Patuxent River Research Articles

SEASONAL PATTERNS AND PLANT-MEDIATED CONTROLS OF SUBSURFACE WETLAND BIOGEOCHEMISTRY

In tidal marshes, spatial and temporal variability in the importance of mi- crobial metabolic pathways influences ecosystem-level processes such as soil carbon stor- age, the regeneration of inorganic nutrients, and the production of atmospherically important trace gases. We measured seasonal changes in rates of microbial Fe(III) reduction, sulfate reduction, and methanogenesis in tidal freshwater and brackish marshes on the Patuxent River, Maryland, USA, and assessed the ability of plant roots to influence these processes by regenerating electron acceptors and supplying electron donors. In both marshes, the importance of microbial Fe(III) reduction was greatest early in the summer and decreased through the study period. Coincident with the seasonal decline in Fe(III) reduction, me- thanogenesis (freshwater marsh) or sulfate reduction (brackish site) increased in importance. At the brackish marsh, the partitioning of anaerobic carbon metabolism between Fe(III) reduction and sulfate reduction was similar within and below the root zone, suggesting that rhizosphere processes did not control anaerobic metabolism at this site. Instead, seasonal biogeochemical patterns at the brackish marsh were affected by factors such as water table depth and iron-sulfur interactions. At the tidal freshwater site, our results suggest that changes in rates of Fe(III) reduction and methanogenesis were directly affected by plant- mediated processes. In midsummer, Fe(III) reduction accounted for a greater fraction of total anaerobic metabolism in rhizosphere-influenced surface soils than in soils below the root zone. High rates of Fe(III) reduction occurred at the expense of methanogenesis. This study documented strong temporal variations in the outcome of microbial competition for electron donors that ultimately affected the balance between Fe(III) reduction and me- thanogenesis within tidal freshwater marsh soils. Our data suggested that variations in microbial metabolic pathways were regulated by physiochemical factors at the brackish site and plant activity at the freshwater site. Plant regulation of Fe(III) reduction is a largely unstudied mechanism by which plants influence wetland carbon cycling and greenhouse gas production.

Application of the nursery-role hypothesis to an estuarine fish

In this paper, we address a recent article by Beck et al. (2001, Bioscience 51:633-641), in which it was proposed that the value of a juvenile fish habitat could be most efficiently measured as the production of adults from each juvenile habitat on a per area basis. Based upon retrospective microchemical analysis of otoliths, we calculated this metric (nursery value) for tidal freshwater and brackish littoral habitats of an estuarine fish, white perch Morone americana, in the Patuxent River estuary, Maryland (USA). In dominant year-classes of the population, brackish habitats had the high- est nursery value, whereas in all other year-classes, freshwater habitats had the highest nursery value. This result highlights the importance of inter-annual variability in habitat use, and conse- quently presents a challenging trade-off for applying the nursery-role hypothesis to facilitate habitat conservation and management. Despite this limitation, the nursery-role hypothesis provides an enriched and much needed perspective on population connectivity.

Measured Concentrations of Herbicides and Model Predictions of Atrazine Fate in the Patuxent River Estuary

The environmental fate of herbicides in estuaries is poorly understood. Estuarine physical transport processes and the episodic nature of herbicide release into surface waters complicate interpretation of water concentration measurements and allocation of sources. Water concentrations of herbicides and two triazine degradation products (CIAT [6-amino-2-chloro-4-isopropylamino-s-triazine] and CEAT [6-amino-2-chloro-4-ethylamino-s-triazine]) were measured in surface water from four sites on 40 d from 4 Apr. through 29 July 19% in the Patuxent River estuary, part of the Chesapeake Bay system. Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) was most persistent and present in the highest concentrations (maximum = 1.29 microg/L). Metolachlor [2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)-o-acetoluidide], CIAT, CEAT, and simazine (1-chloro-3,5-bisethylamino-2,4,6-triazine) were frequently detected with maximum concentration values of 0.61, 1.1, 0.76, and 0.49 microg/L, respectively. A physical transport model was used to interpret atrazine concentrations in the context of estuarine water transport, giving estimates of in situ degradation rates and total transport. The estimated half-life of atrazine in the turbid, shallow upper estuary was t(1/2) = 20 d, but was much longer (t(1/2) = 100 d) in the deeper lower estuary. Although most (93%) atrazine entered the estuary upstream via the river, simulations suggested additional inputs directly to the lower estuary. The total atrazine load to the estuary from 5 April to 15 July was 71 kg with 48% loss by degradation and 31% exported to the Chesapeake Bay. Atrazine persistence in the estuary is directly related to river flows into the estuary. Low flows will increase atrazine residence time in the upper estuary and increase degradation losses.

