> **Session No. 4**
>
> **Course Title: Coastal Hazards Management**
>
> **Session Title: Estuarine Ecosystems**
>
> **Author: Professor John J. Bruno, University of North Carolina at
> Chapel Hill**
>
> **Time: 1.5 hours**
>
> **\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_**

### Objectives:

> 4.1 Describe the physical conditions that define estuaries and
> challenge estuarine
>
> Organisms.
>
> 4.2 Describe some common estuarine communities.
>
> 4.3 Outline some of the ecosystem services provided by estuaries.
>
> 4.4 Summarize the anthropogenic hazards to estuarine communities.
>
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>
> **Scope:**
>
> This session introduces students to the ecology of estuarine
> ecosystems, several important estuarine communities, the services they
> provide, and the hazards they face.
>
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>
> **Readings**
>
> *Instructor and Student Reading:*
>
> Bertness, M.D. 1999. *The Ecology of Atlantic Shorelines*. Sunderland,
> MA: Sinauer.
>
> "Chapter 7: Salt Marsh Communities," pp. 313-375.
>
> *Additional Background Reading:*

Lenihan, H.S., and F. Micheli. 2001. "Soft-Sediment Communities." In
M.D. Bertness,

> M.E Hay, and S.D. Gaines, Eds. *Marine Community Ecology*, Sunderland,
> MA: Sinauer, pp. 253-287.

Williams, S.L. and K.L. Heck, Jr. 2001. "Sea Grass Community Ecology."
In M.D.

> Bertness, M.E. Hay, and S.D. Gaines, Eds. *Marine Community Ecology*.
> Sunderland, MA: Sinauer, pp. 317-337.
>
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#### **PowerPoint Slides**

> **PowerPoint** **4.1** North Carolina Salt Marshes
>
> **PowerPoint** **4.2** A New England Salt Marsh
>
> **PowerPoint** **4.3** New England Salt Marsh Zonation
>
> **PowerPoint** **4.4** A North Carolina Salt Marsh
>
> **PowerPoint** **4.5** Zonation of a Typical North Carolina Salt Marsh
>
> **PowerPoint** **4.6** Bare Patch Generation
>
> **PowerPoint** **4.7** Bare Patch Recolonization

##### **PowerPoint** **4.8** Salt Marsh Dredging

> **PowerPoint** **4.9** Fringing Salt Marshes
>
> **PowerPoint** **4.10** Sea Grass Beds
>
> **PowerPoint** **4.11** Estuarine Habitats
>
> **PowerPoint** **4.12** Oyster Reef
>
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>
> **General Requirements:**
>
> The Instructor should review some of the additional background
> reading.
>
> The content of this session should be presented as a lecture. A
> PowerPoint presentation that illustrates some of the key concepts and
> communities is included with this lecture.
>
> **\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_**
>
> **Objective 4.1 Describe the physical conditions that define estuaries
> and challenge estuarine organisms.**
>
> **Requirements:**

The content should be presented as lecture.

