Dr Michael Williams

Abstract: Chesapeake Bay supports a diverse assemblage of marine and freshwater species of submersed aquatic vegetation (SAV) whose broad distributions are generally constrained by salinity. An annual aerial SAV monitoring program and a bi-monthly to monthly water quality monitoring program have been conducted throughout Chesapeake Bay since 1984. We performed an analysis of SAV abundance and up to 22 environmental variables potentially influencing SAV growth and abundance (1984–2006). Historically, SAV abundance has changed dramatically in Chesapeake Bay, and since 1984, when SAV abundance was at historic low levels, SAV has exhibited complex changes including long-term (decadal) increases and decreases, as well as some large, single-year changes. Chesapeake Bay SAV was grouped into three broad-scale community-types based on salinity regime, each with their own distinct group of species, and detailed analyses were conducted on these three community-types as well as on seven distinct case-study areas spanning the three salinity regimes. Different trends in SAV abundance were evident in the different salinity regimes. SAV abundance has (a) continually increased in the low-salinity region; (b) increased initially in the medium-salinity region, followed by fluctuating abundances; and (c) increased initially in the high-salinity region, followed by a subsequent decline. In all areas, consistent negative correlations between measures of SAV abundance and nitrogen loads or concentrations suggest that meadows are responsive to changes in inputs of nitrogen. For smaller case-study areas, different trends in SAV abundance were also noted including correlations to water clarity in high-salinity case-study areas, but nitrogen was highly correlated in all areas. Current maximum SAV coverage for almost all areas remain below restoration targets, indicating that SAV abundance and associated ecosystem services are currently limited by continued poor water quality, and specifically high nutrient concentrations, within Chesapeake Bay. The nutrient reductions noted in some tributaries, which were highly correlated to increases in SAV abundance, suggest management activities have already contributed to SAV increases in some areas, but the strong negative correlation throughout the Chesapeake Bay between nitrogen and SAV abundance also suggests that further nutrient reductions will be necessary for SAV to attain or exceed restoration targets throughout the bay.

Abstract: In an effort to better portray changing health conditions in Chesapeake Bay and support restoration efforts, a Bay Health Index (BHI) was developed to assess the ecological effects of nutrient and sediment loading on 15 regions of the estuary. Three water quality and three biological measures were combined to formulate the BHI. Water quality measures of chlorophyll-a, dissolved oxygen, and Secchi depth were averaged to create the Water Quality Index (WQI), and biological measures of the phytoplankton and benthic indices of biotic integrity (P-IBI and B-IBI, respectively) and the area of submerged aquatic vegetation (SAV) were averaged to create the Biotic Index (BI). The WQI and BI were subsequently averaged to give a BHI value representing ecological conditions over the growing season (i.e., March-October). Lower chlorophyll-a concentrations, higher dissolved oxygen concentrations, deeper Secchi depths, higher phytoplankton and benthic indices relative to ecological health-based thresholds, and more extensive SAV area relative to restoration goal areas, characterized the least-impaired regions. The WQI, P-BI and BHI were significantly correlated with (1) regional river flow (r = -0.64, -0.57 and -0.49, respectively; p < 0.01), (2) nitrogen (N), phosphorus (P) and sediment loads (all positively correlated with flow), and (3) the sum of developed and agricultural land use (highest annual r(2) = 0.86, 0.71 and 0.68, respectively) in most reporting regions, indicating that the BHI is strongly regulated by nutrient and sediment loads from these land uses. The BHI uses ecological health-based thresholds that give an accurate representation of the health conditions in Chesapeake Bay and was the basis for an annual, publicly released environmental report card that debuted in 2007.

Abstract: The Choptank and Patuxent tributaries of Chesapeake Bay have become eutrophic over the last 50-100 years. Systematic monitoring of nutrient inputs began in similar to 1970, and there have been 2-5-fold increases in nitrogen (N) and phosphor-us (P) inputs during 1970-2004 due to sewage discharges, fertilizer applications, atmospheric deposition, and changes in land use. Hydrochemical modeling and land-use yield coefficients suggest that current input rates are 4-20 times higher for N and P than under forested conditions existing 350 yr ago. Sewage is a major cause of increased nutrients in the Patuxent; agricultural inputs dominate in the Choptank. These loading increases have caused three major water-quality problems: (1) increased nutrients, phytoplankton, and turbidity; (2) decreased submerged grasses due to higher turbidity and epiphyton shading; and (3) bottom-water hypoxia due to respiration of excess organic matter. Oxygen in the Patuxent is consistently < 3 mg L-1 in bottom waters in summer, whereas oxygen in Choptank bottom waters has been decreasing for the last 20 yr and is now approaching 3 mg L-1 in wet years. The low N:P of sewage inputs to the Patuxent results in an N-limited, P-saturated system, whereas the Choptank is primarily limited by N, but with P limitation of phytoplankton during spring river flows. Insufficient action has been taken to improve the water and habitat quality of these estuaries, although reduced eutrophication in dry years suggests that both estuaries will respond to significant decreases in nutrients.

