiwo populations in mixed communities. Quantitative real-time polymerase chain reaction (QPCR) with species-specific primers and probes were used in conjunction with the dilution method to assess grazing pressure on H. akashiwo and other raphidophytes. As a comparison, we measured changes in chlorophyll a (chl a) to determine whole community growth and mortality caused by grazing. We detected grazing on H akashiwo using QPCR in samples where chl a analyses indicated little or no grazing on the total phytoplankton community. Overall, specific microzooplankton grazing pressure on H akashiwo ranged from 0.88 to 1.88 day(-1) at various sites. Experiments conducted on larger sympatric raphidophytes (C. subsalsa, C. cf. verruculosa and F japonica) demonstrated no significant microzooplankton grazing on these species. Grazing pressure on H akashiwo may provide a competitive advantage to other raphidophytes such as Chattonella spp. that are too large to be consumed at high rates by microzooplankton and help to shape the dynamics of this harmful algal bloom consortium. Our results show that QPCR can be used in conjunction with the dilution method for evaluation of microzooplankton grazing pressure on specific phytoplankton species within a mixed community.

Fast microzooplankton grazing on fast-growing, low-biomass phytoplankton: a case study in spring in Chesapeake Bay, Delaware Inland Bays and Delaware Bay

Dilution experiments were performed to examine the growth and grazing mortality rates of picophytoplankton ( 20 μm) at stations in the Chesapeake Bay (CB), the Delaware Inland Bays (DIB) and the Delaware Bay (DB), in early spring 2005. At station CB microphytoplankton, including chain-forming diatoms were dominant, and the microzooplankton assemblage was mainly composed of the tintinnid Tintinnopsis beroidea. At station DIB, the dominant species were microphytoplanktonic dinoflagellates, while the microzooplankton community was mainly composed of copepod nauplii and the oligotrich ciliate Strombidium sp. At station DB, nanophytoplankton were dominant components, and Strombidium and Tintinnopsis beroidea were the co-dominant microzooplankton. The growth rate and grazing mortality rate were 0.13–3.43 and 0.09–1.92 d−1 for the different size fractionated phytoplankton. The microzooplankton ingested 73, 171, and 49% of standing stocks, and 95, 70, and 48% of potential primary productivity for total phytoplankton at station CB, DIB, and DB respectively. The carbon flux for total phytoplankton consumed by microzooplankton was 1224.11, 100.76, and 85.85 μg C l−1 d−1 at station CB, DIB, and DB, respectively. According to the grazing mortality rate, carbon consumption rate and carbon flux turn over rates, microzooplankton in study area mostly preferred to graze on picophytoplankton, which was faster growing but was lowest biomass component of the phytoplankton. The faster grazing on Fast-Growing-Low-Biomass (FGLB) phenomenon in coastal regions is explained as a resource partitioning strategy. This quite likely argues that although microzooplankton grazes strongly on phytoplankton in these regions, these microzooplankton grazers are passive.

A comparison of future increased CO 2 and temperature effects on sympatric Heterosigma akashiwo and Prorocentrum minimum

Very little is known about how global anthropogenic changes will affect major harmful algal bloom groups. Shifts in the growth and physiology of HAB species like the raphidophyte Heterosigma akashiwo and the dinoflagellate Prorocentrum minimum due to rising CO 2 and temperature could alter their relative abundance and environmental impacts in estuaries where both form blooms, such as the Delaware Inland Bays (DIB). We grew semi-continuous cultures of sympatric DIB isolates of these two species under four conditions: (1) 20 °C and 375 ppm CO 2 (ambient control), (2) 20 °C and 750 ppm CO 2 (high CO 2), (3) 24 °C and 375 ppm CO 2 (high temperature), and (4) 24 °C and 750 ppm CO 2 (combined). Elevated CO 2 alone or in concert with temperature stimulated Heterosigma growth, but had no significant effect on Prorocentrum growth. P Bmax (the maximum biomass-normalized light-saturated carbon fixation rate) in Heterosigma was increased only by simultaneous CO 2 and temperature increases, whereas P Bmax in Prorocentrum responded significantly to CO 2 enrichment, with or without increased temperature. CO 2 and temperature affected photosynthetic parameters α, Φ max, E k, and Δ F / F ′ m in both species. Increased temperature decreased and increased the Chl a content of Heterosigma and Prorocentrum, respectively. CO 2 availability and temperature had pronounced effects on cellular quotas of C and N in Heterosigma, but not in Prorocentrum. Ratios of C:P and N:P increased with elevated carbon dioxide in Heterosigma but not in Prorocentrum. These changes in cellular nutrient quotas and ratios imply that Heterosigma could be more vulnerable to N limitation but less vulnerable to P-limitation than Prorocentrum under future environmental conditions. In general, Heterosigma growth and physiology showed a much greater positive response to elevated CO 2 and temperature compared to Prorocentrum, consistent with what is known about their respective carbon acquisition mechanisms. Hence, rising temperature and CO 2 either alone or in combination with other limiting factors could significantly alter the relative dominance of these two co-existing HAB species over the next century.

