[Code of Federal Regulations]

[Title 15, Volume 3]

[Revised as of January 1, 2006]

From the U.S. Government Printing Office via GPO Access

[CITE: 15CFR921.82]



[Page 109-116]

 

                  TITLE 15--COMMERCE AND FOREIGN TRADE

 

CHAPTER IX--NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION, DEPARTMENT 

                               OF COMMERCE

 

PART 921_NATIONAL ESTUARINE RESEARCH RESERVE SYSTEM REGULATIONS--Table 

of Contents

 

            Subpart I_General Financial Assistance Provisions

 

Sec. 921.82  Amendments to financial assistance awards.



    Actions requiring an amendment to the financial assistance award, 

such as a request for additional Federal funds, revisions of the 

approved project budget or original scope of work, or extension of the 

performance period must be submitted to NOAA on Standard Form 424 and 

approved in writing.



       Appendix I to Part 921--Biogeographic Classification Scheme



                                 Acadian



    1. Northern of Maine (Eastport to the Sheepscot River.)

    2. Southern Gulf of Maine (Sheepscot River to Cape Cod.)



                                Virginian



    3. Southern New England (Cape Cod to Sandy Hook.)

    4. Middle Atlantic (Sandy Hook to Cape Hatteras.)

    5. Chesapeake Bay.



                               Carolinian



    6. North Carolinas (Cape Hatteras to Santee River.)

    7. South Atlantic (Santee River to St. John's River.)

    8. East Florida (St. John's River to Cape Canaveral.)



                               West Indian



    9. Caribbean (Cape Canaveral to Ft. Jefferson and south.)

    10. West Florida (Ft. Jefferson to Cedar Key.)



[[Page 110]]



                               Louisianian



    11. Panhandle Coast (Cedar Key to Mobile Bay.)

    12. Mississippi Delta (Mobile Bay to Galveston.)

    13. Western Gulf (Galveston to Mexican border.)



                               Californian



    14. Southern California (Mexican border to Point Conception.)

    15. Central California (Point Conception to Cape Mendocino.)

    16. San Francisco Bay.



                                Columbian



    17. Middle Pacific (Cape Mendocino to the Columbia River.)

    18. Washington Coast (Columbia River to Vancouver Island.)

    19. Puget Sound.



                               Great Lakes



    20. Lake Superior (including St. Mary's River.)

    21. Lakes Michigan and Huron (including Straits of Mackinac, St. 

Clair River, and Lake St. Clair.)

    22. Lake Erie (including Detroit River and Niagara Falls.)

    23. Lake Ontario (including St. Lawrence River.)



                                  Fjord



    24. Southern Alaska (Prince of Wales Island to Cook Inlet.)

    25. Aleutian Island (Cook Inlet Bristol Bay.)



                               Sub-Arctic



    26. Northern Alaska (Bristol Bay to Damarcation Point.)



                                 Insular



    27. Hawaiian Islands.

    28. Western Pacific Island.

    29. Eastern Pacific Island.



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[GRAPHIC] [TIFF OMITTED] TC12SE91.000



    Appendix II to Part 921--Typology of National Estuarine Research 

                                Reserves



    This typology system reflects significant differences in estuarine 

characteristics that are not necessarily related to regional location. 

The purpose of this type of classification is to maximize ecosystem 

variety in the selection of national estuarine reserves. Priority will 

be given to important ecosystem types as yet unrepresented in the 

reserve system. It should be noted that any one site may represent 

several ecosystem types or physical characteristics.



                        Class I--Ecosystem Types



                           Group I--Shorelands



    A. Maritime Forest-Woodland. That have developed under the influence 

of salt spray. It can be found on coastal uplands or recent features 

such as barrier islands and beaches, and may be divided into the 

following biomes:



    1. Northern coniferous forest biome: This is an area of 

predominantly evergreens such as the sitka spruce (Picea), grand fir 

(Abies), and white cedar (Thuja), with poor development of the shrub and 

herb leyera, but high annual productivity and pronounced seasonal 

periodicity.