Measured Concentrations of Herbicides and Model Predictions of Atrazine Fate in the Patuxent River Estuary

The environmental fate of herbicides in estuaries is poorly understood. Estuarine physical transport processes and the episodic nature of herbicide release into surface waters complicate interpretation of water concentration measurements and allocation of sources. Water concentrations of herbicides and two triazine degradation products (CIAT [6-amino-2-chloro-4-isopropylamino-s-triazine] and CEAT [6-amino-2-chloro-4-ethylamino-s-triazine]) were measured in surface water from four sites on 40 d from 4 Apr. through 29 July 1996 in the Patuxent River estuary, part of the Chesapeake Bay system. Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) was most persistent and present in the highest concentrations (maximum = 1.29 μg/L). Metolachlor [2-chloro-6′-ethyl-N-(2-methoxy-1-methylethyl)-o-acetoluidide], CIAT, CEAT, and simazine (1-chloro-3,5-bisethylamino-2,4,6-triazine) were frequently detected with maximum concentration values of 0.61, 1.1, 0.76, and 0.49 μg/L, respectively. A physical transport model was used to interpret atrazine concentrations in the context of estuarine water transport, giving estimates of in situ degradation rates and total transport. The estimated half-life of atrazine in the turbid, shallow upper estuary was t1/2 = 20 d, but was much longer (t1/2 = 100 d) in the deeper lower estuary. Although most (93%) atrazine entered the estuary upstream via the river, simulations suggested additional inputs directly to the lower estuary. The total atrazine load to the estuary from 5 April to 15 July was 71 kg with 48% loss by degradation and 31% exported to the Chesapeake Bay. Atrazine persistence in the estuary is directly related to river flows into the estuary. Low flows will increase atrazine residence time in the upper estuary and increase degradation losses.

Dynamics of white perch Morone americana population contingents in the Patuxent River estuary, Maryland, USA

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 279:247-259 (2004) - doi:10.3354/meps279247 Dynamics of white perch Morone americana population contingents in the Patuxent River estuary, Maryland, USA Richard T. Kraus1,2,*, David H. Secor1 1Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, One Williams Street, Solomons, Maryland 20688, USA 2Present address: Department of Marine Biology, Texas A & M University, 5007 Avenue U, Galveston, Texas 77551, USA *Email: krausr@tamug.edu ABSTRACT: Alternative migratory pathways in the life histories of fishes can be difficult to assess but may have great importance to the dynamics of spatially structured populations. We used Sr/Ca in otoliths as a tracer of time spent in freshwater and brackish habitats to study the ontogenetic movments of white perch Morone americana in the Patuxent River estuary. We observed that, soon after the larvae metamorphose, juveniles either move to brackish habitats (brackish contingent) or take up residency in tidal fresh water (freshwater contingent) for the first year of life. In one intensively studied cohort of juveniles, the mean age at which individuals moved into brackish environments was 45 d (post-hatch), corresponding to the metamorphosis of lavae to juveniles and settlement in littoral habitats. Back-calculated growth rates of the freshwater contingent at this same age (median = 0.6 mm d-1) were significantly higher than the brackish contingent (median = 0.5 mm d-1). Strong year-class variability (>100-fold) was evident from juvenile surveys and from the age composition of adults sampled during spawning. Adult samples were dominated by the brackish contingent (93% of n = 363), which exhibited a significantly higher growth rate (von Bertalanffy, k = 0.67 yr-1) than the freshwater contingent (k = 0.39 yr-1). Combined with evidence that the relative frequency of the brackish contingent has increased in year-classes with high juvenile recruitment, these results implicate brackish environments as being important for maintaining abundance and productivity of the population. By comparison, disproportionately greater recruitment to the adult population by the freshwater contingent during years of low juvenile abundance suggested that freshwater habitats sustain a small but crucial reproductive segment of the population. Thus, both contingents appeared to have unique and complementary roles in the population dynamics of white perch. KEY WORDS: Population contingents · Estuarine-dependency · Otolith strontium · Electron microprobe · Back-calculation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 279. Online publication date: September 28, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Effects of barley straw (Hordeum vulgare) on freshwater and brackish phytoplankton and cyanobacteria