**Remarks:**

### Physical setting

> An **estuary** is a semi-enclosed body of water where river water from
> the **terrestrial watershed** mixes with ocean water. An estuary is
> different from a **lagoon,** which is semi-enclosed but receives
> little or no river water. Estuaries are often formed behind **barrier
> islands** (session 3) and in **drowned river valleys.** But they can
> also form in steep valleys created by the scouring of glaciers and
> along the boundaries of tectonic plates.
>
> **Horizontal gradients**
>
> A key feature of an estuary is a **gradient of physical conditions**
> from being purely oceanic near the mouth of the estuary, where
> salinity can be 35 ppt to the sections close to the freshwater input
> where salinity is very low (e.g., 2 ppt). Other physical factors
> including pH, oxygen saturation, temperature, and water clarity also
> vary along this gradient.
>
> ("ppt" denotes *parts per thousand*, which refers to the concentration
> of salts in the water)
>
> **Vertical gradients**
>
> Characteristic patterns of currents and the mixing of seawater with
> freshwater from the river(s) complicate this gradient by overlaying a
> **vertical zonation** of these physical factors onto the horizontal
> gradient.
>
> The freshwater from the river is less salty and lighter than the ocean
> water, so when it flows into the estuary, it floats on top of the
> seawater flowing in the opposite direction from the ocean. This
> creates a **halocline**, with the less salty water on the surface.
>
> Tidal flow of the seawater rushing into the estuary can create
> **turbulent mixing** which can break up this **stratification**.
>
> **Estuaries can be characterized based on physical gradients and the
> degree of mixing**
>
> The balance of water input between the river and the ocean is dictated
> by the size of the river (and the volume of water moving through it)
> and the shape of the estuary.
>
> Some estuaries are dominated by freshwater input from rivers. They are
> **highly stratified** with little vertical mixing between the ocean
> and river water and are called **salt wedge** **estuaries**.
>
> **Well-mixed estuaries** are dominated by the ocean and tidal mixing
> and are largely **unstratified**.
>
> These characteristics can change during the year and the rate of
> freshwater input changes, largely as a result of changes in rainfall
> and snowmelt in the **watershed** that feeds the estuary. Increase
> input from the river can also increase nutrient concentrations, which
> can in turn cause phytoplankton blooms in the estuarine water column.
>
> Phytoplankton populations are generally limited by the availability of
> nutrients such as nitrogen and phosphorus but can also be controlled
> by their predators; zooplankton and filter-feeders such as oysters.
>
> **Living in an estuary**
>
> Species diversity (the number of species living in an area) is
> generally lower in estuaries than in the river of ocean connected to
> the estuary. This is because few species have evolved the
> physiological adaptations necessary to handle such spatially and
> temporally variable temperature and salinity. Salinity at a given
> location can vary by as much as 20 ppt during a single day. When the
> tide is coming in, the water in the estuary can be dominated by the
> ocean, and oceanic conditions prevail. The opposite can be true after
> periods of heavy rainfall in the watershed and when the tide is low or
> going out.

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> **Objective 4.2 Describe some common estuarine communities.**

**Requirements:**

The content should be presented as lecture, supported by PowerPoint
slides.

\[**PowerPoint** **4.1** North Carolina Salt Marshes\]

> \[**PowerPoint** **4.2** A New England Salt Marsh\]
>
> \[**PowerPoint** **4.3** New England Salt Marsh Zonation\]
>
> \[**PowerPoint** **4.4** A North Carolina Salt Marsh\]
>
> \[**PowerPoint** **4.5** Zonation of a Typical North Carolina Salt
> Marsh\]
>
> \[**PowerPoint** **4.6** Bare Patch Generation\]
>
> \[**PowerPoint** **4.7** Bare Patch Recolonization\]

#####  \[**PowerPoint** **4.8** Salt Marsh Dredging\]

> \[**PowerPoint** **4.9** Fringing Salt Marshes\]
>
> \[**PowerPoint** **4.10** Sea grass Beds\]
>
> \[**PowerPoint** **4.11** Estuarine Habitats\]

\[**PowerPoint** **4.12** Oyster Reef\]

­­­­­­

> **Remarks**:

#### **Estuaries** are short-lived on a geological time scale because they are quickly filled with sediments and organic matter. 

#### 

#### But this input from rivers also makes estuaries highly productive ecosystems.

#### 

#### They are often shallow (1-10 meters in depth) and protected from large ocean waves so plant and animal biomass can accumulate and expansive **biogenically** created habitats dominate the landscape. 

### Salt marsh communities

### Salt marshes are always found in protected estuarine habitats. They are created by grasses, which stabilize the soft sediment substrate and provide habitat for other species including flowering plants, birds, worms, fish, crabs, and snails. 

