LITMIR.BIZ

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This has yet to induce long‐term changes in the annual discharge of the Po River, which has greatly oscillated over the past three decades without showing clear trends (Cozzi & Giani, 2011). However, flow dynamics of Po River are characterized by a shift towards early spring peaks of runoff (Zampieri et al., 2015) and a decline in summer flows (Cozzi et al., 2019). At the same time, the other NAS rivers have shown a strong reduction in flow (Cozzi et al., 2012). It is important to note that reductions in river flow can result from both a greater anthropogenic use of continental waters as well as from climate‐driven changes. For example, annual runoff to Adriatic rivers of Slovenia were reduced (6%) in 1971–2000 due to increased evapotranspiration of the soils (11%), even in the presence of relatively constant precipitation (Frantar, 2007).

      2.5.3. Reservoir Filling

      A major challenge that is unique to CB is the filling of the Conowingo Reservoir of the Susquehanna River which has neared its sediment storage capacity after 90 years of operation. As sediment accumulates in this reservoir, the cross‐sectional area available for flow, and the vertical depth from water surface to sediment bed, decreases, thereby increasing the average horizontal flow velocity. Consequently, sediment trapping by the reservoir decreases and sediment load to CB increases. Numerous studies have demonstrated the declining trapping performance of this reservoir in recent decades (Hirsch, 2012; Langland, 2015; Zhang et al., 2013; Zhang, Hirsch, et al., 2016). Moreover, Zhang, Hirsch, et al. (2016) reported that such decline in reservoir trapping has occurred under a wide range of flow conditions. These changes, if not addressed, can hinder the attainment of the Chesapeake Bay Total Maximum Daily Load goals because the reservoir was expected to continue trapping sediments and nutrients at historical rates for another 20–30 years when those goals were established in 2010. Thus, the Chesapeake Bay Program partnership has worked to incorporate recent scientific understanding in upgrading its watershed model to better capture the temporal changes in reservoir function (Linker, Batuik, et al., 2013; Shenk & Linker, 2013), which will be used to adjust the goals of nutrient and sediment reductions by each jurisdiction.

2.6. IMPLICATIONS AND RECOMMENDATIONS

      Anthropogenic riverine inputs of N, P, and sediment have led to undesirable consequences in the coastal marine environment, including eutrophication and associated oxygen depletion, declines in water transparency, loss of submerged aquatic vegetation, and shifts in the composition of plankton communities (Boesch et al., 2001; Breitburg et al., 2018; Cloern, 2001; Degobbis, 1989; Diaz & Rosenberg, 2008; Giani et al., 2012; Kemp et al., 2005). Therefore, reduction of watershed inputs has been a management priority for many coastal marine systems, including CB and the NAS. A review of parallel time‐series data on hypoxia and watershed loading rates in coastal ecosystems shows that oxygen conditions tend to improve rapidly and linearly when the primary driver targeted for control is nutrients from wastewater treatment plants (Kemp et al., 2009). In larger more open systems, where nonpoint nutrient loads are more important in fueling eutrophication, responses to remediation tend to be nonlinear with hysteresis and time‐lags. Nonetheless, there have been some signs of ecosystem recovery. For CB, water quality improved with time during 1985–2016, which is statistically linked to the reduction of riverine inputs of TN (Zhang et al., 2018). For the NAS, the reduction of riverine loads of P has been an effective method to alleviate eutrophication, even with high inputs of N and silicates (Djakovac et al., 2012; Giani et al., 2012). However, ecosystem conditions in this posteutrophic phase are still not comparable to those in pristine environments due to the occurrence of hypoxia and degraded benthic habitats in shallow coastal zones (Alvisi & Cozzi, 2016; Stachowitsch, 2014). Thus, continued reduction of watershed loads is indispensable for both CB and the NAS.