A short-term laboratory study was conductedto investigate the effect of barley strawin controlling several common phytoplanktonand cyanobacterial species. Following aone-month incubation of barley straw incoarsely filtered fresh Potomac River andbrackish Patuxent River waters, the growthof six autotrophic taxa was followed inculture. Barley straw slurry reduced theyield of three taxa (Ankistrodesmusfalcatus, Chlorella capsulata, Isochrysis sp.) in comparison withcultures not receiving the slurry. Although no significant changes in growthwere detected with three other taxa (Cyclotella sp., Prorocentrumminimum, freshwater Pseudanabaenasp.), some patterns indicated potentialimpacts of the barley straw. First, ahigher addition of straw to Cyclotella sp. resulted in a lower biomassaccumulation than in cultures receivinglower levels. Second, the bloom-formingdinoflagellate Prorcentrum minimumwas apparently stimulated at low barleystraw levels, perhaps suggesting conditionsassociated with the straw(metals-chelation, bacterial-producednutrients) might stimulate dinoflagellategrowth. Third, species shifts wereobserved in two of the cultures, withbarley straw favoring shifts from Isochrysis to a Cyclotella sp. –Thalassiosira sp. mixture and shiftsfrom Pseudanabaena to a Pseudanabaena – Scenedesmus mixture. These results provide new records for thesusceptibility of freshwater and brackishphytoplankton taxa to barley strawexposure, including species-specificresponses and shifts in species dominancein mixed assemblages.

Interactive effects of animal disturbance and elevation on vegetation of a tidal freshwater marsh

We studied interactions between animal disturbance (geese, carp, and muskrat) and elevation in a field experiment in tidal freshwater marshes of the Patuxent River, Maryland, United States. Vegetation changes were recorded in fenced and unfenced plots in high and low marsh community types for 2 yr using measurements of areal cover and within-plot frequency (which were averaged to create a dominance index), Leaf Area Index (LAI), and aboveground biomass. We related light environment to differences in vegetation using below-canopy measurements of Photosynthetically Active Radiation (PAR). In the low marsh, total cover of all species, cover of annual species, biomass, and LAI were significantly higher in plots fenced to exclude animals (exclosures) than in unfenced plots (fenced/unfenced total cover=76/40%, annual cover=45/10%, biomass=936/352 g m−2, LAI=3.3/1.4). PAR was significantly lower in fenced than unfenced plots (fenced/unfenced=115/442 μmol s-1 m−2). Despite the strong effect of fencing on biomass, species richness per plot (i.e., the number of species per plot, or species density) was not affected significantly by fencing in the low marsh. Most of the observed differences in cover, biomass, LAI, and PAR were due to variation in the abundance of the herbaceous annual speciesBidens laevis (dominance index fenced/unfenced=45/10%) andZizania aquatica (30/12%). In the high marsh community, fencing had only minor effects on plant community composition and did not significantly affect species richness, cover, biomass, PAR, or LAI. Our results show that animals can dramatically affect low marsh vegetation, primarily via physical disturbance or herbivory of shallowly rooted seedlings of annual species.

Numerical modelling of shoal pattern formation in well-mixed elongated estuaries

The formation of channel and shoal patterns in a schematic estuary is investigated using a 2-D depth-averaged numerical model based on a description of elementary flow and sediment transport processes. The schematisations apply to elongated inland estuaries, sandy, well-mixed and tide-dominated. The model results show how, due to non-linear interactions, a simple and regular pattern of initially grown perturbations merges to complex larger-scale channel/shoal patterns. The emerging patterns are validated with field observations. The overall pattern agrees qualitatively with patterns observed in the Westerschelde, The Netherlands, and in the Patuxent River estuary, Virginia. Quantitative comparison of the number of channels and meander length scales with observations and with an analytical model gives reasonable accordance. Complementary to other research approaches, this model provides a tool to study the morphodynamic behaviour of channels and shoals in estuaries.