### 

### Salt marshes are intertidal habitats like rocky shore communities and are characterized by a gradient of submergence time. Lower elevations of the salt marsh are usually submerged daily for several hours, while the upper border of the salt marsh is only submerged once or twice a month. 

### This creates a strong physical gradient of salinity, oxygen concentration and water logging of the soil.

### 

### From the perspective of the plants living in the marsh, this creates a stress gradient, extending from the relatively low stress high marsh to the highly physiologically stressful low marsh. 

### 

### Note this is the opposite stress gradient found in rocky intertidal habitats. This is because to the marine invertebrates and algae that live on rocky shore, being exposed to air is physiologically stressful. The opposite is true for terrestrial plants. 

> **Salt marsh zonation**
>
> Like many other intertidal habitats, salt marshes are characterized by
> **striking biological zonation**. This pattern is largely driven by
> life history and evolutionary trade offs. Few species can be good at
> everything. Salt marsh grasses are usually either good competitors or
> able to tolerate the physiologically stressful conditions of the low
> salt marsh.
>
> Grasses and flowering plants that are good stress tolerators like
> *Spartina alterniflora* dominate the **low marsh**.
>
> The middle and high marsh is occupied by good competitors like *Juncas
> geradii*, which out compete stress tolerators for freshwater and
> nutrients and exclude them to the lower salt marsh.
>
> Theoretically, *Juncas geradii* and other good competitors should
> dominate the middle and upper marsh, and they largely do. However,
> many inferior competitors also thrive in this zone. Their presence is
> facilitated by **physical disturbance** agents that remove the
> **dominant competitor** and create new **bare patches** of unoccupied
> habitat. Due to the absence of plant shading, salt builds up quickly
> in these bare patches, and only stress tolerators can initially
> colonize them. They then shade the soil surface, **modify the
> environment**, and facilitate the colonization of more competitive
> grasses like *Juncas geradii.* This whole sequence is called
> **facilitative succession**.
>
> Because they are so physiologically stressful, such **positive
> interactions** or **facilitations** are frequent and very important in
> salt marshes.
>
> For example, grasses facilitate the presence of birds and insects
> (which nest, feed, and live in the marsh) and even other plants, by
> shading the soil surface, thereby reducing evaporation and salinity.

### Sea grass beds

> Sea grass beds are shallow sub tidal habitats created by marine
> grasses. Sea grass habitats are found in estuaries and lagoons in both
> temperate and tropical systems throughout the world.
>
> Sea grasses **modify the environment** by reducing current velocity,
> which can in turn affect sedimentation rates and soil grain size.
> Grass beds also increase habitat complexity, which reduces predation
> intensity.
>
> All of these factors act synergistically to increase the number of
> individuals and species in the grass bed, compared to unvegitated
> bottom outside the bed.
>
> For example, fish, crabs, shrimp, and scallops are much more common
> inside sea grass beds. Some species, such as clams and scallops, also
> grow faster within a sea grass bed.

### Oyster reefs

> Oysters historically created vast intertidal and shallow subtidal
> reefs in estuaries. Oysters are filter-feeders and remove plankton
> from the water. This can affect water clarity, having cascading
> effects on other **foundation and associated species**.
>
> For example, sea grasses are intolerant of low light levels, so
> improving water clarity favors sea grass growth. Oysters also provide
> habitat for numerous sessile and mobile invertebrate animals that
> utilize the spaces among their shells as a **predation refuge**.

### Soft sediment habitats

> Not all of the substrate in estuaries is covered by biogenic habitats
> like marshes, sea grass beds, and oyster reefs. Much of the bottom is
> covered by soft sediments, which also house a diverse community of
> invertebrate animals.
>
> For instance, segmented worms are common in soft sediment habitats.
> They move through the bottom, mixing deeper, oxygen-starved sediments
> with surface sediments.
>
> This **bioturbation** is a critical process in soft sediment habitats.
>
> Bivalve clams are also common in this environment.

