Pamlico River Estuary Research Articles

Meteorological Influences on Nitrogen Dynamics of a Coastal Onsite Wastewater Treatment System

On-site wastewater treatment systems (OWTS) can contribute nitrogen (N) to coastal waters. In coastal areas with shallow groundwater, OWTS are likely affected by meteorological events. However, the meteorological influences on temporal variability of N exports from OWTS are not well documented. Hydrogeological characterization and seasonal monitoring of wastewater and groundwater quality were conducted at a residence adjacent to the Pamlico River Estuary, North Carolina, during a 2-yr field study (October 2009-2011). Rainfall was elevated during the first study year, relative to the annual mean. In the second year, drought was followed by extreme precipitation from Hurricane Irene. Recent meteorological conditions influenced N speciation and concentrations in groundwater. Groundwater total dissolved nitrogen (TDN) beneath the OWTS drainfield was dominated by nitrate during the drought; during wetter periods, ammonium and organic N were common. Effective precipitation (precipitation [P] minus evapotranspiration [ET]) affected OWTS TDN exports because of its influence on groundwater recharge and discharge. Groundwater nitrate-N concentrations beneath the drainfield were typically higher than 10 mg/L when total biweekly precipitation was less than evapotranspiration (precipitation deficit: P < ET). Overall, groundwater TDN concentrations were elevated above background concentrations at distances >15 m downgradient of the drainfield. Although OWTS nitrate inputs caused elevated groundwater nitrate concentrations between the drainfield and the estuary, the majority of nitrate was attenuated via denitrification between the OWTS and 48 m to the estuary. However, DON originating from the OWTS was mobile and contributed to elevated TDN concentrations along the groundwater flowpath to the estuary.

Influence of Wind-Driven Inundation and Coastal Geomorphology on Sedimentation in Two Microtidal Marshes, Pamlico River Estuary, NC

Marsh sediment accumulation is predominately a combination of in situ organic accumulation and mineral sediment input during inundation. Within the Pamlico River Estuary (PRE), marsh inundation is dependent upon event (e.g., storms) and seasonal wind patterns due to minimal astronomical tides (<10 cm). A better understanding of the processes controlling sediment deposition and, ultimately, marsh accretion is needed to forecast marsh sustainability with changing land usage, climate, and sea level rise. This study examines marsh topography, inundation depth, duration of inundation, and wind velocity to identify relationships between short-term deposition (tile-based) and long-term accumulation (210Pb and 137Cs) recorded within and adjacent to the PRE. The results of this study indicate (1) similar sedimentation patterns between the interior marsh and shore-side marsh at different sites regardless of elevation, (2) increased sedimentation (one to two orders of magnitude, 0.04–4.54 g m−2 day−1) within the interior marsh when the water levels exceeded the adjacent topography (e.g., storm berm), and (3) that short-term sea level changes can have direct effects on sediment delivery to interior marshes in wind-driven estuarine systems.

Numerical study on salinity stratification in the Pamlico River Estuary

The variations of current circulation, salt intrusion, and vertical stratification under different river flow and wind conditions in the Pamlico River Estuary (PRE) were investigated in this paper using a three-dimensional numerical model. The model was calibrated and verified against water level variation, temperature, and salinity variations during 2003 and 2001, respectively. Eight sensitivity tests were conducted with different river flow and wind conditions specified in the model. Model results show that salinity intruded further upstream under scenarios with low flow, downriver local wind, and remote-wind-caused water level set-up conditions. In contrast, the responses of salinity stratification to different environmental forcing functions were different in different portions of the estuary. Salinity stratification was enhanced under high flow condition at the lower part of the estuary, under upriver wind near the river mouth, under downriver wind at the upstream to middle portion of the estuary, and under remote-wind-caused water level set-up condition at the majority of the estuary except near the river mouth. Model results also show that across-channel wind tended to reduce salt intrusion and salinity stratification in the PRE through increased vertical mixing.