A framework for estimating ecological risks posed by nutrients and trace elements in the Patuxent River

The continued urbanization of coastal watersheds can influence the quality of water that enters rivers and estuaries. Intelligent management of aquatic resources will require the capability to quantitatively assess and evaluate the impacts of alterations in surface waters that result from changes in patterns of land use. An aquatic ecosystem model was developed and linked to an empirical landscape model to estimate ecological risks posed by nutrients and potentially toxic trace elements (copper [Cu], cadmium [Cd], arsenic [As]) in the Patuxent River, Maryland. The empirical landscape model translated reductions in croplands within the Patuxent River watershed into corresponding changes in nitrate estimated to enter the river. Trace element concentrations were increased in relation to urbanization associated with the loss of agricultural lands in the watershed. The aquatic ecosystem model used the altered inputs of nutrients and trace elements to estimate changes in the annual production dynamics of selected producer and consumer populations within the Patuxent River. The models were implemented for four mainstem locations that defined a transect from the upper freshwater portion of the river to downstream estuarine locations. Ecological impacts were estimated for 4 hypothetical changes in land use that consisted of 10%, 7.5%, 5%, and 2.5% watershed coverage by cropland. Impacts were estimated as the probability (risk) of different magnitudes of increases or decreases in total annual production of populations representative of freshwater and estuarine food webs in the Patuxent River.

Sources of nutrient inputs to the Patuxent River estuary

We quantified annual nutrient inputs to the Patuxent River estuary from point and nonpoint sources and from direct atmospheric deposition. We also compared nonpoint source (NPS) discharges from Piedmont and Coastal Plain regions and from agricultural and developed lands. Using continuous automated-sampling, we measured discharges of water, nitrogen, phosphorus, organic carbon (C), and suspended solids from a total of 23 watersheds selected to represent various proportions of developed land and cropland in the Patuxent River basin and the neighboring Rhode River basin. The sampling period spanned two years that differed in annual precipitation by a factor of 1.7. Water discharge from the watershed to the Patuxent River estuary was 3.4 times higher in the wet year than in the dry year. Annual water discharges from the study watersheds increased as the proportion of developed land increased. As the proportion of cropland increased, there were increases in the annual flow-weighted mean concentrations of nitrate (NO3 −), total nitrogen (TN), dissolved silicate (Si), total phosphate (TPO4 3−), total organic phosphorus (TOP), total P (TP), and total suspended solids (TSS) in NPS discharges. The effect of cropland on the concentrations of NO3 − and TN was stronger for Piedmont watersheds than for Coastal Plain watersheds. As the proportion of developed land increased, there were increases in annual mean concentrations of NO3 −, total ammonium (TNH4 +), total organic N (TON), TN, total organic C (TOC), TPO4 3−, TOP, TP, and TSS and decreases in concentrations of Si. Annual mean concentrations of TON, TOC, forms of P, and TSS were highest in the wet year. Annual mean concentrations of NO3 −, TNH4 +, TN, and Si did not differ significantly between years. We directly measured NPS discharges from about half of the Patuxent River basin and estimated discharges from the other half of the basin using statistical models that related annual water flow and material concentrations to land cover and physiographic province. We compared NPS discharges to public data on point source (PS) discharges. We estimated direct atmospheric deposition of forms of N, P, and organic C to the Patuxent River estuary based on analysis of bulk deposition near the Rhode River. During the wet year, most of the total terrestrial and atmospheric inputs of forms of N and P came from NPS discharges. During the dry year, 53% of the TNH4 + input was from atmospheric deposition and 58% of the NO3 − input was from PS discharges; NPS and PS discharges were about equally important in the total inputs of TN and TPO4 3−. During the entire 2-yr period, the Coastal Plain portion of the Patuxent basin delivered about 80% of the NPS water discharges to the estuary and delivered similar proportions of the NPS TNH4 +, TN, TOP, and TSS. The Coastal Plain delivered greater proportions of the NPS TON, TOC, Si, and TP (89%, 90%, 93%, and 95%, respectively) than of water, and supplied nearly all of the NPS TPO4 3− (99%). The Piedmont delivered 33% of the NPS NO3 − while delivering only 20% of the NPS water to the stuary. We used statistical models to infer the percentages of NPS discharges supplied by croplands, developed lands, and other lands. Although cropland covers only 10% of the Patuxent River basin, it was the most important source of most materials in NPS discharge, supplying about 84% of the total NPS discharge of NO3 −; about three quarters of the TPO4 3−, TOP, TP, and TSS; and about half of the TNH4 + and TN. Compared to developed land, cropland supplied a significantly higher percentage of the NPS discharges of NO3 −, TN, TPO4 3−, TOP, TP, and TSS, despite the fact development land covered 12% of the basin.