    2. Moist temperate (Mesothermal) coniferous forest biome: Found 

along the west coast of North America from California to Alaska, this 

area is dominated by conifers, has relatively small seasonal range, high 

humidity with rainfall ranging from 30 to 150 inches, and a well-

developed understory of vegetation with an abundance of mosses and other 

moisture-tolerant plants.



    3. Temperate deciduous forest biome: This biome is characterized by 

abundant, evenly distributed rainfall, moderate temperatures which 

exhibit a distinct seasonal pattern,



[[Page 112]]



well-developed soil biota and herb and shrub layers, and numerous plants 

which produce pulpy fruits and nuts. A distinct subdivision of this 

biome is the pine edible forest of the southeastern coastal plain, in 

which only a small portion of the area is occupied by climax vegetation, 

although it has large areas covered by edaphic climax pines.



    4. Broad-leaved evergreen subtropical forest biome: The main 

characteristic of this biome is high moisture with less pronounced 

differences between winter and summer. Examples are the hammocks of 

Florida and the live oak forests of the Gulf and South Atlantic coasts. 

Floral dominants include pines, magnolias, bays, hollies, wild tamarine, 

strangler fig, gumbo limbo, and palms.



    B. Coast shrublands. This is a transitional area between the coastal 

grasslands and woodlands and is characterized by woody species with 

multiple stems and a few centimeters to several meters above the ground 

developing under the influence of salt spray and occasional sand burial. 

This includes thickets, scrub, scrub savanna, heathlands, and coastal 

chaparral. There is a great variety of shrubland vegetation exhibiting 

regional specificity:



    1. Northern areas: Characterized by Hudsonia, various erinaceous 

species, and thickets of Myricu, prunus, and Rosa.

    2. Southeast areas: Floral dominants include Myrica, Baccharis, and 

Iles.

    3. Western areas: Adenostoma, arcotyphylos, and eucalyptus are the 

dominant floral species.

    C. Coastal grasslands. This area, which possesses sand dunes and 

coastal flats, has low rainfall (10 to 30 inches per year) and large 

amounts of humus in the soil. Ecological succession is slow, resulting 

in the presence of a number of seral stages of community development. 

Dominant vegetation includes mid-grasses (5 to 8 feet tall), such as 

Spartina, and trees such as willow (Salix sp.), cherry (Prunus sp.), and 

cottonwood (Pupulus deltoides.) This area is divided into four regions 

with the following typical strand vegetation:

    1. Arctic/Boreal: Elymus;

    2. Northeast/West: Ammophla;

    3. Southeast Gulf: Uniola; and

    4. Mid-Atlantic/Gulf: Spartina patens.

    D. Coastal tundra. This ecosystem, which is found along the Arctic 

and Boreal coasts of North America, is characterized by low 

temperatures, a short growing season, and some permafrost, producing a 

low, treeless mat community made up of mosses, lichens, heath, shrubs, 

grasses, sedges, rushes, and herbaceous and dwarf woody plants. Common 

species include arctic/alpine plants such as Empetrum nigrum and Betula 

nana, the lichens Cetraria and Cladonia, and herbaceous plants such as 

Potentilla tridentata and Rubus chamaemorus. Common species on the 

coastal beach ridges of the high arctic desert include Bryas 

intergrifolia and Saxifrage oppositifolia. This area can be divided into 

two main subdivisions:

    1. Low tundra: Characterized by a thick, spongy mat of living and 

undecayed vegetation, often with water and dotted with ponds when not 

frozen; and

    2. High Tundra: A bare area except for a scanty growth of lichens 

and grasses, with underlaying ice wedges forming raised polygonal areas.