Inter-annual variability of hypoxic conditions in a shallow estuary

Water quality data from two monitoring programs in the Pamlico River Estuary (PRE) were analyzed for dissolved oxygen (DO), salinity, temperature, and nutrient concentrations. Data were collected bi-weekly at 8 stations from 1997 to 2003 by East Carolina University and continuously at three stations from 1999 to 2003 by the U.S. Geological Survey. Hypoxic conditions were observed mostly in the upper to middle estuary, but the frequency of hypoxic events varied between years. During June to October in 1997–1999 (referred to as the oxic summers) bottom water hypoxia (DO < 2 mg l − 1 ) was found in 8.7% of the observations. By contrast, during June to October in 2001–2003 (referred to as the hypoxic summers), 37.9% of the total measurements had DO concentrations less than 2 mg l − 1 . The more frequent and/or prolonged hypoxic conditions during the hypoxic summers were closely associated with stronger salinity stratification and greater loadings of nutrient and particulate matter. Salinity stratification appeared to be governed by patterns of freshwater discharge, and frequency of wind mixing events. The “oxic” summers were characterized by continuous low freshwater inflow (except one extremely high flow event due to hurricanes), stronger northeastward wind, and more frequent wind mixing events. In contrast, the hypoxic summers were characterized by frequent moderate freshwater inflow events, and fewer wind mixing events. The greater loadings of nutrient (nitrate, ammonium, and phosphate) and particulate matter during the hypoxic summers were primarily due to higher river discharges. At the head of the PRE, no significant differences were found in concentrations of nutrient and particulate nitrogen between the oxic and the hypoxic summers. In addition, chlorophyll a concentrations were averaged above 30 μg l − 1 (maximum 167 μg l − 1 ) during the hypoxic summers, significantly higher than those during the oxic summers (averaged around 15 μg l − 1 ).

Dissolved oxygen stratification in two micro-tidal partially-mixed estuaries

The controlling physical factors for vertical oxygen stratification in micro-tidal, partially-mixed estuaries are discussed in this paper. A theoretical deduction shows that vertical stratification of dissolved oxygen (DO) concentration can be explained by the extended Hansen and Rattray's Central Region theory, which suggests that in addition to biological factors such as photosynthesis, biochemical oxygen demand (BOD), sediment oxygen demand (SOD), vertical DO profiles are mainly controlled by physical factors such as surface re-aeration, river flow, and estuarine gravitational circulation. Vertical mixing of DO from surface re-aeration and photosynthesis sets a DO profile of higher concentration near the surface and lower near the bottom. With a positive seaward longitudinal DO gradient, strong river flow and estuarine gravitational circulation can cause lower DO concentrations near the surface and higher near the bottom. The actual vertical oxygen profile is then determined by the relative magnitude of the above-mentioned mechanisms. It is sensitive to two parameters: (1) the strength of the gravitational circulation ( u E); and (2) the relative importance between biochemical oxygen demand and vertical diffusivity ( α). Vertical DO stratification usually becomes weaker as u E increases. The impact of gravitational circulation on vertical oxygen distribution becomes more important for a larger α. The impact of α on oxygen stratification is profound. As u E (and river flow) increases, DO stratification appears to be less sensitive to the value of α. Surface-to-bottom differences in DO concentrations (ΔDO) is negligible when α is small ( α < 0.5). As α increases, ΔDO increases under a weak to moderate gravitational circulation mode ( u E ≤ 5 cm s −1). Under a strong gravitational circulation mode, ΔDO becomes negative with a small α ( α < 2), and as α continues to increase, ΔDO becomes positive. The newly-deduced governing equation for vertical oxygen stratification is applied to two micro-tidal, partially-mixed estuarine systems: the Cape Fear River Estuary (CFRE) and the Pamlico River Estuary (PRE) of North Carolina. In the CFRE, although strong vertical salinity stratification exists, DO concentrations are usually well mixed. De-coupling between salinity stratification and oxygen stratification is mainly due to a relatively stronger estuarine gravitational circulation and higher freshwater inflow in the system. It appears that river flow and gravitational circulation are the dominant factors in controlling oxygen stratification in the CFRE. In contrast, vertical stratification of DO concentrations is closely correlated with that of salinity in the PRE. In the PRE, the estuarine gravitational mode and river flow are often both very weak, and DO stratification is very sensitive to the value of α. With negligible influence from tidal mixing, the system is more sensitive to vertical mixing regulated by salinity stratification and wind. As a result, vertical DO stratification is closely correlated with salinity stratification in the PRE.