      After decades of management efforts, the goals of CB and the NAS restoration have not yet been fulfilled (Volf et al., 2018; Zhang et al., 2018). Moving forward, we provide the following recommendations:

       continue monitoring river flows and water quality in the major tributaries to CB and the NAS;

       improve statistical approaches for quantifying riverine constituent loads and trends, including associated uncertainties;

       increase understanding of watershed factors that influence riverborne loads and trends (e.g., land use, hydrology, source controls) and their relative importance;

       develop consensus and solutions among stakeholders to address the major challenges that hinder the achievement of restoration goals in a timely fashion (e.g., legacy sources, climate change, and reservoir filling);

       increase understanding of the effects of land‐based inputs on downstream water quality and ecological responses (e.g., dissolved oxygen, water clarity, chlorophyll‐a);

       enhance public awareness of the impacts of anthropogenic nutrient loading, management goals and actions, progress toward achieving these goals, and major challenges.

      In a world with seemingly ubiquitous nutrient enrichment and water‐quality degradation, past and future advancement in our scientific understanding on these two coastal ecosystems can be valuable resources that may guide and facilitate the protection and restoration of estuarine and coastal ecosystems in other geographical locations.

ACKNOWLEDGMENTS

      The authors thank many institutions for making the river monitoring data available, including the US Geological Survey, the Chesapeake Bay Program, the Environmental Agency of the Republic of Slovenia, the Croatian Meteorological and Hydrological Service, the European Environmental Agency, the Autorità di Bacino Distrettuale del fiume Po, the Regional Environmental Protection Agencies of Emilia Romagna, Veneto and Friuli Venezia Giulia for the NAS rivers. The editors and anonymous reviewers are greatly appreciated for their comments on an earlier draft of this chapter. This is contribution CN5869 of the University of Maryland Center for Environmental Science.

REFERENCES

      1 Alcamo, J., Flörke, M., & Märker, M. (2007). Future long‐term changes in global water resources driven by socio‐economic and climatic changes. Hydrological Sciences Journal, 52(2), 247–275. https://doi:10.1623/hysj.52.2.247

      2 Alvisi, F., & Cozzi, S. (2016). Seasonal dynamics and long‐term trend of hypoxia in the coastal zone of Emilia Romagna (NW Adriatic Sea, Italy). Science of the Total Environment, 541, 1448–1462. https://doi:10.1016/j.scitotenv.2015.10.011

      3 Ator, S.W., Brakebill, J.W., & Blomquist, J.D. (2011). Sources, fate, and transport of nitrogen and phosphorus in the Chesapeake Bay watershed: An empirical model (Scientific Investigations Report 2011‐5167, 27 pp.). Reston, VA: US Geological Survey.

      4 Bachman, L.J., Lindsey, B., Brakebill, J., & Powars, D.S. (1998). Ground‐water discharge and base‐flow nitrate loads of nontidal streams, and their relation to a hydrogeomorphic classification of the Chesapeake Bay Watershed, middle Atlantic coast (Water‐Resources Investigations Report 98‐4059, 71 pp.). Baltimore, MD: US Geological Survey.

      5 Basu, N.B., Destouni, G., Jawitz, J.W., Thompson, S.E., Loukinova, N.V., Darracq, A., et al. (2010). Nutrient loads exported from managed catchments reveal emergent biogeochemical stationarity. Geophysical Research Letters, 37(23), L23404. https://doi:10.1029/2010gl045168

      6 Bloschl, G., Hall, J., Parajka, J., Perdigao, R.A.P., Merz, B., Arheimer, B., et al. (2017). Changing climate shifts timing of European floods. Science, 357(6351), 588–590. https://doi:10.1126/science.aan2506

      7 Boesch, D.F., Brinsfield, R.B., & Magnien, R.E. (2001). Chesapeake Bay eutrophication: Scientific understanding, ecosystem restoration, and challenges for agriculture. Journal of Environmental Quality, 30(2), 303–320. Скачать книгу