The salinity content of water or soil has an important influence on plant and animal distribution. Some of the best examples of this control can be seen in coastal environments. The plant species that dominate in coastal habitats are typically halophytic, that is they are adapted to high salinity environments. There is a question here as to whether or not they are "obligate halophytes," in other words are they necessarily limited to high salinity environments or could they extend their ranges inland if they were given the chance? We will consider this question later.
Cox and Moore (1993, pp 54-55) use Gammarus, a crustacean, to show how the salinity gradient in an estuary can determine the distribution of species. They point out how osmosis, the process whereby water moves from a dilute salt concentration to a higher salt concentration can cause physical problems for animals as they across to wide a range of salinities. For example, salt water fish become bloated if they swim too far up an estuary; similarly freash water fish will become dehydrated if they swim too far out to sea. In the case of Gammarus different species have apparently adapted to different sections of the salinity gradient (see Figure 3.20). Just how this speciation occurred is not easy to visualize.
Perhaps the different "species" are simply ecotypes?
Estuaries are good places in which to study salinity effects because the salinity gradient is not as abrupt as along the open coast. In lecture the local example of the San Francisco Estuary was discussed as an example of how the salinity gradient influences the distribution of plants in the intertidal zone.
|The San Francisco Estuary||
The San Francisco Estuary is the largest estuary on the West Coast extending some ? km inland from the Golden Gate to the Sacramento- San Joaquin Delta. Sometimes the Estuary is referred to as "San Francisco Bay" but this is inappropriate because San Francisco Bay is only part of the Estuary; also included in the estuary are San PabloBay, Suisun Bay, and the Delta.
The salinity gradient in the Estuary is largely a response to two key controls: the inflow of saline water from the Pacific and the outflow of freshwater from the river systems that drain into the Bay. We should note that the average tidal flow (ebb or flood) through the Golden Gate is 2,300,000 cubic feet per second whereas the freshwater outflow through the Delta is 32,000 cfs (cubic feet per second) in winter and only 6,000 cfs in the summer. The dramatic imbalance is the basic reason why saltwater enters the Estuary. For purposes of comparison it is interesting to note that the average discharge of the Mississippi River is just less than 1,000,000 cfs.
The way in which saltwater enters the Estuary is largely determined by tides. The Estuary has a mixed tidal cycle with two high tides and two low tides every day. The tidal range, however, is not very large. The vertical distance between Mean Lower Low Water and Mean Higher High Water in the southern part of San Francisco Bay is about 3 meters; in Suisun Bay it is only about 2 meters. The lower range in the northern part of the Estuary means that the tidal marshes there are more restricted in their vertical extent.
The early ecologists who worked on the tidal marshes of the Estuary were strongly influenced by the concept of the "vegetation type." Typically they described marsh vegetation in terms of zones, such as the Spartina Zone or the Salicornia Zone. These zones were assumed to have sharp elevation limits and were thought to be controlled by the degreee of tidal submergence. More recently ecologists working in the marshes have tended to drop the idea of zones and have focussed more on the autecology of individual species.
Brian Atwater, then a scientist at the US Geological Survey pioneered much of the research on the basic distribution of tidal marsh plants. His graphic shows clearly how the distribution of the dominant taxa is closely tied to the salinity gradient. Note that the average salinity at the Golden Gate is 35 ppt (parts per thousand) whereas in the Delta it is 0 ppt.We should also note that the vertical range occupied by different genera is also largely controlled by salinity. Paradoxically, salinity levels are actually highest in the upper part of the inter-tidal zone because longer periods between tidal submergence leads to evaporation and concentration of dissolved salts. In the saltier areas of the Estuary salt ponds form naturally on the higher marsh surface and provide important feeding grounds for several species of birds.
The Dominant Marsh Plants
Brian Atwater's graphic shows how 4 genera dominate the inter-tidal marshes of the Estuary. Note how each genus, in most cases only one species is involved, occupies a different part of the salinity gradient.
Cord grass (Spartina foliosa ) is the dominant in the lower part of the marshes around San Francisco Bay and San Pablo Bay. This species is able to survive being largely submerged at high tide because its stems have specialized air-conducting cells that carry oxygen down to its roots. Like many salt marsh species it can reproduce asexually; broken off fragments of root are capable of getting established independently of the parent plant if the appropriate substrate is available. Reproduction by seed also occurs, especially when salinity levels are reduced by increased freshwater flow.