The pattern and influence of low dissolved oxygen in the Patuxent River, a seasonally hypoxic estuary

Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River, a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models, to configure a spatially-explicit individual-based model to predict how chang- ing land use within the Patuxent watershed may affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated.

The cost of stress: Low dissolved oxygen and economic benefits of recreational striped bass (Morone saxatilis) fishing in the Patuxent River

A Poisson catch rate model for striped bass (Morone saxatilis) anglers in Chesapeake Bay was developed that incorporates the effect of bottom temperature and dissolved oxygen (DO). Angler catch rates are shown to be negatively affected by low DO. Predicted angler catch rates were then used in a random utility model of striped bass fishing location choice. Where anglers choose to fish is significantly related to expected catch rate and the travel cost and time from the anglers residence to the fishing location. Results from the random utility model were then used to simulate the economic welfare changes that result from changing DO levels in the Patuxent River. Since there are many substitute sites for fishing in the Patuxent River, the welfare effects are small. Increases in DO from current levels have a small effect on angler welfare, but if levels are allowed to deteriorate so they never exceed 5 mg l−1, the welfare effects are much larger. Under this latter scenario, the net present value of angler losses exceeds $100,000, and are almost $300,000 if the fishing grounds are anoxic. Losses are considerably higher as the area impacted by low oxygen conditions increases.

A watershed perspective on nutrient enrichment, science, and policy in the Patuxent River, Maryland: 1960–2000

The Patuxent River, Maryland, is a nutrient-overenriched tributary of the Chesapeake Bay. Nutrient inputs from sewage outfalls and nonpoint sources (NPS) have grown substantially during the last four decades, and chlorophyll a levels have increased markedly with concomitant reductions in water quality and dissolved oxygen concentrations. The Patuxent has gained national attention because it was one of the first river basins in the U.S. for which basin-wide nutrient control standards were developed. These included a reduction in NPS inputs and a limit on both nitrogen (N) and phosphorus (P) loadings in sewage discharges intended to return the river to 1950s conditions. Full implementation of point source controls occurred by 1994, but population growth and land-use changes continue to increase total nutrient loadings to the river. The present paper provides the perspectives of scientists who participated in studies of the Patuxent River and its estuary over the last three decades, and who interacted with policy makers as decisions were made to develop a dual nutrient control strategy. Although nutrient control measures have not yet resulted in dramatic increases in water quality, we believe that without them, more extensive declines in water quality would have occurred. Future reductions will have to come from more effective NPS controls since future point source loadings will be difficult to further reduce with present technology. Changing land use will present a challenge to policy makers faced with sprawling population growth and accelerated deforestation.

Effect of nutrient loading on Atlantic menhaden (Brevoortia tyrannus) growth rate potential in the Patuxent River

We linked a 2-dimensional water quality model of the Patuxent River with a spatially-explicit model of fish growth to simulate how changes in land use in the Patuxent River Basin would affect the growth rate potential (GRP) of Atlantic menhaden (Brevoortia tyrannus). Simulations of three land-use patterns that reflected current nutrient loadings, increased nutrient loadings, and decreased nutrient loadings were used to drive the water quality model. Changes in nutrient loadings caused changes in the timing and intensity of phytoplankton concentrations and the region of hypoxia increased during summer with increased nutrient loading. The spatial distribution of menhaden GRP was highly correlated with phytoplankton concentrations and localized in the middle on third of the Patuxent River. Menhaden growth rate was highest in early June and late summer. During June, menhaden GRP (and phytoplankton concentration) was lowest at the lower nutrient loading simulation. During late summer, mean menhaden growth rates were inversely proportional to nutrient loading rates and menhaden grew best when nutrient loadings were the lowest. Upriver to mid-river phytoplankton patches drove overall mean calculations. Model results suggest that more research is needed on water quality model predictions of phytoplankton levels at a high level of spatial and temporal resolution, menhaden foraging, and menhaden habitat selection.

