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Estuaries. Tidal Potomac Teacher Training Workshop June 2008 (with thanks to Leila Hamdan). Outline. What is an estuary Key Features of Estuaries in general Examining our famous Estuary: Chesapeake Bay Nutrients and Pollutants Habitat Human Interaction. What is an Estuary?.

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Tidal Potomac Teacher Training Workshop

June 2008

(with thanks to Leila Hamdan)

  • What is an estuary
  • Key Features of Estuaries in general
  • Examining our famous Estuary: Chesapeake Bay
    • Nutrients and Pollutants
    • Habitat
    • Human Interaction
what is an estuary
What is an Estuary?
  • Webster's Dictonary
    • 1.) A water passage where the tide meets a river current
    • 2.) an arm of the sea at the lower end of a river
  • Oxford Dictonary
    • 1.) An arm of the sea
  • Classification based on the organisms present
classic definition of estuary
Classic Definition of Estuary

“ An estuary is a semi-enclosed coastal body of water having a free connection with the open sea and within which the sea-water is measurably diluted with fresh water deriving from land drainage”

Pritchard’s treatment of estuaries based upon the common physical characteristics shared by this type of ecosystem

Cameron, W.M. and D.W. Pritchard. 1963. Estuaries. InThe Sea: Ideas and Observations on Progress in the Study of the Seas. Volume 2. M.N. Hill (ed) 1963. Interscience Publishers, New York.

types of estuaries
Types of Estuaries
  • Fjords
  • Bar-built estuaries
  • Estuaries produced by tectonic processes
  • Drowned river valleys (Chesapeake)

Pritchard, D.W. 1967. What is an Estuary: Physical viewpoint. In Estuaries. Publication # 83. AAAS, Washington, D.C.

  • Interface between the land and the sea
  • Due to geomorphology, climate, and biodiversity, each estuary is unique.
  • Common names include Bays, Lagoons, or Sounds
  • Some well known Examples:
    • Chesapeake Bay, Delaware Bay, San Francisco Bay, Boston Harbor, Tampa Bay and Puget Sound.
basic concepts salts
Basic Concepts: Salts
  • Salinity
    • Salinity is the level of salts (mainly NaCl) dissolved in water. It is measured in parts per thousand (‰).
    • Salinity affects
      • Osmosis for organisms
      • Stratification (formation of pycnocline)
  • Salinity often varies longitudinally, horizontally, and vertically in estuaries.
  • This can be a challenging environment for biota.
review of terms
Reviewof Terms
  • Anoxia/Hypoxia
  • Eutrophication
  • Geomorphology
  • Residence Time
basin morphology




Western Shore

Eastern Shore









Basin Morphology
  • Drowned River Valley:
    • V shaped profile
    • Deep main channel- shallow flanks
      • Average depth: 6m
      • Channel depth: 15m
  • ca. 20,000 years ago the Susquehanna River carved a deep valley through PA, MD and VA across the dry continent. Joined by tributaries each with deep V shaped channel
  • ca,. 10,000 years ago, sea level rise filled in old river channels and beds.
  • Deepest portion of Chesapeake and all tributaries located just “upstream” of mouth.
  • Often a shallow sill where tributaries open to the main-stem and where Bay opens to ocean

Basin Morphology

  • 50 rivers drain into bay
  • 8 major tributaries account for majority of fresh water delivery:
  • Susquehanna 48%
  • Potomac 13%
  • James 12%
  • Rappahannock 3%
  • York 3%
  • Patuxent 1%
  • Choptank 1%
  • Nanticoke 1%
circulation patterns of the chesapeake
Circulation Patterns of the Chesapeake
  • Moderately stratified estuary
    • Fresh water flow towards the sea
    • Salt water intrusion towards land
    • Opposing currents of water nearly equal in volume and velocity
  • Distance of salt intrusion into bay depends upon season and volume of runoff

Salinity Distribution in the Chesapeake Bay During Spring

High fresh water

Runoff from snow melt into tidal rivers pushes higher salinity

water down Bay

Schubel, J.R. and D.W. Pritchard. 1987


Salinity Distribution in the Chesapeake Bay During Autumn Months

Fresh water run-off lessens during summer and autumn.