    E. Coastal cliffs. This ecosystem is an important nesting site for 

many sea and shore birds. It consists of communities of herbaceous, 

graminoid, or low woody plants (shrubs, heath, etc.) on the top or along 

rocky faces exposed to salt spray. There is a diversity of plant species 

including mosses, lichens, liverworts, and ``higher'' plant 

representatives.



                       Group II--Transition Areas



    A. Coastal marshes. These are wetland areas dominated by grasses 

(Poacea), sedges (Cyperaceae), rushes (Juncaceae), cattails (Typhaceae), 

and other graminoid species and is subject to periodic flooding by 

either salt or freshwater. This ecosystem may be subdivided into: (a) 

Tidal, which is periodically flooded by either salt or brackish water; 

(b) nontidal (freshwater); or (c) tidal freshwater. These are essential 

habitats for many important estuarine species of fish and invertebrates 

as well as shorebirds and waterfowl and serve important roles in shore 

stabilization, flood control, water purification, and nutrient transport 

and storage.

    B. Coastal swamps. These are wet lowland areas that support mosses 

and shrubs together with large trees such as cypress or gum.

    C. Coastal mangroves. This ecosystem experiences regular flooding on 

either a daily, monthly, or seasonal basis, has low wave action, and is 

dominated by a variety of salt-tolerant trees, such as the red mangrove 

(Rhizophora mangle), black mangrove (Avicennia Nitida), and the white 

mangrove (Laguncularia racemosa.) It is also an important habitat for 

large populations of fish, invertebrates, and birds. This type of 

ecosystem can be found from central Florida to extreme south Texas to 

the islands of the Western Pacific.

    D. Intertidal beaches. This ecosystem has a distinct biota of 

microscopic animals, bacteria, and unicellular algae along with 

macroscopic crustaceans, mollusks, and worms with a detritus-based 

nutrient cycle. This area also includes the driftline communities found 

at high tide levels on the beach. The dominant organisms in this 

ecosystem include crustaceans such as the mole crab (Emerita), amphipods 

(Gammeridae), ghost crabs (Ocypode), and bivalve mollusks such



[[Page 113]]



as the coquina (Donax) and surf clams (Spisula and Mactra.)

    E. Intertidal mud and sand flats. These areas are composed of 

unconsolidated, high organic content sediments that function as a short-

term storage area for nutrients and organic carbons. Macrophytes are 

nearly absent in this ecosystem, although it may be heavily colonized by 

benthic diatoms, dinoflaggellates, filamintous blue-green and green 

algae, and chaemosynthetic purple sulfur bacteria. This system may 

support a considerable population of gastropods, bivalves, and 

polychaetes, and may serve as a feeding area for a variety of fish and 

wading birds. In sand, the dominant fauna include the wedge shell Donax, 

the scallop Pecten, tellin shells Tellina, the heart urchin 

Echinocardium, the lug worm Arenicola, sand dollar Dendraster, and the 

sea pansy Renilla. In mud, faunal dominants adapted to low oxygen levels 

include the terebellid Amphitrite, the boring clam Playdon, the deep sea 

scallop Placopecten, the Quahog Mercenaria, the echiurid worm Urechis, 

the mud snail Nassarius, and the sea cucumber Thyone.

    F. Intertidal algal beds. These are hard substrates along the marine 

edge that are dominated by macroscopic algae, usually thalloid, but also 

filamentous or unicellular in growth form. This also includes the rocky 

coast tidepools that fall within the intertidal zone. Dominant fauna of 

these areas are barnacles, mussels, periwinkles, anemones, and chitons. 

Three regions are apparent:

    1. Northern latitude rocky shores: It is in this region that the 

community structure is best developed. The dominant algal species 

include Chondrus at the low tide level, Fucus and Ascophylium at the 

mid-tidal level, and Laminaria and other kelplike algae just beyond the 

intertidal, although they can be exposed at extremely low tides or found 

in very deep tidepools.

