Unhealthy Chesapeake
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A science-based look at the Chesapeake's eutrophication problem
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Unhealthy Chesapeake
- 2. The Chesapeake is one of the largest estuaries on the planet 200 miles 2
- 3. The Chesapeake has an average depth of ~21 feet A narrow deep channel runs north/south down the Bay The Chesapeake’s deep channel corresponds to the original Susquehanna River ~15,000 years ago before the continental glaciers melted and sea level rose. This channel is now ~50-120 feet deep. ~100 feet ~4-20 miles West East Chesapeake profile looking northward (note difference in horizontal and vertical scales) 3
- 4. The Chesapeake watershed extends northward into New York state, and westward into West Virginia 4
- 5. Fresh water flow into the Chesapeake is variable— both seasonally and annually Mar-06 Mar-07 Mar-08 5
- 6. Nitrogen- and Phosphorous-based nutrients from the watershed flow into the Chesapeake where they drive phytoplankton growth that depletes the Bay’s oxygen 6
- 7. Because Sea Water Contains Dissolved Salt, it is ~3% Heavier than Fresh Water 1 liter fresh water weighs 997 grams 1 liter sea water weighs ~1023 grams At room temperature 7
- 8. Warm water is less dense than cold water 1 liter warm sea water (30 o C) weighs 1021 grams 1 liter cold sea water (5 o C) weighs 1027 grams 8
- 9. Fresh water flowing into an estuary creates a gradient with sea water Seaward flow of lighter, fresher water from the watershed Estuarine flow of denser, saltier water River Ocean 9
- 10. Pycnoclines are a strong barrier to vertical mixing 10
- 11. 11
- 12. Salinity Maps from Spring and Fall show how the Bay responds to variation in fresh water inflow Chesapeake Bay Program High river flows Low river flows Havre de Grace Norfolk 12
- 13. During the summer, a pycnocline forms in the central Chesapeake Pycnocline : A sharp change in density (due to salinity and temperature changes) The Chesapeake’s summer pycnocline is typically 20-40 feet below the surface, i.e., largely but not exclusively associated with the deep trench Deg C Temperature Salinity Havre de Grace Norfolk Chesapeake Bay Program 13
- 14. Phytoplankton can grow vigorously in estuaries where they comprise the base of the food chain Phytoplankton—free- floating microscopic organisms utilizing photosynthesis for energy Dinoflagellates Diatoms Cyanobacteria = Blue-green algae etc Zooplankton Fish Crabs Phytoplankton N- and P-based nutrients from watershed Oysters Jellyfish Small fish 14
- 15. Chlorophyll a monitoring shows how phytoplankton can grow and die quickly Chesapeake Bay Program Feb Mar Apr 15
- 16. Phytoplankton growth is usually limited by either nitrogen- and phosphorous-based nutrients +P +N+P +N +P +N+P Day 0 Day 4 +N 16
- 17. Chesapeake phytoplankton are limited by either phosphorous- or nitrogen-based nutrients 17
- 18. The critical nutrients for phytoplankton growth are nitrogen and phosphorous compounds Chesapeake Bay Program 18
- 19. Nutrients from watershed Zooplankton Oysters Jellyfish Small fish Fish Crabs Phytoplankton Zooplankton Fish Crabs Phytoplankton High nutrient loads foster the rapid growth of phytoplankton, but zooplankton cannot increase their numbers rapidly enough to expand the whole food pyramid Oysters Jellyfish Small fish 19
- 20. Nitrogen- and Phosphorous-based nutrients from the watershed flow into the Chesapeake where they drive phytoplankton growth that depletes the Bay’s oxygen 20
- 21. Multicellular organisms require oxygen Anoxia 0.0 – 0.2 mg/liter Hypoxia 0.2-2.0 mg/liter 21 (Anaerobic bacteria only)
- 22. 2005 Hypoxia/Anoxia Develops in the deep Chesapeake in Summer Annapolis Potomac Chesapeake Bay Program 22 March April May June July August Sept October Nov
- 23. 23 Levels of anoxia vary from year to year
- 24. 24 In the summer fish suffer from a bidirectional “habitat squeeze” Warm water Low DO U MD CES
- 25. Dissolved oxygen, mg/liter Depth, meters Dissolved oxygen depth profiles at the Bay Bridge 2005 Hypoxia Anoxia 25
- 26. MD DNR Chesapeake Benthic life is Decimated by Anoxia 26 Deep Channel Mainstem Edge Northern Bay
- 27. Blocking nutrient influx as the only way to stop low oxygen problems 27
- 28. 28
- 29. Farms are a major source of nutrients going into the Chesapeake 29
- 30. 30
- 31. DDT wiped out osprey populations, but banning it led to recovery 31
Notas do Editor
- Most of the Chesapeake is fairly shallow—its average depth is only 21 feet. However, there is a *deep channel corresponding to the former Susquehanna River bed, that runs down the length of the Bay, more-or-less in the middle. This diagram depicts a cross-section of the Chesapeake looking northward. It is important to emphasize that the diagram is not to scale—the horizontal dimension is actually much, much bigger than what is illustrated here relative to the 100 foot depth scale. In understanding this shape, it helps to think about how the Bay was formed. Geologically the Chesapeake is only ~10,000 years old. It was created at the end of the last glacial period—when sea level rose as a result of the melting continental glaciers. What had been the Susquehanna River flood plain then became permanently flooded and is now the broad shallow sections of the Bay to the east and west of the deep channel. The former route of flowing Susquehanna River then became the Bay’s deep channel. This kind of explanation also applies to the major Chesapeake tidal tributaries like the Potomac. The overall shallow water dominating the Chesapeake sets up its rich biological life, because the microscopic plants called phytoplankton form the base of the estuary’s food chain. Phytoplankton generate energy by photosynthesis, and in a shallow estuary they have better access to sunlight than in deeper estuaries.