Nutrient limitation of phytoplankton in Chesapeake Bay: Development of an empirical approach for water-quality management Archives

Entry Thumbnail

Nutrient limitation of phytoplankton in Chesapeake Bay: Development of an empirical approach for water-quality management

Understanding the temporal and spatial roles of nutrient limitation on phytoplankton growth is neces- sary for developing successful management strategies. Chesapeake Bay has well-documented seasonal and spatial variations in nutrient limitation, but it remains unknown whether these patterns of nutrient limitation have changed in response to nutrient management efforts. We analyzed historical data from nutrient bioassay experiments (1992–2002) and data from long-term, fixed-site water-quality monitoring program (1990–2017) to develop empirical approaches for predicting nutrient limitation in the surface waters of the mainstem Bay. Results from classification and regression trees (CART) matched the seasonal and spatial patterns of bioassay-based nutrient limitation in the 1992–2002 period much better than two simpler, non-statistical approaches. An ensemble approach of three selected CART models satisfactorily reproduced the bioassay-based results (classification rate = 99%). This empirical approach can be used to characterize nutrient limitation from long-term water-quality monitoring data on much broader geo- graphic and temporal scales than would be feasible using bioassays, providing a new tool for informing water-quality management. Results from our application of the approach to 21 tidal monitoring stations for the period of 2007–2017 showed modest changes in nutrient limitation patterns, with expanded ar- eas of nitrogen-limitation and contracted areas of nutrient saturation (i.e., not limited by nitrogen or phosphorus). These changes imply that long-term reductions in nitrogen load have led to expanded areas with nutrient-limited phytoplankton growth in the Bay, reflecting long-term water-quality improvements in the context of nutrient enrichment. However, nutrient limitation patterns remain unchanged in the majority of the mainstem, suggesting that nutrient loads should be further reduced to achieve a less nutrient-saturated ecosystem.

Published in the Journal of Water Research, Volume 188, January 2021:

Entry Thumbnail

Planning Assistance to States: Jennings Randolph Lake Scoping Study Phase II Report

The watershed of the North Branch Potomac River experienced severe environmental degradation and flooding in the 20th century. A dam across the river mainstem was completed in 1982, creating Jennings Randolph Lake. The lake and dam are operated by the U. S. Army Corps of Engineers for four authorized purposes: control floods, dilute downstream pollution, supply drinking to Washington DC during droughts, and provide recreation. Water quality in the North Branch watershed has improved considerably since the dam was built due to many factors, including regulatory enforcement, mine runoff mitigation, wastewater treatment, infrastructure improvements, forest regrowth and the abatement of acid rain (see ICPRB report 19-4). This pilot study was done to determine if an update of the 1997 Reservoir Regulation manual is appropriate at this time. The report reviews and evaluates each of the authorized purposes in terms of their original management goals and objectives, current relevance, and future application.

A copy of the report is available here.

Entry Thumbnail

A water quality binning method to infer phytoplankton community structure and function

Aspects of phytoplankton community structure (e.g., taxonomic composition, biomass) and function (e.g., light adaptation, net oxygen production, exudation) can be inferred with a binning method that uses water transparency (Secchi depth), dissolved inorganic nitrogen, and ortho-phosphate to classify phytoplankton habitat conditions in the surface mixed layer. The method creates six habitat categories, forming a disturbance scale from turbid, nutrient-enriched waters (“degraded”) to clear waters with bloom-limiting nutrient concentrations (“reference”). Across this disturbance scale, estuarine phytoplankton exhibit strong differences in chlorophyll a, count-based biomass, trophic mode, average cell size, photopigment cell content, taxonomic dominance, and the frequency of algal blooms. Differences in ambient dissolved oxygen and dissolved organic carbon are also observed. Two alternate states are apparent, separated primarily by water transparency, or clarity.Water transparency determines cellular light-adaptation and the potential for photosynthesis and growth; nutrient concentrations determine how much of that potential can be realized if and when light becomes available. In Chesapeake Bay, Secchi depth thresholds separating the two states are 0.7–0.9 m in shallow, well-mixed, low salinity waters and 1.2–2.1 m in deeper, stratified, higher salinity waters. The water quality binning method offers a conceptual framework that can be used to infer the overall state of a phytoplankton population more accurately than chlorophyll a alone.