    2. Southern latitudes: The communities in this region are reduced in 

comparison to those of the northern latitudes and possesses algae 

consisting mostly of single-celled or filamentour green, blue-green, and 

red algae, and small thalloid brown algae.

    3. Tropical and subtropical latitudes: The intertidal in this region 

is very reduced and contains numerous calcareous algae such as 

Porolithon and Lithothamnion, as well and green algae with calcareous 

particles such as Halimeda, and numerous other green, red, and brown 

algae.



                      Group III--Submerged Bottoms



    A. Subtidal hardbottoms. This system is characterized by a 

consolidated layer of solid rock or large pieces of rock (neither of 

biotic origin) and is found in association with geomorphological 

features such as submarine canyons and fjords and is usually covered 

with assemblages of sponges, sea fans, bivalves, hard corals, tunicates, 

and other attached organisms. A significant feature of estuaries in many 

parts of the world is the oyster reef, a type of subtidal hardbottom. 

Composed of assemblages of organisms (usually bivalves), it is usually 

found near an estuary's mouth in a zone of moderate wave action, salt 

content, and turbidity. If light levels are sufficient, a covering of 

microscopic and attached macroscopic algae, such as keep, may also be 

found.

    B. Subtidal softbottoms. Major characteristics of this ecosystem are 

an unconsolidated layer of fine particles of silt, sand, clay, and 

gravel, high hydrogen sulfide levels, and anaerobic conditions often 

existing below the surface. Macrophytes are either sparse or absent, 

although a layer of benthic microalgae may be present if light levels 

are sufficient. The faunal community is dominated by a diverse 

population of deposit feeders including polychaetes, bivalves, and 

burrowing crustaceans.

    C. Subtidal plants. This system is found in relatively shallow water 

(less than 8 to 10 meters) below mean low tide. It is an area of 

extremely high primary production that provides food and refuge for a 

diversity of faunal groups, especially juvenile and adult fish, and in 

some regions, manatees and sea turtles. Along the North Atlantic and 

Pacific coasts, the seagrass Zostera marina predominates. In the South 

Atlantic and Gulf coast areas, Thalassia and Diplanthera predominate. 

The grasses in both areas support a number of epiphytic organisms.



                   Class II--Physical Characteristics



                            Group I--Geologic



    A. Basin type. Coastal water basins occur in a variety of shapes, 

sizes, depths, and appearances. The eight basic types discussed below 

will cover most of the cases:

    1. Exposed coast: Solid rock formations or heavy sand deposits 

characterize exposed ocean shore fronts, which are subject to the full 

force of ocean storms. The sand beaches are very resilient, although the 

dunes lying just behind the beaches are fragile and easily damaged. The 

dunes serve as a sand storage area making them chief stabilizers of the 

ocean shorefront.

    2. Sheltered coast: Sand or coral barriers, built up by natural 

forces, provide sheltered areas inside a bar or reef where the ecosystem 

takes on many characteristics of confined waters-abundant marine 

grasses, shellfish, and juvenile fish. Water movement is reduced, with 

the consequent effects pollution being more severe in this area than in 

exposed coastal areas.

    3. Bay: Bays are larger confined bodies of water that are open to 

the sea and receive strong tidal flow. When stratification is pronounced 

the flushing action is augmented by



[[Page 114]]



river discharge. Bays vary in size and in type of shorefront.

    4. Embayment: A confined coastal water body with narrow, restricted 

inlets and with a significant freshwater inflow can be classified as an 

embayment. These areas have more restricted inlets than bays, are 

usually smaller and shallower, have low tidal action, and are subject to 

sedimentation.

    5. Tidal river: The lower reach of a coastal river is referred to as 

a tidal river. The coastal water segment extends from the sea or estuary 

into which the river discharges to a point as far upstream as there is 

significant salt content in the water, forming a salt front. A 

combination of tidal action and freshwater outflow makes tidal rivers 

well-flushed. The tidal river basin may be a simple channel or a complex 

of tributaries, small associated embayments, marshfronts, tidal flats, 

and a variety of others.