Higher salinity waters surge in from the Bay mouth and into upper reaches of the Bay and its sub-estuaries (rivers).

Schubel, J.R. and D.W. Pritchard. 1987.


Circulation Pattern of Chesapeake Bay

From: Schubel, J.R. and D.W. Pritchard. 1987

chesapeake bay
Chesapeake Bay
  • Moderately Stratified Estuary
    • Significant mixing of both salt and fresh water layers
    • Water flushes out of the estuary at equal velocity with water flushing in
    • Residence time of water in Bay is approximately 12months
      • What does this mean for flushing?
flushing of pollutants in the chesapeake bay
Flushing of Pollutants in the Chesapeake Bay
  • Pollutants run in, pollutants flush out?
    • Not quite in the Chesapeake
    • Circulation of the Chesapeake ensures that following the initial introduction of a pollutant, it has the potential to be dispersed at a much greater extent, and will not soon flush out
  • This finding needs to be considered in all management decisions concerning pollution of the Bay
net effect of urbanization of cb watershed
Net Effect of Urbanization of CB Watershed
  • Increased Nutrient Loading - Eutrophication
  • Increased Sediment Delivery
  • Toxic Chemicals
  • Habitat Loss

Sedimentation Sources

Sediment delivery to CB increased during urbanization of watershed (ca 1950s) and continues today

Shading due to sediment was key factor to early decline in SAV

Impervious surfaces (highways) increase sediment delivery

construction and in stream scouring
Construction and In-stream Scouring
  • Construction sites deliver substantial loadings of sediment, particularly when not using best management practices
  • In stream scouring increases as watersheds develop and there is increased impervious area.
toxic compounds in the chesapeake bay
Toxic compounds in the Chesapeake Bay
  • Examples of toxics (current or historical) in the Chesapeake Bay system
    • PAHs-polycyclic aromatic hydrocarbons
    • PCBs-polychlorinated biphenyls
    • Mercury and other metals
Four Regions of Concern-areas with known chemical contaminant-related impacts
    • James River
    • Elizabeth River
    • Anacostia River
    • Baltimore Harbor/Patapsco River

Habitat Loss

  • Decline of Submersed Aquatic Vegetation (SAV), also known as aquatic macrophytes
  • Since the 1960s, over 50% of the SAV has disappeared from the Bay waters.
  • Causes:
    • Declining water quality (nutrient loading, sedimentation)
    • Disturbance of SAV beds (Hurricane Agnes-1972)
    • Alteration of shallow water habitat (epiphyte growth)
  • Initial decline of SAV most severe in the upper Chesapeake and western shore tributaries.
  • Today, all areas of the Bay have experienced the decline of these important grasses.
sav in gunston cove
SAV in Gunston Cove
  • 2000 Distribution
sav in gunston cove29
SAV in Gunston Cove
  • 2005 Distribution
importance of sav
Importance of SAV
  • Provides habitat for benthic organisms (Blue Crab) and fish populations
  • Stabilizes benthic sediment
  • Filters water column (settling particles)
  • Oxygenates deep portions of water column
chesapeake bay important living resources
Chesapeake Bay Important Living Resources
  • Shellfish (Invertebrates)
    • American oyster
    • Blue crab
  • Fish
    • American shad
    • Striped bass
    • Menhaden
american oyster
American Oyster
  • The “trademark” species of the bay from the 1600’s to the 1960’s
  • A bivalve mollusc
  • Larva is planktonic and spends first few weeks of life drifting in the currents
  • When it reaches the size of the head of a pin, it settles to the bottom and attaches to a hard surface and is called “spat”
  • The spat grows larger and in the process extends the size of its shell
american oyster35
American Oyster
  • Oysters pump water through their shell and as the water passes over their gills, plankton (microscopic plants and animals) are filtered out
  • The plankton provides the food for the oyster
  • In the process the oyster clears the water and makes it easier for SAV (submersed aquatic vegetation) to grow
  • Population levels in the 1930’s were calculated to filter the entire volume of the Bay in 2-3 days.
american oyster36
American Oyster
  • Oyster shell also forms the major hard surface substrate in the bay
  • Oyster “reefs” were once so common that they caused ship wrecks
  • Oyster reefs provide essential habitat for new oysters and for other estuarine organisms
american oyster37
American Oyster

What Happened?