Effects of hurricane floyd inland flooding, September–October 1999, on tributaries to Pamlico Sound, North Carolina

Hurricane Floyd in September 1999 caused disastrous flooding from South Carolina to Massachusetts in the United States, with particularly severe and prolonged flooding in eastern North Carolina resulting in record flood- flow loadings of freshwater and contaminants to Pamlico Sound, North Carolina. The inland flooding, water quality, and loadings to Pamlico Sound were determined as part of a multi-agency response to the floods and in an effort to under- stand the effects of the floods on the greater Pamlico Sound Basin. All major river basins draining to Pamlico Sound experienced floods at the 500-yr recurrence level. The volume of flood waters entering Pamlico Sound during September- October 1999 was estimated to be equivalent to about 95% of the volume of Pamlico Sound, meaning that flood waters could have essentially displaced most of the water present in Pamlico Sound. Nitrogen and phosphorus loads to the Pamlico River estuary and Neuse River estuary, the two principal estuaries draining to Pamlico Sound, in a 36-d period during the flooding were between 50-90% of the long term average annual loads. Pesticide concentrations in flood waters were surprisingly high, given the amount of dilution produced by the floodwaters.

Fifteen Years of Vegetation and Soil Development after Brackish‐Water Marsh Creation

AbstractAboveground biomass, macro‐organic matter (MOM), and wetland soil characteristics were measured periodically between 1983 and 1998 in a created brackish‐water marsh and a nearby natural marsh along the Pamlico River estuary, North Carolina to evaluate the development of wetland vegetation and soil dependent functions after marsh creation. Development of aboveground biomass and MOM was dependent on elevation and frequency of tidal inundation. Aboveground biomass of Spartina alterniflora, which occupied low elevations along tidal creeks and was inundated frequently, developed to levels similar to the natural marsh (750 to 1,300 g/m2) within three years after creation. Spartina cynosuroides, which dominated interior areas of the marsh and was flooded less frequently, required 9 years to consistently achieve aboveground biomass equivalent to the natural marsh (600 to 1,560 g/m2). Aboveground biomass of Spartina patens, which was planted at the highest elevations along the terrestrial margin and seldom flooded, never consistently developed aboveground biomass comparable with the natural marsh during the 15 years after marsh creation. MOM (0 to 10 cm) generally developed at the same rate as aboveground biomass. Between 1988 and 1998, soil bulk density decreased and porosity and organic C and N pools increased in the created marsh. Like vegetation, wetland soil development proceeded faster in response to increased inundation, especially in the streamside zone dominated by S. alterniflora. We estimated that in the streamside and interior zones, an additional 30 years (nitrogen) to 90 years (organic C, porosity) are needed for the upper 30 cm of created marsh soil to become equivalent to the natural marsh. Wetland soil characteristics of the S. patens community along upland fringe will take longer to develop, more than 200 years. Development of the benthic invertebrate‐based food web, which depends on organic matter enrichment of the upper 5 to 10 cm of soil, is expected to take less time. Wetland soil characteristics and functions of created irregularly flooded brackish marshes require longer to develop compared with regularly flooded salt marshes because reduced tidal inundation slows wetland vegetation and soil development. The hydrologic regime (regularly vs. irregularly flooded) of the “target” wetland should be considered when setting realistic expectations for success criteria of created and restored wetlands.

Assessment of function in an oligohaline environment: Lessons learned by comparing created and natural habitats

Assessments of nursery area function were carried out over a 10-year period in a 3-ha oligohaline marsh and creek system (‘Project Area 2’) and four natural ‘control’ creeks (Drinkwater, Jacks, Jacobs, and Tooley) located in the Pamlico River estuary, North Carolina. Habitat function was assessed by comparing (1) growth and survival of fish; (2) long-term monitoring of water quality, sediment organic carbon, and the benthic infaunal community; and (3) measurement of benthic food availability. Growth (weight gain) and survival of the fish Leiostomus xanthurus held within enclosures were similar in both created and natural habitats. Species composition, total fauna density, and species richness of the infaunal community of the Project Area and the natural creeks were comparable within 3 years after construction of the Project Area. However, the sediments of the Project Area lacked the woody detrital cover, high peat content, and predominance of silt and clay characteristic of the natural creek sediments. There was no evidence of significant accretion of total organic carbon in the Project Area during the course of the study. This study has heuristically inspired four recommendations concerning assessment criteria of mitigation success. (1) Direct experimentation is needed to assess habitat function for motile species such as fish. (2) Studies of community structure need to be carried out long enough to permit testing of community stability, especially when working in areas exposed to stochastic abiotic and biotic stressors. (3) Measurements of nutritional content of the sediments should include estimates of overall organic quantity and nutritional quality. (4) Site design or restoration techniques should be included in the experimental design of each mitigation effort. Specifically, the lack of replication in these aspects of the mitigation process limits the inferential potential of the study, constrains the ability to make accurate predictions about the probability of success of future mitigation endeavors, and impedes our understanding of the critical mechanisms governing successful habitat creation, restoration, and enhancement.