    6. Lagoon: Lagoons are confined coastal bodies of water with 

restricted inlets to the sea and without significant freshwater inflow. 

Water circulation is limited, resulting in a poorly flushed, relatively 

stagnant body of water. Sedimentation is rapid with a great potential 

for basin shoaling. Shores are often gently sloping and marshy.

    7. Perched coastal wetlands: Unique to Pacific islands, this wetland 

type found above sea level in volcanic crater remnants forms as a result 

of poor drainage characteristics of the crater rather than from 

sedimentation. Floral assemblages exhibit distinct zonation while the 

faunal constituents may include freshwater, brackish, and/or marine 

species. Example: Aunu's Island, American Samoa.

    8. Anchialine systems: These small coastal exposures of brackish 

water form in lava depressions or elevated fossil reefs have only a 

subsurface connection in the ocean, but show tidal fluctuations. 

Differing from true estuaries in having no surface continuity with 

streams or ocean, this system is characterized by a distinct biotic 

community dominated by benthis algae such as Rhizoclonium, the mineral 

encrusting Schiuzothrix, and the vascular plant Ruppia maritima. 

Characteristic fauna which exhibit a high degree of endemicity, include 

the mollusks Theosoxus neglectus and Tcariosus. Although found 

throughout the world, the high islands of the Pacific are the only areas 

within the U.S. where this system can be found.

    B. Basin structure. Estuary basins may result from the drowning of a 

river valley (coastal plains estuary), the drowning of a glacial valley 

(fjord), the occurrence of an offshore barrier (bar-bounded estuary), 

some tectonic process (tectonic estuary), or volcanic activity (volcanic 

estuary).

    1. Coastal plains estuary: Where a drowned valley consists mainly of 

a single channel, the form of the basin is fairly regular forming a 

simple coastal plains estuary. When a channel is flooded with numerous 

tributaries an irregular estuary results. Many estuaries of the eastern 

United States are of this type.

    2. Fjord: Estuaries that form in elongated steep headlands that 

alternate with deep U-shaped valleys resulting from glacial scouring are 

called fjords. They generally possess rocky floors or very thin veneers 

of sediment, with deposition generally being restricted to the head 

where the main river enters. Compared to total fjord volume river 

discharge is small. But many fjords have restricted tidal ranges at 

their mouths due to sills, or upreaching sections of the bottom which 

limit free movement of water, often making river flow large with respect 

to the tidal prism. The deepest portions are in the upstream reaches, 

where maximum depths can range from 800m to 1200m while sill depths 

usually range from 40m to 150m.

    3. Bar-bounded estuary: These result from the development of an 

offshore barrier such as a beach strand, a line of barrier islands, reef 

formations a line of moraine debris, or the subsiding remnants of a 

deltaic lobe. The basin is often partially exposed at low tide and is 

enclosed by a chain of offshore bars of barrier islands broken at 

intervals by inlets. These bars may be either deposited offshore or may 

be coastal dunes that have become isolated by recent seal level rises.

    4. Tectonic estuary: These are coastal indentures that have formed 

through tectonic processes such as slippage along a fault line (San 

Francisco Bay), folding or movement of the earth's bedrock often with a 

large inflow of freshwater.

    5. Volcanic estuary: These coastal bodies of open water, a result of 

volcanic processes are depressions or craters that have direct and/or 

subsurface connections with the ocean and may or may not have surface 

continuity with streams. These formations are unique to island areas of 

volcanic orgin.

    C. Inlet type. Inlets in various forms are an integral part of the 

estuarine environment as they regulate to a certain extent, the velocity 

and magnitude of tidal exchange, the degree of mixing, and volume of 

discharge to the sea.

    1. Unrestricted: An estuary with a wide unrestricted inlet typically 

has slow currents, no significant turbulence, and receives the full 

effect of ocean waves and local disturbances which serve to modify the 

shoreline. These estuaries are partially mixed, as the open mouth 

permits the incursion of marine waters to considerable distances 

upstream, depending on the tidal amplitude and stream gradient.