Spartina foliosa does not cover large areas in the San Francisco Estuary; it is largely restricted to a narrow fringe along the edge of the marshes and along tidal channels. In this respect it is very different from its East Coast relative Spartina alterniflora which dominates extensive areas of marshland as for example behind the Barrier Islands of Georgia. The difference here probably reflects climatic differences betwen the East and West Coasts of North America.
The dominant plant speces in the higher area of the San Francisco and San Pablo Bay marshes is Pickleweed (Salicornia virginica ). In this case it is the same species on the West Coast as on the East Coast. Pickleweed is a low growing perennial with succulent, jointed stems and much reduced leaves. Salicornia can withstand higher salinities that Spartina. For example, it has been reported as as growing in a diked area of Suisun Bay where the sediment salinity was measured at 52 ppt. This was in July - winter salinities are, of course, much lower.
Pickleweed, like Spartina , can reproduce both asexually and by seed. In the high marsh areas of San Francisco and San Pablo Bays Salicornia is by far the most dominant species. It forms extensive monospecific stands. Salicornia is also of interest in that it grows actively during the spring and summer and becomes dormant in winter. During its growth cycle the plant is dark green in colour; during the winter it turns red as chlorophyll is lost from the leaves and replaced by anthocyanin (red pigment). In other words, Pickleweed's growth cycle is out of phase with California's summer-dry climate.
In the brackish areas of the Estuary, for example: Suisun Bay, the floristic compostion of the tidal marshes changes. Tules (Scirpus spp.) become dominant in the lower part of the marshes and Saltgrass (Distichlis spicata) and several other species become more common on the higher marsh.
Several different species of Scirpus are present in these marshes. Scirpus californicus takes over from Spartina in the Carquinez Straits area. Apparently it can stand significant submergence because it grows down to below MLLW (Mean Lower Low Water) in the Delta. The Alkali Bulrush (Scirpus robustus) is common at slightly higher elevations especially so in Suisun Bay where it is actively encouraged as a food source for ducks. Large areas of Suisun Bay marsh are managed for duck hunting. According to one study Alkali Bulrush grows best in marshes where the seasonal salinity range is from 0 to 25 ppt (Pearcy et al., 1981). In Suisun Bay and the Delta, Olney's Bulrush (Scirpus olneyi also now referred to as Scirpus americanus ) now is often the dominant along the edges of tidal channels.
At MHHW in the Suisun Bay Marshes several different species are likely to be present; brackish marshes are floristically much more diverse than the saltmarshes. These species include: saltgrass (Distichlis spicata ), fat-hen (Atriplex patula ssp. hastata, gumplant (Grindelia humilis ) and the baltic rush (Juncus balticus ).
Most of the Delta is now converted to agricultural land, the marshes having been diked and reclaimed. Tules (Scirpus spp.) and Catttails (Typha spp.) were the dominants here. The process of reclamation has led to considerable subsidence because marsh peat is typically 75 to 80 percent water. In the western part of the Delta the pre-reclamation peat was typically over 20 feet thick and the land surface is now over 15 feet below sea level.
For current data regarding tides, winds, currents, and salinity in the Estuary go to the USGS Physical Oceanography Web Site. For a comprehensive web site that contains a great deal of information regarding the ecology and history of the Estuary check out the San Francisco Estuary Institute web page.
At the beginning of the lecture the question was raised as to whether or not halophytes are of necessity restricted to saline habitats and if not, What stops them expanding their ranges into freshwater habitats?
Experimental evidence has been produced that shows that many halophytes can grow in freshwater habitats. In fact, they often grow faster and reproduce more effectively in freshwater habitats than in salt. Professor Barbour at UC Davis has therefore concluded that there is no such "thing" as a obligate halophyte. He also suggests that competition not the physical environment is what prevents halophytes from expanding their ranges on the freshwater end of the salinity gradient.
The question here though is whether or not it is the change in the physical environment that gives the freshwater plants the advantage.
What do you think?