The article was published in Estuaries and Coasts (2020). DOI link: Please contact us for a full copy of the report.


Entry Thumbnail

Disentangling the potential effects of land‐use and climate change on stream conditions

Land‐use and climate change are significantly affecting stream ecosystems, yet understanding of their long‐term impacts is hindered by the few studies that have simultaneously investigated their interaction and high variability among future projections. We modeled possible effects of a suite of 2030, 2060, and 2090 land‐use and climate scenarios on the condition of 70,772 small streams in the Chesapeake Bay watershed, United States. The Chesapeake Basin‐wide Index of Biotic Integrity, a benthic macroinvertebrate multimetric index, was used to represent stream condition. Land‐use scenarios included four Special Report on Emissions Scenarios (A1B, A2, B1, and B2) representing a range of potential landscape futures. Future climate scenarios included quartiles of future climate changes from downscaled Coupled Model Intercomparison Project ‐ Phase 5 (CMIP5) and a watershed‐wide uniform scenario (Lynch2016). We employed random forests analysis to model individual and combined effects of land‐use and climate change on stream conditions. Individual scenarios suggest that by 2090, watershed‐wide conditions may exhibit anywhere from large degradations (e.g., scenarios A1B, A2, and the CMIP5 25th percentile) to small degradations (e.g., scenarios B1, B2, and Lynch2016). Combined land‐use and climate change scenarios highlighted their interaction and predicted, by 2090, watershed‐wide degradation in 16.2% (A2 CMIP5 25th percentile) to 1.0% (B2 Lynch2016) of stream kilometers. A goal for the Chesapeake Bay watershed is to restore 10% of stream kilometers over a 2008 baseline; our results suggest meeting and sustaining this goal until 2090 may require improvement in 11.0%–26.2% of stream kilometers, dependent on land‐use and climate scenario. These results highlight inherent variability among scenarios and the resultant uncertainty of predicted conditions, which reinforces the need to incorporate multiple scenarios of both land‐use (e.g., development, agriculture, etc.) and climate change in future studies to encapsulate the range of potential future conditions. DOI link:

Entry Thumbnail

The Influence of Jennings Randolph Lake and Dam Operations on River Flow and Water Quality in the North Branch Potomac River

A multi-year study began in 2018 to determine if an update of the Army Corps’ 1997 Water Control Plan for Jennings Randolph Lake is needed. Watershed and river conditions have improved significantly since the turn of the century, an outcome of regulatory enforcement, mine runoff mitigation, wastewater treatment,  infrastructure improvements, forest regrowth and the abatement of acid rain. The Commission, in partnership with the Corps, has produced a draft Scoping Study report that reviews the dam’s long-running operational objectives and procedures, and assesses the current importance of these procedures in achieving the four mandates. It also reviews various modeling approaches that incorporate modern science and technology for better future management. Learn more…

Entry Thumbnail

Biological Surveys of Three Potomac River Mainstem Reaches (2012-2014) with Considerations for Large River Sampling

The Interstate Commission on the Potomac River Basin (ICPRB) conducted a study to describe the biological composition of three under-represented reaches in the mainstem Potomac River Basin and determine the effort required to accurately assess large river sites for freshwater mussel and benthic macroinvertebrate populations. Located at Knoxville (MD), Carderock (MD), and Little Falls (MD), these reaches were selected because they are difficult to sample and represent gaps in spatial coverage of the mainstem in the otherwise comprehensive Maryland Department of Natural Resources (MD-DNR) Core Trend Monitoring Program. Data from the Knoxville reach will improve our understanding of the mixing zones below the confluence of the Shenandoah and Potomac rivers and the relative importance of each river at the Potomac water supply intakes downstream. The Carderock and Little Falls reaches are important in identifying stresses on the river’s biological communities that could relate to upstream consumptive losses and water supply withdrawals during severe droughts. The Little Falls reach is in the only stretch of the Potomac River with a minimum flow-by requirement.

Surveys of freshwater mussel and benthic macroinvertebrate populations were conducted during late-summer low-flow periods of 2012, 2013, and 2014. The three years of the study had moderate flows overall and did not experience extreme drought or floods, so managers and researchers should view the results as a characterization of biological communities unaffected by flow extremes. In addition to recording mainstem Potomac species distributions, biological collections underwent post-collection analyses that provided an informed baseline for the collection effort required to achieve sufficiently accurate data in the future.