Organisms living in an aquatic environment face specific stresses which, through evolutionary time, have given rise to ingenious structural and physiological adaptations. These stresses include life in an anaerobic (anoxic) environment, high salinity, concentration of toxic compounds, and extreme fluctuations in water level. For many plants and microorganisms the most severe stress is shortage or absence of oxygen in the soil. Without oxygen, plant and animal respiration occurs only through anaerobic metabolic pathways, slower and less efficient than aerobic pathways. Many bacteria, essential decomposers in wetland food webs, are obligatory or facultative anaerobes. They use metabolic pathways based on sulfur compounds—not oxygen—as the electron acceptors. Many plants have stems, leaves and roots which contain air spaces permitting transfer of oxygen from above water structures to submerged plant parts. The submerged basal leaves of Water Lobelia (
), a common plant of coastal pond shores, are hollow. Emergent flower-bearing stems pump oxygen down to these hollow leaves and to growing roots. Floating leaves of water lilies do the same.
Wetland plants have far higher percentages of pore space in their tissues than do terrestrial plants. Trees of bottom land hardwood forests, cypress swamps, and coastal mangrove swamps have adventitious roots, emergent “knees” and prop roots which facilitate the transfer of atmospheric oxygen to submerged trunks and roots. Life history strategies in plants and animals often are adapted to life in aquatic environments. Pond shore plants growing in moist peaty sand above the water’s edge, such as the carnivorous sundews (Drosera), lie dormant on pond shores in years of high water and germinate, grow quickly, flower and set seed when drought years expose expanses of shore.
Plants growing in salty coastal environments have the difficulty of surviving salt stress. Salt can be toxic to plant cells and tissue, or it can be disruptive to cellular osmotic balance. Salt marsh plants and plants of the coastal prairie in the Everglades are halophytic, or salt loving organisms. They have adaptations for removal of salt from tissues and salt separation from sensitive tissues. Vertebrate and invertebrate animals also have structural, physiological and behavioral modifications which allow them to thrive in wetland environments. Some of these are mentioned in the site descriptions below.