Importance of food availability for growth of juvenile southern flounder ( Paralichthys lethostigma) in the Pamlico River Estuary, North Carolina, USA

Results presented in an earlier paper showed that differences in temperature and salinity could explain approximately 30% of the variability in growth rates of juvenile southern flounder ( Paralichthys lethostigma) in different creeks of the Pamlico River estuary. In addition, the maximal growth rate was lower than expected from published laboratory results, suggesting food limitation. In the present paper, the obtained mean and maximal growth rates are related to simultaneously collected data on the amount of food available in the creeks. Thirteen different groups of food items were observed in the stomachs of the flounder. Most of the stomachs of flounder collected outside the cages contained only mysids (65 to 75%). Inside the cages, 25 to 45% of the flounder stomachs contained only mysids, while 85 to 100% of the stomachs contained both mysids and other food categories. Individual growth rates of the founder were only significantly related to the number of mysids in the stomachs, and not to any of the other food categories. Stomachs of other fish species mostly showed food categories other than mysids. This indicates that interspecific competition for food did not occur. The four creeks of the cage experiment showed significant differences in abundance of mysids. However, in only one of the two trials the abundance of mysids was significantly related to the observed growth rates. It can be concluded that mysid abundance does not unequivocally explain the variability in growth rates of juvenile southern flounder in low salinity nursery areas of Pamlico Sound, and that the hypothesis of food limiting maximal growth is not supported by the results of this study.

Growth potential of juvenile southern flounder, Paralichthys lethostigma, in low salinity nursery areas of Pamlico sound, North Carolina, USA

Field growth experiments were conducted in cages during June–August 1993, to compare growth rates of juvenile southern flounder, Paralichthys lethostigma (standard length (SL) 37 to 70 mm), in historically utilized (two creeks) and underutilized (two creeks) low salinity nursery areas within the Pamlico River Estuary of Pamlico Sound, North Carolina. Growth rates from five sequential trials were used to estimate the nursery areas' productive capacities for juveniles of this species. Instantaneous daily growth rates, measured as a function of changes in weight, showed that southern flounder placed in underutilized creeks exhibited significantly higher mean instantaneous daily growth rates (3.94·10 −2±0.47) than southern flounder placed in utilized creeks (1.66·10 −2±0.49). Thus, other factors such as colonization rate must be limiting production in nursery areas. Temperature was positively correlated with growth rate and accounted for about 20% of the variability in growth rates. The approach used in this study provides an alternative method of assessing critical nursery habitats for juvenile flatfish in estuarine environments. Assessment of nursery value in terms of potential productive capacity rather than realized production should prevent underassessment of value due to low levels of colonization in some years.

Invasions and Declines of Submersed Macrophytes in the Tidal Potomac River and Estuary, the Currituck Sound-Back Bay System, and the Pamlico River Estuary

ABSTRACT Long-term changes in biomass, species composition, and distribution of submersed aquatic macrophytes have been documented and studied at two sites in the mid-Atlantic region: the tidal Potomac River and Estuary in Maryland, Virginia, and the District of Columbia, and the Currituck Sound-Back Bay system in Virginia and North Carolina. Additional information based on a shorter time period is available for the Pamlico River Estuary in North Carolina. This paper briefly describes the study areas and summaries the history of declines and increases in each area and factors implicated in these changes. The remainder of the paper is devoted to a discussion of factors influencing invasion/establishment success and the current status of submersed macrophytes in the three areas.