Simultaneous enumeration of multiple raphidophyte species by quantitative real-time PCR: capabilities and limitations

Quantitative real-time PCR (QPCR) is a powerful method for species-specific detection and enumeration of harmful algal bloom (HAB) algae, but can be quite time-consuming and expensive for multiple species detection when only a single species is analyzed in each reaction. Rapid enumeration methods would be valuable for the investigation of multiple HAB events such as the mixed blooms of harmful raphidophyte species (Chattonella cf verruculosa, C. subsalsa, and Heterosigma akashiwo) that co-occur in the Delaware Inland Bays (DIB), USA. This technology uses multiple species-specific probes that fluoresce at different wavelengths and allows for the simultaneous detection and enumeration of more than one species, limited only by the detection capabilities of the instrument. The approach can cut analysis time and cost at least in half, depending on the number of species that can be accurately detected and enumerated in the mixture. Here, we compared multiprobing—or using a single primer set with species-specific probes—to multiplexing, in which species-specific primers and probes are used, targeting the 18S rDNA of 3 raphidophyte species and an internal standard. Although both methods were effective for multispecies detection compared to simplex results, multiplexing was more accurate than multiprobing for this particular group of species. We investigated the accuracy and sensitivity of multiplex/multiprobe QPCR using plasmids, genomic DNA from cultures, or DNA extracted from environmental samples as template. Smaller amplicon size and sequence heterogeneity between amplicons increased the accuracy of the results. We found that environmental samples of raphidophytes can be successfully multiplexed or multiprobed with only minimal losses in sensitivity. In addition, we explored probe reporter dyes and quenchers for their compatibility, robustness, and reproducibility in multitemplate detection, and we compared and evaluated the capabilities and limitations of this method for detecting and enumerating multiple phytoplankton species simultaneously.

Evaluating vertical migration behavior of harmful raphidophytes in the Delaware Inland Bays utilizing quantitative real-time PCR

Mixed blooms of 4 species of harmful raphidophytes (Chattonella cf. verruculosa, Chat- tonella subsalsa, Heterosigma akashiwo, and Fibrocapsa japonica) occur in the shallow (1 to 2 m) Delaware Inland Bays (DIB), USA. Raphidophytes vertically migrate in other deeper water ecosys- tems to utilize deep nutrient stocks at night, and thus obtain an advantage over non-migrating algae. Anoxic DIB sediments release high levels of bioavailable phosphate, which could potentially be used by vertically migrating flagellates. This study aimed to characterize and understand the migration patterns of DIB raphidophytes, and determine whether benthic phosphate fluxes could provide the cells with P. We demonstrated vertical migration of isolated DIB raphidophyte cultures in the labora- tory, where differences in the response of C. subsalsa and H. akashiwo to light:dark period manipu- lations suggested possible differences in external versus endogenous regulation of migration behav- ior in the 2 species. Natural blooms in the field (enclosed in a mesocosm system) also exhibited patterns of diel vertical migration, as determined by quantitative real-time PCR (QPCR) used to enumerate the diel vertical distributions of each species. Our data suggested that these 2 photoauto- trophic species spend daylight hours near the surface and are found directly on the sediment surface at night. However, diel changes in particulate C:P ratios did not support the hypothesis that there is preferential uptake of sedimentary phosphate at night. Our results also suggested that the migration behavior may have important implications for designing sampling strategies for monitoring pro- grams. QPCR has a number of decisive advantages over traditional microscopic counting methods, making this a powerful tool for fine spatial and temporal scale detection and enumeration of vertically migrating harmful algal species.

The roles of anoxia, H2S, and storm events in fish kills of dead-end canals of Delaware inland bays

In 2001, the development of seasonal anoxia was studied in two waterways located at the head of Delaware’s northern inland bay, Rehoboth Bay. Bald Eagle Creek is a northern tributary of the bay, which has tidal exchange with Torquay Canal (a dead-end canal) via a short channel with a 1.4 m sill. Mean low water depth in Torquay Canal is about 2 m, but dredging produced over a dozen depressions with a total water depth of 5.5 m. During the summer of 2000, four major fish kills were reported in Torquay Canal and Bald Eagle Creek with more than 2.5 million juvenile menhaden (Brevoortia tyrannus) killed. Low O2 concentration was assumed to be the problem but production of toxic H2S is more likely. From late spring 2001, we conducted in situ determination of temperature, salinity, pH, dissolved O2, and H2S in Torquay Canal and Bald Eagle Creek. During spring, water column stratification began in the depressions with warmer and less salty water observed in the upper layer, and cooler, saltier water below 2 m. O2 was at saturation levels in the surface waters but was not detectable below 2 m by the end of May. The depressions were anoxic with H2S accumulating to mM concentrations in June. A storm event prior to July 12 mixed these two layers with a subsequent loss of H2S. The H2S levels again increased in the deep water due to stratification and reached another maximum in late August. Another storm event occurred at this time resulting in no detectable O2 and up to 400 μM H2S in surface waters. H2S appears to be the primary reason for fish kills in these tributaries. Aerators installed in Torquay Canal on June 21 had no significant effect on abating stratification and anoxic conditions beyond their immediate area.