    2. Restricted: Restrictions of estuaries can exist in many forms: 

Bars, barrier islands, spits, sills, and more. Restricted inlets result 

in decreased circulation, more pronounced longitudinal and vertical 

salinity gradients, and more rapid sedimentation. However, if



[[Page 115]]



the estuary mouth is restricted by depositional features or land 

closures, the incoming tide may be held back until it suddenly breaks 

forth into the basin as a tidal wave, or bore. Such currents exert 

profound effects on the nature of the subtrate, turbidity, and biota of 

the estuary.

    3. Permanent: Permanent inlets are usually opposite the mouths of 

major rivers and permit river water to flow into the sea.

    4. Temporary (Intermittent): Temporary inlets are formed by storms 

and frequently shift position, depending on tidal flow, the depth of the 

sea, and sound waters, the frequency of storms, and the amount of 

littoral transport.

    D. Bottom composition. The bottom composition of estuaries attests 

to the vigorous, rapid, and complex sedimentation processes 

characteristic of most coastal regions with low relief. Sediments are 

derived through the hydrologic processes of erosion, transport, and 

deposition carried on by the sea and the stream.

    1. Sand: Near estuary mouths, where the predominating forces of the 

sea build spits or other depositional features, the shore and substrates 

of the estuary are sandy. The bottom sediments in this area are usually 

coarse, with a graduation toward finer particles in the head region and 

other zones of reduced flow, fine silty sands are deposited. Sand 

deposition occurs only in wider or deeper regions where velocity is 

reduced.

    2. Mud: At the base level of a stream near its mouth, the bottom is 

typically composed of loose muds, silts, and organic detritus as a 

result of erosion and transport from the upper stream reaches and 

organic decomposition. Just inside the estuary entrance, the bottom 

contains considerable quantities of sand and mud, which support a rich 

fauna. Mud flats, commonly built up in estuarine basins, are composed of 

loose, coarse, and fine mud and sand, often dividing the original 

channel.

    3. Rock: Rocks usually occur in areas where the stream runs rapidly 

over a steep gradient with its coarse materials being derived from the 

higher elevations where the stream slope is greater. The larger 

fragments are usually found in shallow areas near the stream mouth.

    4. Oyster shell: Throughout a major portion of the world, the oyster 

reef is one of the most significant features of estuaries, usually being 

found near the mouth of the estuary in a zone of moderate wave action, 

salt content, and turbidity. It is often a major factor in modifying 

estuarine current systems and sedimentation, and may occur as an 

elongated island or peninsula oriented across the main current, or may 

develop parallel to the direction of the current.



                         Group II--Hydrographic



    A. Circulation. Circulation patterns are the result of combined 

influences of freshwater inflow, tidal action, wind and oceanic forces, 

and serve many functions: Nutrient transport, plankton dispersal, 

ecosystem flushing, salinity control, water mixing, and more.

    1. Stratified: This is typical of estuaries with a strong freshwater 

influx and is commonly found in bays formed from ``drowned'' river 

valleys, fjords, and other deep basins. There is a net movement of 

freshwater outward at the top layer and saltwater at the bottom layer, 

resulting in a net outward transport of surface organisms and net inward 

transport of bottom organisms.

    2. Non-stratified: Estuaries of this type are found where water 

movement is sluggish and flushing rate is low, although there may be 

sufficient circulation to provide the basis for a high carrying 

capacity. This is common to shallow embayments and bays lacking a good 

supply of freshwater from land drainage.

    3. Lagoonal: An estuary of this type is characterized by low rates 

of water movement resulting from a lack of significant freshwater influx 

and a lack of strong tidal exchange because of the typically narrow 

inlet connecting the lagoon to the sea. Circulation whose major driving 

force is wind, is the major limiting factor in biological productivity 

within lagoons.