Long‐term trends in Pamlico River Estuary nutrients, chlorophyll, dissolved oxygen, and watershed nutrient production

Trends in Pamlico River estuary ammonia nitrogen (NH4), nitrate nitrogen (NO3), phosphate phosphorus (PO4), chlorophyll a (chl a) and dissolved oxygen (DO) during the past 20–24 years were analyzed, and estimates of annual N and P production in the watershed over the past century were computed. The goal of the study was to determine whether or not the estuary is becoming more eutrophic. NO3 has decreased in the upper and middle regions of the estuary by 3–6% yr−1 since 1970, and NH4 has decreased throughout the estuary at an annual rate of 5.5–7.7% yr−1. Since 1967 PO4 has increased by 2% yr−1 in the lower two thirds of the estuary due to discharges from a phosphate mining facility. Thus the inorganic N:P ratio has decreased, which suggests that N is now potentially more limiting than in the past. In the upper estuary, chl a increased at a rate of 6.6% yr−1 since 1970, and bottom water DO decreased very slightly; neither showed trends in the middle and lower estuary regions. The weight of the evidence is that the Pamlico has not become more eutrophic during the past two decades. This finding is corroborated by the lack of a trend since 1970 in calculated N and P production from point and nonpoint sources in the watershed. Watershed nutrient production is estimated to have increased severalfold between 1880 and 1970, but appears to have stabilized after 1970, due primarily to decreased application of fertilizer on croplands.

Stratification and Bottom-Water Hypoxia in the Pamlico River Estuary

Relationships among bottom-water dissolved oxygen (DO), vertical stratification, and the factors responsible for stratification-destratification in this shallow, low tidal-energy estuary were studied using a 15-yr set of biweekly measurements, along with some recent continuous-monitoring data. Hypoxia develops only when there is both vertical water-column stratification and warm water temperature (>15°C). In July, 75% of the DO readings were <5 mg 1−1, and one-third were <1 mg 1−1. Severe hypoxia occurs more frequently in the upper half of the estuary than near the mouth. Both the time series data and correlation analysis results indicate that stratification events and DO levels are tightly coupled with variations in freshwater discharge and wind stress. Stratification can form or disappear in a matter of hours, and episodes lasting from one to several days seem to be common. Estimated summertime respiration rates in the water and sediments are sufficient to produce hypoxia if the water is mixed only every 6–12 d. There has been no trend toward lower bottom water DO in the Pamlico River Estuary over the past 15 yr. *** DIRECT SUPPORT *** A01BY059 00002

Isolation of infectious pancreatic necrosis virus (serotype Ab) from diverse species of estuarine fish

Two significant fish kills occurred in the Pamlico River estuary (North Carolina, USA), one in December 1981 and January 1982, and the other in June 1982. The first involved only the southern flounder(Paralichthys lethostigma). Histopathologic examination of morbid and moribund flounder revealed extensive sloughing and necrosis of the mucosa of the pyloric caeca and intestine, and inflammation of the submucosa of the pyloric caeca. Brain and internal organ homogenates from morbid and moribund flounder were assayed on CHSE-214 cells, and a virus was isolated. Virus titers ranged from≤8.4 · 102 to 6.3 · 107 TCID50 per gram of tissue. Cross-plaque neutralization assays indicated that the southern flounder virus was infectious pancreatic necrosis virus serotype Ab. Immersion challenge showed the isolate is only slightly virulent for fry of brook trout(Salvelinus fontinalis). The second fish kill involved the southern flounder and six other species: hogchoker(Trinectes maculatus), Atlantic silverside(Menidia menidia), spot(Leiostomus xanthurus), Atlantic croaker(Micropogon undulatus), silver perch(Bairdiella chrysura), and striped mullet(Mugil cephalus). Virus was isolated from southern founder, hogchoker, Atlantic silverside, and spot. Neutralization assays indicated that the four isolates were nearly identical; however, the diversity of species affected suggests that the virus might not have been the specific cause of mortality.

