Relatively short, fine-stemmed grasses dominate high marsh vegetation. Transitions between low marsh, high marsh and upper border may be gradual or quite abrupt and, although linked by tidal flooding, these three regions are characterized by distinctive assemblages of plants and animals and they are, in many ways, very different environments. Flooding tides also enhance plant growth since they deliver oxygen enriched water to marsh soils and remove potentially toxic materials as they exit. Tidal waters are also the vehicle carrying plant nutrients onto the marsh in periods of abundance, and transporting excess nutrients back to the estuary at other times of the year. Flooding frequency and duration, and marsh surface relief also strongly influence patterns of soil saturation and resulting oxygen availability, which in turn are major factors controlling the distribution of plant species within these wetlands.
Tides also link the marshes to estuarine waters and the offshore coastal zone environment.
They deliver saltwater from the marine environment, which mixes with and is diluted by freshwater from uplands, to produce salinity gradients which also act to characterize and help organize these systems. Finally, tides are critical to animals that live on the marsh as well as those in estuarine waters, both for nutrition and reproduction. Reproduction of a number of invertebrates is also dependent upon the spring tides which regularly flood the high marsh in two week cycles. Salinity in Long Island Sound varies seasonally and with proximity to major sources of fresh water such as the Connecticut, Housatonic and Thames Rivers, but is generally between 27 and 32 ppt. Sea water is carried into estuaries and embayments by tidal action, where it mixes with and is diluted by freshwater from rivers and streams.