    B. Tides. This is the most important ecological factor in an estuary 

as it affects water exchange and its vertical range determines the 

extent of tidal flats which may be exposed and submerged with each tidal 

cycle. Tidal action against the volume of river water discharged into an 

estuary results in a complex system whose properties vary according to 

estuary structure as well as the magnitude of river flow and tidal 

range. Tides are usually described in terms of the cycle and their 

relative heights. In the United States, tide height is reckoned on the 

basis of average low tide, which is referred to as datum. The tides, 

although complex, fall into three main categories:

    1. Diurnal: This refers to a daily change in water level that can be 

observed along the shoreline. There is one high tide and one low tide 

per day.

    2. Semidiurnal: This refers to a twice daily rise and fall in water 

that can be observed along the shoreline.

    3. Wind/Storm tides: This refers to fluctuations in water elevation 

to wind and storm events, where influence of lunar tides is less.

    C. Freshwater. According to nearly all the definitions advanced, it 

is inherent that all estuaries need freshwater, which is drained from 

the land and measurably dilutes seawater to create a brackish condition. 

Freshwater enters an estuary as runoff from the



[[Page 116]]



land either from a surface and/or subsurface source.

    1. Surface water: This is water flowing over the ground in the form 

of streams. Local variation in runoff is dependent upon the nature of 

the soil (porosity and solubility), degree of surface slope, 

vegetational type and development, local climatic conditions, and volume 

and intensity of precipitation.

    2. Subsurface water: This refers to the precipitation that has been 

absorbed by the soil and stored below the surface. The distribution of 

subsurface water depends on local climate, topography, and the porosity 

and permeability of the underlying soils and rocks. There are two main 

subtypes of surface water:

    a. Vadose water: This is water in the soil above the water table. 

Its volume with respect to the soil is subject to considerable 

fluctuation.

    b. Groundwater: This is water contained in the rocks below the water 

table, is usually of more uniform volume than vadose water, and 

generally follows the topographic relief of the land being high hills 

and sloping into valleys.



                           Group III--Chemical



    A. Salinity. This reflects a complex mixture of salts, the most 

abundant being sodium chloride, and is a very critical factor in the 

distribution and maintenance of many estuarine organisms. Based on 

salinity, there are two basic estuarine types and eight different 

salinity zones (expressed in parts per thousand-ppt.)

    1. Positive estuary: This is an estuary in which the freshwater 

influx is sufficient to maintain mixing, resulting in a pattern of 

increasing salinity toward the estuary mouth. It is characterized by low 

oxygen concentration in the deeper waters and considerable organic 

content in bottom sediments.

    2. Negative estuary: This is found in particularly arid regions, 

where estuary evaporation may exceed freshwater inflow, resulting in 

increased salinity in the upper part of the basin, especially if the 

estuary mouth is restricted so that tidal flow is inhibited. These are 

typically very salty (hyperhaline), moderately oxygenated at depth, and 

possess bottom sediments that are poor in organic content.

    3. Salinity zones (expressed in ppt):

    a. Hyperhaline--greater than 40 ppt.

    b. Euhaline--40 ppt to 30 ppt.

    c. Mixhaline--30 ppt to 0.5 ppt.

    (1) Mixoeuhaline--greater than 30 ppt but less than the adjacent 

euhaline sea.

    (2) Polyhaline--30 ppt to 18 ppt.

    (3) Mesohaline--18 ppt to 5 ppt.

    (4) Oligohaline--5 ppt to 0.5 ppt.

    d. Limnetic: Less than 0.5 ppt.

    B. pH Regime: This is indicative of the mineral richness of 

estuarine waters and falls into three main categories:

    1. Acid: Waters with a pH of less than 5.5.

    2. Circumneutral: A condition where the pH ranges from 5.5 to 7.4.

    3. Alkaline: Waters with a pH greater than 7.4.