Distribution of selected trace elements in sediments of Pamlico river estuary, north Carolina

Sixty-eight samples of sediment collected on a variably-spaced grid pattern from Pamlico River Estuary of North Carolina were analyzed for As, Cd, Co, Cr, Cu, F, Ni, Pb, U, Zn, clay, and organic matter. The major objectives of the study were to determine background and anomalous levels of trace elements in the sediments, and the effects of human activities on concentration and distribution of trace elements in the sediments. Clay and organic matter are more concentrated near the center of the estuary. This causes the highest concentration of trace elements in the sediments to be located there also owing to their preferential uptake of these elements. Highest trace element concentrations were observed in clay and organic matter near industrial sites, housing developments, and tributary mouths that drain areas of human activity. The apparent increase in trace element contents of fine sediments in Pamlico River Estuary owing to human activities is 4 to 1,750 times normal background levels.

Clay Distributions in Recent Estuarine Sediments

AbstractThe distribution of clay minerals in Recent sediments can be explained by the relative stability of the clays. The rates of particle aggregation for three clays were determined in the laboratory in synthetic estuarine solutions; from the kinetic studies stability values were calculated. The results indicate that illite is more stable than kaolinite which is more stable than montmorillonite. The distribution of the clays in the Pamlico River Estuary can be explained on the basis of relative clay stability where kaolinite which aggregates rapidly (relatively unstable clay) is found upstream of illite.

The Utilization of Dissolved Free Amino Acids by Estuarine Microorganisms

The importance of vacteria in the cycling of carbon in the Pamlico River estuary was studied by measuring the rates of uptake of organic compounds. Our methods allowed analysis with the Michaelis—Menten kinetics equations, and both the rates of uptake of dissolved free amino acids (DFAA) and glucose as well as the percentage of carbon subsequently respired as CO2 were determined. In addition, the concentrations of the amino acids in the water were determined using ion exchange chromatography. Other tests included measurements of primary productivity and of the effects of the other amino acids in the water upon the uptake of one amino acid. There was considerable variation in the heterotrophic activity over time and distance probably caused by patchiness in distribution of plankton and dissolved compounds in the water. Although there is some competition between amino acids being taken up, the effect upon kinetics measurements is probably negligible. Tests made every 3 hr showed a coefficient of variability (CV) of the measured maximum velocity of uptake (Vmax) of aspartic acid to be only 26%, and a similar CV was found for daily samples. In several instances the uptake of one amino acid was found to be competitively inhibited by the presence of another amino acid, but the concentrations necessary to inhibit were far above natural concentrations and such effects are probably unimportant in nature. Mutual inhibition was found between the similar amino acid pairs glutamic acid and aspartic acid, threonine and serine, glycine and alanine, and leucine and alanine. Highest Vmax values were found during the summer months and early fall and ranged from a high of 69.42 mg C/1°hr for alanine in August to less than 0.20 mg C/1°hr for most of the substrates tested in the colder months. The Vmax values for glucose uptake (0.06 to 9.64 mg C/1°hr) indicate that this estuarine system is one of the most microbially—active environments tested. The DFAA were presented in the water at concentrations of from 10 to 30 mg C/1; over half of this was ornithine, glycine, and serine. The DFAA were only about 0.2% of the total dissolved organic carbon in the water. Further, seasonal variations of DFAA concentrations, generally paralleling those of primary productivity, suggested that the amino acids originated from algal excretion and the decay of algal cells. The orders of abundance and concentrations of individual amino acids were similar to those reported for other bodies of water. When the natural concentration of a substrate is known the actual velocity of uptake (Vn) or flux for that substrate may be found. Flux rates were only 1%—10% of the Vmax values in the coldest months; the highest values were found in the warmest months. At each experimental concentration of amino acid, a certain amount was taken up, and a percentage of this amount was oxidized to caron dioxide. This percentage was constant for a particular amino acid in spite of varying experimental times, substrate concentrations, and temperatures. Leucine had the lowest percent respired (13%) while aspartic and glutanic acids had the highest (50%). Failure to correct uptake data for this respiratory loss introduces significant underestimation. The production of particulate material was calculated by correcting total uptake figures for each amino acid by its characteristic respiration percentage. Over 60% of the particulate production from amino acids was by uptake of alanine, leucine, valine, serine, glycine, aspartic acid, and glutamic acid. Such particulate production averaged 0.79 mg C/1.hr for the year and ranged from 0..6 to 2.37 mg C/1.hr; this is about 10% of the rate of production by algae during the summer months. This amount of particulate organic material is a significant contribution to this estuarine food chain.