Injury to Birds and Diamondback Terrapins Resulting from the Chalk Point Oil Spill, Maryland

ABSTRACT A pipeline spill of 126,000 gallons of a mixture of No. 6 and No. 2 fuel oils into the Patuxent River on 7 April 2000 affected over-wintering waterfowl, resident birds, and diamondback terrapins, as well as intertidal habitats, fish and shellfish, benthic habitats, and muskrats. The final estimate of bird impacts was the sum of the numbers of birds collected dead, dying in rehabilitation, and estimated to have died after release and in the field. The result was an estimated 495 total birds killed, with 67 known dead. The total terrapin mortality was estimated by projecting: a) the number of terrapin-years lost from mortality of adults and juveniles killed directly (estimated to be 122 animals) by the spill, b) the estimated number of young that would have been born and survived to adults from those directly killed, and c) increased hatchling mortality. The final estimate of direct terrapin mortality was 616 lost discounted terrapin-years (DTYs), while lost production from the next generation contributed 3,793 lost DTYs. The loss due to increased hatchling mortality was 836 DTYs. The total injury to Patuxent River diamondback terrapins was estimated to be a loss of 5,245 DTYs.

Seasonal variability in response of estuarine phytoplankton communities to stress: Linkages between toxic trace elements and nutrient enrichment

We examined individual and interactive effects of two stressors—nutrients (nitrogen [N] and phosphorus [P]) and trace elements (a mix of arsenic [As], copper [Cu], and cadmium [Cd], and in a second experiment also zinc [Zn] and nickel [Ni])—on phytoplankton of the mesohaline Patuxent River, a tributary of Chesapeake Bay. Experiments were conducted in twenty 1-m3 mesocosms. Four mesocosm runs used two levels of nutrient loadings (0.7–1.0 × ambient N loading and enriched to 1.3–1.6 × ambient N loading) crossed with two levels of trace elements (ambient and enriched approximately 2–5 × higher than ambient concentrations) crossed with five progressive levels of ecosystem complexity. To examine seasonal patterns of responses to stressors, data from these experiments were combined with results of a similar experiment conducted during 1996 (Breitburg et al. 1999a). A second mesocosm experiment examined effects of individual and mixed trace elements, both alone and in combination with nutrients, to further examine which nutrient-trace element interactions were important. Nutrients consistently increased phytoplankton productivity and biomass. Most of the increased biomass was created by large centric diatoms, which increased the mean cell size of the phytoplankton community. Trace element additions decreased phytoplankton productivity and biomass, as well as the contribution of large centric diatoms to phytoplankton biomass. When both trace elements and nutrients were added, trace elements reduced nutrient stimulation. Although the magnitude of the response to nutrient additions tended to be somewhat greater in spring, the seasonal patterns of trace element effects, and nutrient-trace element interactions were far more striking with significant responses restricted to spring mesocosm runs. The second experiment indicated that both As and Cu were more inhibitory to phytoplankton in spring than in summer, but As was more inhibitory in the low nutrient treatments and Cu was more inhibitory in the nutrient enrichment treatments. The potential for strong seasonal patterns and high temporal variability in stressor effects and multiple stressor interactiosn will require close attention in the design and interpretation of management-relevant research and monitoring and may indicate the need for seasonally varying management strategies.