Abstract: The Patuxent River watershed is a heavily impacted basin (2290 km(2)) and estuarine tributary (120 km(2)) of the Chesapeake Bay, USA. To assist management of the basin, we are testing a coupled modelling system composed of a watershed model (HSPF), an estuarine circulation model (CH3D), and an estuarine water-quality model (CE-QUAL-ICM). The modelling system is being tested to guide the development of Total Maximum Daily Loads (TMDLs), and therefore errors in the models must be carefully evaluated. A comparison of daily total nitrogen (TN) concentrations simulated in HSPF with observations indicated that there was no significant bias, with an rms error of 37%. In contrast, modelled total phosphorus (TP) and total suspended solids (TSS) had significant bias with larger rms errors (65% and 259%, respectively). In the estuary, CH3D accurately simulated tides, temperature, and salinity. CE-QUAL-ICM overestimated nitrogen (N) and phosphorus (P) in the upper estuary and underestimated in the lower estuary, primarily because intertidal marshes are not currently a model component. Model errors declined from short (<= 1 day) to long (multi-year) timescales as under- and overestimations cumulatively cancelled. Watershed model errors propagate into the estuarine models, interacting with each subsequent model's errors, which limits the effectiveness of this TMDL management tool at short timescales.

Abstract: The hydrochemical responses to slash-and-bum agriculture in a small rainforest catchment of the central Amazon were investigated for one year. Disturbances in the partially deforested catchment began in 1987, and during the study a 2-ha plot was cut (July 1989) and burned (October 1989) in preparation for the cultivation of manioc; the partially deforested catchment was approximately 80% deforested at the time of this study. Solute fluxes exported by base flow were estimated from solute concentrations of stream water measured at least once per week. Solute fluxes for storm flow were estimated by measuring streamwater concentrations during two storms. Baseflow runoff represented about 94% of the water outflow from the study basin acid was the dominant pathway of solute export. Total rainfall during the study period was 2754 mm of which 2080 mm was exported from the partially deforested catchment as stream runoff. The ratio of surface runoff to annual rainfall for a similar study conducted in the same catchment while completely forested in 1984 was lower than after the catchment was 80% deforested in 1990 (0.57 versus 0.76), while evapotranspiration (ET) was lower by about a factor of two in 1990 compared to 1984. Particulate removal from the partially deforested catchment was 151 kg ha(-1) yr(-1). Nutrient losses from the partially deforested catchment were higher than those measured when the catchment was undisturbed in 1984 by factors of 1.4, 1.8, and 2.1 for total inorganic nitrogen (TIN), total dissolved nitrogen (TDN), and total nitrogen (TN); and by factors of 4.0, 6.6, and 7.9 for soluble reactive phosphate (PO43-), total dissolved phosphorus (TDP), and total phosphorus (TP), respectively. These data show that deforestation and colonization in upland catchments of the central Amazon alter the hydrochemical balance of streams by decreasing ET, thereby increasing discharge and solute export.

Abstract: Hydrochemical changes caused by slash-and-bum agricultural practices in a small upland catchment in the central Amazon were measured. Solute concentrations were analyzed in wet deposition, overland flow, shallow throughflow, groundwater and bank seepage in a forested plot (about 5 ha) and an adjacent plot (about 2 ha) which had been deforested in July 1989 and planted to manioc, and in stream water in partially deforested and forested catchments. Measurements were made from November 1988 to June 1990. The effects of slash-and-bum agricultural practices observed in the experimental plot included increased overland flow, erosion, and large losses of solutes from the rooted zone. Concentrations of NO3-, Na+, K+, SO42-, Cl- and Mn in throughflow of the experimental plot were higher than those of the control plot by more thana factor of 10. Extensive leaching occurred after cutting and burning, but solute transfers were diminished along pathway stages of throughflow to groundwater, and particularly within the riparian zone of the catchment. High concentrations of N and P in overland flow indicate the importance of using forested riparian buffers to mitigate solute inputs to receiving waters in tropical catchments.