Tidal Wetlands Primer: An Introduction to Their Ecology, Natural History, Status, and Conservation

3. The Dynamic Intertidal Environment

46 The intertidal zone is a unique ­ environment where conditions fluctuate between aquatic habitat and semiterrestrial habitat as­ frequently as twice daily and for most tidal wetlands at least a few times a month. A variable pattern of often sinuous creeks typify the landscape of many tidal ­ wetlands and serve as conduits for the tides to bring water, sediments, and nutrients into and out of coastal marshes. Such creeks provide transient marine and estuarine nekton (fishes and aquatic invertebrates) access to the marsh interior as well. Tidal wetlands fluctuate between aquatic habitat when inundated and semiterrestrial habitat when exposed. As such they are excellent examples of a pulsating ecosystem (Odum et al. 1995; Odum 2000). The alternating flooding and exposure also produce variations in salinity, soil saturation, aeration, temperature , and use by fish and wildlife. These fluctuations create a harsh environment for plants and sedentary animals attempting to colonize these habitats. ­ Special adaptations are required for permanent residency.­ Salinity changes from one part of the­ estuary to another, within marshes, seasonally , and even daily in some places depending on rainfall and groundwater influences. The most upstream reaches in the tidal zone are strictly freshwater habitats. From a vegetation standpoint, these wetlands have more in common with nontidal wetlands than the salt or brackish wetlands downstream. Substrates differ among tidal wetlands and­ represent another significant component of the intertidal environment affecting plant and animal life. Coastal processes are constantly reshaping the intertidal zone. Episodic events, for example, hurricanes, can have devastating effects on tidal wetland vegetation but may actually help build marsh elevations to keep pace with rising sea level. Consequently, the intertidal environment is one shaped by tides and coastal processes and further influenced by salinity gradients, soil properties, and other factors. Superimposed on the intertidal environment are differences in climate including the frequency of wet (pluvial) periods and droughts that further add to the dynamics and rigors of the tidal wetland environment. The end result of all of these factors is a pulsating or dynamic ecosystem offering a wide range of intertidal habitats for plants and animals. Wetland Hydrology Flooding by tidal water is the unifying­ property or common denominator of all tidal wetlands. It is the driving force that creates, shapes, and maintains these habitats and makes them different from their nontidal counterparts—the inland wetlands. The intertidal zone is periodically flooded at regular or irregular intervals creating environmental conditions that appear to be in a constant state of flux (e.g., wetting, drying , and rewetting). Some marsh areas are­ continuously saturated or nearly so, while 3 The Dynamic Intertidal Environment The Dynamic Intertidal Environment   47 other places are naturally drained to some depth for varying periods. Water Budget Before addressing tides and their effect on the intertidal zone, an introduction to the water budget of tidal wetlands is worthwhile to demonstrate that other factors affect site wetness besides the tides. The “water budget” is an accounting of water inflows (gains or inputs) and outflows (losses or outputs). A simple model can show the dynamics of these flows (Figure 3.1). Site wetness at any particular time is related to its water budget. Inflows increase site wetness with water coming from four sources: 1) ­ precipitation (P)—rain, snow, sleet, hail, or fog; 2) surface water inflow (Si)—rivers, streams, and surface water runoff from the land; 3) groundwater inflow (Gi)—water discharged to the wetland surface (seepage); and 4) flood or rising tides (Ti). Outflows reflect water losses through four circumstances: 1) evapotranspiration (ET)—the combination of evaporation and plant uptake of water and loss through transpiration; 2) surface water outflow (So)—water draining to rivers or streams; 3) ground­ water outflow (Go)—water recharging underground aquifers; and 4) ebb tides (To)—­ falling tides that drain water off the surface and lower the water tables. The water budget is expressed by a simple formula that yields a net change in volume (∆V): ∆V = [P + Si + Gi + Ti] – [ET + So + Go + To] When the inputs are greater than the outputs, a positive value indicates water storage (saturated substrate and/or inundation ) at the site, whereas a negative value reflects a net loss of wetness favoring drier conditions (lower water tables). From this equation, one can see that the tides are just one factor affecting site wetness, although the dominant one for tidal wetlands as illustrated by the following example. A 34-day water budget for a California tidal wetland island in Suisun Bay revealed mean flow inputs of 0...