The responses of Patuxent River upper trophic levels to nutrient and trace element induced changes in the lower food web

As a result of human activities, coastal waters can be exposed to multiple stressors that affect primary producers and their interactions with higher trophic levels. Mesocosm experiments were conducted during spring and summer 1996–1998 to investigate the responses of natural populations of primary producers to multiple stressors and the potential for these responses to be transmitted to higher trophic levels (i.e., copepods, bivalves, anemones, and fish). The effects of two stressors, elevated nutrient and trace element loadings, were examined individually and in combination. Nutrient additions had a positive effect on biomass, productivity, and abundance of primary producers (Breitburg et al. 1999; Riedel et al. 2003). Growth or abundance of consumers increased with nutrient additions, but the magnitude of the response was reduced relative to that of their prey. Responses to trace element additions varied seasonally and among taxa. The responses of zooplankton and bivalves to stressor additions were affected by the biomass and changes in species composition of phytoplankton assemblages. The presence of fish predators did not alter zooplankton responses to stressor additions. These results suggest that the extent to which nutrient and trace element effects are transmitted from primary producers to higher trophic levels depends on the capacity of consumers to respond to stressor-induced changes in abundance and species composition of prey, on the absolute abundance of prey, and on the ability of predators to feed on alternative prey. The magnitude of the effects of stressors on estuarine food webs may depend on seasonal variability in species composition of phytoplankton assemblages, whether sensitive species dominate, and whether these species are important prey for secondary consumers. Because spatial and temporal patterns in nutrient and trace element loadings to the estuary can affect species composition of primary producers, it is critically important to examine the magnitude, timing, and spatial relationships of loadings of multiple stressors to coastal waters in order to understand the impacts of these stressors on higher trophic levels.

Effects of land-use change on nutrient discharges from the Patuxent River watershed

We developed an empirical model integrating nonpoint source (NPS) runoff, point sources (PS), and reservoir management to predict watershed discharges of water, sediment, organic carbon, silicate, nitrogen, and phosphorus to the Patuxent River in Maryland. We estimated NPS discharges with linear models fit to measurements of weekly flow and 10 material concentrations from 22 study watersheds. The independent variables were the proportions of cropland and developed land, physiographic province (Coastal Plain or Piedmont), and time (week). All but one of the NPS models explained between 62% and 83% of the variability among concentration or flow measurements. Geographic factors (land cover and physiographic province) accounted for the explained variability in largely dissolved material concentrations (nitrate [NO3], silicate [Si], and total nitrogen [TN]), but the explained variability in flow and particulates (sediment and forms of phosphorus) was more strongly related to temporal variability or its interactions with land cover and province. Average concentrations of all materials increased with cropland proportion and also with developed land (except Si), but changes in cropland produced larger concentration shifts than equivalent changes in developed land proportion. Among land cover transitions, conversions between cropland and forest-grassland cause the greatest changes in material discharges, cropland and developed land conversions are intermediate, and developed land and forest-grassland conversions have the weakest effects. Changing land cover has stronger effects on NO3 and TN in the Piedmont than in the coastal Plain, but for all other materials, the effects of land-use change are greater in the Coastal Plain. We predicted the changes in nutrient load to the estuary under several alternate land cover configurations, including a state planning scenario that extrapolates current patterns of population growth and land development to the year 2020. In that scenario, declines in NPS discharges from reducing cropland are balanced by NPS discharge increases from developing an area almost six times larger than the lost cropland. When PS discharges are included, there are net increases in total water, total phosphorus, and TN discharges.

Preface—From ecology to economics: Tracing human influence in the Patuxent River estuary and its watershed

Preface—From ecology to economics: Tracing human influence in the Patuxent River estuary and its watershed

The interrelationships among trace element cycling, nutrient loading, and system complexity in estuaries: A mesocosm study

Biogeochemical interactions between a suite of trace elements and nutrients were examined in a series of experimental mesocosm experiments to understand how multiple stressors affect estuarine environments and how these effects are modified by the complexity of the system used to examine them. Experimental treatment included additions of nutrients and trace elements separately and combined, along with a gradient in experimental system complexity. Eight mesocosm experiments were carried out from 1996 through 1998. Increased nutrients generally decreased dissolved trace element concentrations, in large part through an increase in phytoplankton biomass, but also by increasing the concentration of metals in the particles. Trace element additions increased dissolved nutrients by decreasing phytoplankton biomass. The presence of sediments reduced both dissolved trace element and nutrient concentrations. Other complexity treatments had weaker effects on both dissolved nutrients and trace elements. Many of the observed effects appeared to be seasonal, occurring only in spring, or their magnitude was greater in spring. This may be linked to a change from phosphorus to nitrogen limitation that often occurs in the Patuxent River estuary in the late spring or early summer period.