The Subtidal Lagrangian Current in Delaware's Inland Bays Under Low Wind Conditions

A series of field surveys were conducted in Delaware's inland bays to investigate the subtidal current in a Lagrangian framework. Drifters tracked by the LORAN-C and satellite (ARGOS) positioning techniques were released in January and April of 1992. The LORAN-C position data were first locally corrected, and then inter-compared with the concurrent ARGOS data. Subtidal currents were then examined based on LORAN-C position data obtained under low wind conditions. Current velocities, calculated from a central finite-difference algorithm, were generally less than 3·0 Cm s -1 in the bays, although larger currents (5·5 cm s -1) were observed in the vicinity of the Indian River Inlet. Correlation between the subtidal current and wind was also systematically examined. In January, subtidal current and wind were favourably correlated with a complex correlation coefficient of 0·78 and an average veering of about 3°. In April, however, a poor correlation was found between the current and wind vector time series.

Low‐frequency variability in Delaware's inland bays

The low‐frequency variability in Delaware's inland bays is examined based on observational data derived from a combination of tide gauges, moored current meters, and free‐floating Woodhead drifters. The currents and surface elevations in the bays exhibit considerable fluctuations at subtidal timescales of 2–7 days. These subtidal fluctuations are forced primarily by the remote atmospheric effects through the imposition of coastal sea level setup or setdown at the entrance of the bays. To first order, the current and sea level fluctuations in the interior of the system respond to coastal forcing in a pumping mode fashion. With a coastal setup (or setdown), water is pumped into (or out of) the system from all open boundaries. This pumping mode response produces vigorous subtidal current fluctuations (of the order of 25 cm/s) in the Indian River Inlet. In addition to the subtidal fluctuations, the data also indicate the presence of significant residual currents of the order of 10 cm/s in the Indian River Inlet. The residual currents have a pattern of surface outflow and bottom inflow that is consistent with the density‐induced gravitational circulation.

Delaware's inland bays: A case study

Abstract A management plan has been developed for the Inland Bays which is a major coastal estuary. Eutrophication and closure of shellfish areas are two major water quality problems in the Inland Bays. The largest source of nutrients entering the bays is from nonpoint sources. Agriculture is the largest contributor of nitrogen and phosphorus. Best Management practices (BMP's) such as manure nutrient analysis, calibration of manure spreaders, timing of application and applying only enough nitrogen to be removed by the crop are being emphasized to make more efficient use of nutrients in poultry manure and commercial fertilizer. Erosion and runoff control BMP's would have little effect on reducing nitrogen and phosphorus loads to the bays because of the porous soils.

The response of Delaware's inland bays to ocean forcing

Sea level and current observations made during a 2‐month summer period in Delaware's inland bays are used for the examination of the tidal and subtidal variability in the bays. To first order, the coupled inland bays are found to be forced primarily by coastal sea level fluctuations at both tidal and subtidal frequencies. The observations suggest that the systems behave as low‐pass filters in response to coastal forcing. Overall, tidal variance decreases sharply inside the bays, with the semidiurnal tides suffering significantly higher attenuation than the diurnal tides. The subtidal variability, however, experiences no appreciable attenuation within the bays. The systems are also found to be only weakly nonlinear in nature. A linearized frequency‐dependent pumping mode model is developed, and results indicate that the model adequately describes the first‐order response of the inland bays to coastal sea level forcing.

Tidal and Subtidal Variability in Delaware's Inland Bays

Sea level observations made during a three-month period in late 1984 at four stations in and near Delaware's inland bays are used for the examination of tidal and subtidal variability in the bays. Five tidal Constituents are found to be active in the bays, With M2 being the most important one. The inland bays, with their narrow inlets and shallow depths, behave like low-pass filters for oceanic disturbances propagating into the interior of the bays. Overall, tidal variance decreases sharply inside the bays, with the semidiurnal tides suffering significantly higher attenuation than the diurnal tides. At subtidal frequencies, sea level fluctuations in the interior of the bays are primarily forced by coastal sea level fluctuations generated by atmospheric forcing on the adjacent shelf. Local wind forcing within the bays appears to play only a secondary role in modifying the coastally forced sea level. The active subtidal sea level fluctuations induced by the bay-shelf coupling effect experience no appreciable attenuation within the bays. As a consequence, sea level variance at subtidal frequencies is even greater than that at tidal frequencies in one of the inland bays.