  • Over-harvesting: not only depleted the stock of live oysters, but also wrecked the reefs
  • Algal blooms and toxic chemicals
  • Siltation from the land
  • Parasites (protozoans)
blue crab
Blue Crab
  • The most important commercial species in the bay now
  • A crustacean, crabs have an exoskeleton or shell
  • As they grow, they must molt or shed their shell
  • It takes the new shell a few days to harden. In the interim they are “soft shell” clams
blue crab39
Blue Crab
  • Crabs spawn in the ocean just outside the Chesapeake Bay
  • The larvae and young crabs move up into the estuary and some even into the tidal rivers where they spend most of their life cycle
  • Mating occurs and the gravid (with eggs) females move back through the estuary and into the ocean
blue crab40
Blue Crab
  • How are they doing?
    • Harvests increased in the 1980’s as watermen shifted from oysters to crabs and demand for crab from consumers increased
    • However, by 2000 harvests had declined, not because effort or demand had declined, but because the resource was becoming more scarce
    • This could signal a problem for the blue crab
striped bass rockfish
Striped Bass (Rockfish)
  • Striped bass is the most sought-after fish in the Chesapeake Bay
  • They can grow to a length of nearly 5 ft and reach 100 lbs
  • They are valued by recreational fisherman for their size, fighting ability, and their taste
striped bass rockfish42
Striped Bass (Rockfish)
  • The striped bass is an semianadromous fish
  • That means that it spawns in tidal freshwater
  • This occurs in May and early June
  • The eggs are fertilized and hatch in open water where they are subject to predation even by oysters
  • The small larvae are unable to swim and are moved by currents
striped bass rockfish43
Striped Bass (Rockfish)
  • By later in the summer, they reach juvenile stage and can swim
  • At this time they move back down into the estuary and even into the open ocean
  • By age 3-4 they reach sexual maturity and can return to spawn
  • They can return multiple times
striped bass rockfish44
Striped Bass (Rockfish)
  • How are they doing?
    • Striped bass landings remained steady through mid-1970’s
    • But by mid-1980’s landings had decreased markedly
  • 1985 The Atlantic Striped Bass Conservation Act
    • requires that states implement a 55% harvest reduction
    • states respond by placing a moratorium on the taking of striped bass
  • Bass recover quickly
    • Limited fishing is resumed and by 1997 landings reach a new record
  • Lingering issues
    • Bass have started showing signs of a new disease (mycobacteriosis)
    • Also water quality continues to decline and food supplies (menhaden) are being heavily exploited by humans
  • Menhaden are small fish that serve as forage (food) for the larger game fish like striped bass (max 7.3 inches long)
  • Native americans called menhaden “munnawhateaug” which means “fertilizer”
  • These were probably this fish that were planted in the mounds used to grow corn
  • Today these fish are heavily exploited for fish meal, fish oil, and bait
  • Menhaden spawn from March to May and Sept to Oct in the bay proper
  • The larvae live as plankton in the open water and grow rapidly
  • During winter they migrate out of the bay and stay off North Carolina before returning to the bay in the early spring
  • Menhaden, like oysters, feed heavily on phytoplankton and zooplankton
  • Huge amounts of menhaden are harvested each year from the midAtlantic region
  • Harvests have declined from the 600,000 mt range in the late 1950’s to current levels below 200,000 mt
  • Menhaden probably benefited from the oyster decline, but fishing pressure has increased
  • Some have begun to question whether the decline in menhaden due to human fishing may be limiting the food for fish such as striped bass
lessons learned
Lessons Learned
  • Human resource utilization tends to focus on a single species until it is reduced beyond it’s sustainability
    • First oysters and shad, then striped bass, then crabs and now, maybe menhaden?
    • Some species can spring back quickly (striped bass), others may require much longer, more intervention or never? (oyster)
    • Exploitation of one species has an effect on others:
      • Oyster overfishing may have release menhaden
      • Oyster disappearance resulted in more exploitation of crabs and their overfishing
  • More and longer data are needed to fully understand human impacts vs. those of climatic and other environmental cycles and changes