NITRATE REDUCTASE ACTIVITY DURING A DINOFLAGELLATE BLOOM1

Phytoplankton nitrate reductase activity in the Pamlico River estuary, North Carolina, paralleled the development of a winter dinoflagellate bloom. High enzyme activity was associated not only with high algal biomass, primarily of Peridinium triquetrum, but also with low nitrate concentrations in the typically nitrate‐rich waters. Nitrate was an important source of nitrogen for the bloom organisms in spite of the abundance of ammonium. The initiation and cessation of nitrate assimilation seemed closely related to the appearance and disappearance of the bloom. Although nitrate metabolism may be an important factor in bloom timing, temperature, salinity, and flushing of the system are also involved.

Brackish-water phytoplankton response to temperature elevation

An experiment was carried out to determine the effects of a temperature increase on the composition of brackish-water phytoplankton populations. Treatments were partitioned in duplicate and results were tested at the 95% confidence level with a randomized-block analysis of variance. The studies were made between 9 February and 18 April 1970 in four 14-m 3 plastic pools located adjacent to the Pamlico River estuary, North Carolina, U.S.A. Temperatures in the two control pools ranged from 5 to 19 °C and in heated pools from 10 to 27 °C during the study. Heated pools averaged 5·5 °C warmer than unheated. The only statistically significant effect noted on phytoplankton class composition was an increase in the dinoflagellate cell concentrations in the heated pools. Phytoplankton populations in the heated pools were more diverse and were composed of larger cells (cells averaged 5·2 times larger in volume) than in controls. It appeared that the warming tended to accelerate the succession of the late winter-early spring population and produced a more mature successional stage. Thus the warming of brackish water, possibly by heated effluents from electric generating plants, can alter the composition of phytoplankton populations.

Macrobenthos of the Pamlico River Estuary, North Carolina

A study of the macrobenthos and its environment in the Pamlico River estuary, North Carolina, included measurements of salinity, temperature, and dissolved oxygen, as well as the particle size and concentrations of organic matter of the sediments. Salinity ranged from 1% to 20%, and temperature from 5° to 31° C. Anoxic conditions in the bottom water over—lying the deeper portions of the estuary occurred in the late summer. Two distinct sediment types were found: a sand substrate in the shallow area and a silty clay substrate in the deep area of the estuary. A preliminary sampling program in July 1968 showed that the macrobenthos was characterized by both low diversity and low density because of unfavorable environmental conditions of salinity and sediment. The statistical sampling design for this study was stratification of the estuary and equal random allocation of samples. Only 18 species were found, and the Shannon—Weaver information diversity index (H) showed low values of 0.66—1.77. In addition, low density values (maximum number of organisms at any station was 629/m2) further indicated that stressing salinity conditions and unstable sediments are factors that make this estuary unfavorable for the benthos. The absence of benthic life in much of the deeper portions of the estuary correlated directly with anoxic conditions in these areas. On the basis of trellis diagrams, the estuary was divided into three areas: (1) and oligohaline zone dominated by Rangia cuneata and Nereis succinea; (2) a mesohaline zone with a Macoma balthica—Heteromastus filiformis—Nereis succinea association; and (3) a polyhaline zone at the mouth of the estuary characterized by a Macoma phenax—Mulinia lateralis—Glycera dibranchiata association. A seasonal sampling program, with transects located in these communities, showed vast seasonal changes in species composition, range of distribution, density, and community delimitation caused by larva set in the estuary. A total of 36 species was found during this 1968—69 sampling program. Although a few species, such as Rangia cuneata and Nereis sucinea, showed relatively little change in densities and range of distribution, most of the benthos underwent great fluctuations due to recruitment from new set. Settling periods in the fall and spring added large numbers of juveniles to the benthos, not only in the deeper portions where re—establishment of life was found after the late fall mixing destroyed the anoxic conditions, but also in the shallow areas. Due to this recruitment, the distribution of communities in the estuary changed so drastically during the year that a great homogeneity was found throughout the estuary during the winter and spring sampling periods. Ho ever, the communities noted in the preliminary sampling reappeared in the summer. Thus the macrobenthos undergoes significant seasonal cycles in the Pamlico River estuary.