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> **Objective 4.3** **Outline some of the ecosystem services provided by
> estuaries.**

**Requirements:**

The content should be presented as lecture, supported by PowerPoint
slides.

**Remarks**:

### Water filtration 

### Salt marshes can draw nutrients out of terrestrial runoff and reduce the eutrophication of estuaries. Oysters filter plankton out of the water and increase water clarity. 

### Reduction of coastal erosion

> Salt marshes (and in some cases sea grass beds and oyster reefs) can
> dramatically reduce coastal erosion cased by storms and boat traffic.
>
> **Provision of nursery habitat for commercial species**
>
> Estuaries in general serve as critical nursery habitats for
> commercially important species that spend most of their adult lives in
> the open ocean.
>
> Many species targeted by commercial and recreational fishers (e.g.,
> shrimp, blue crabs, striped bass, red drum, etc.) live almost
> exclusively in estuaries.
>
> More specifically, most of these species (as well as countless
> non-game species) are dependent both on the estuary itself as well as
> the habitats within it; namely sea grass beds, oyster reefs, and salt
> marshes.

### Recreation and tourism 

> Estuaries provide valuable harbors that facilitate shipping and
> international trade.
>
> Many major cities are located on estuaries (e.g., Manhattan, San
> Francisco, Washington D.C.).
>
> Estuaries also provide for a wide variety of recreational
> opportunities including sailing, boating, fishing, swimming, and
> kayaking.
>
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>
> **Objective 4.4 Summarize the anthropogenic hazards to estuarine
> communities.**

**Requirements:**

The content should be presented as lecture, supported by PowerPoint
slides.

**Remarks**:

> **Over harvesting** during the last century has reduced the abundance
> of oysters in most North American estuaries by approximately 95%.
>
> Fisheries also have a variety of **indirect effects** on estuarine
> ecosystems. For example, the over fishing of oysters has indirectly
> reduced water clarity and sea grass abundance. Fisheries that use
> trawling or other destructive techniques (e.g., the scallop fishery)
> can indirectly damage or destroy sea grass beds. This collateral
> damage also reduces the productivity of the fishery by removing the
> habitats upon which the target species depends. Finally, **by catch**
> is a major problem in some commercial estuarine fisheries. The
> **bycatch:catch ratio** in the estuarine shrimp industry ranges from
> 5:1 to 10:1, one of the highest in commercial fishing. The by catch is
> mainly non-target fish.

### Coastal development

> Estuarine wetlands were once used extensive as waste disposal sites
> and for agriculture (e.g., in New England cows were grazed in salt
> marshes).
>
> They have also long been filled in to accommodate coastal development.
> Many major cities are built in part or completely on former wetlands
> (e.g., Manhattan and Boston). As a result in some locations, only a
> fraction of former estuarine wetlands remain. Coastal development
> continues to reduce salt marsh cover in most regions. During the last
> century most salt marshes were ditched to increase draining for
> ultimate development and in an attempt to control mosquito
> populations.

### 

> **Disease outbreaks** have also had substantial impacts on estuarine
> ecosystems by reducing the abundance of foundation species including
> sea grasses and oysters.

### Nutrient pollution

> Nutrient enrichment is one of the greatest threats to estuarine
> ecosystems. There are numerous point and diffuse sources that increase
> nitrogen and phosphorus concentrations in estuaries including
> agriculture, cleaning detergents, sewage release, and residential
> fertilizers.
>
> This often leads to **eutrophication**, which is an increase in
> phytoplankton or ephemeral algal abundance as a response to increased
> nutrient availability.
>
> Eutrophication usually leads to decreased water clarity, light
> penetration, and seagrass cover, and cascading effects on nearly every
> aspect of the ecosystem.
>
> A major indirect effect is the reduction on dissolved oxygen
> (**hypoxia**) cased by the decay of phytoplankton after the bloom is
> over. This in turn can cause fish kills and mass mortality events for
> invertebrates including oysters.

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