Large River Floodplain Ecology

Construction of levees along the Mississippi River and many of its tributaries has severed the river from over 90% of its floodplain, denying fish and other aquatic species access to millions of acres of foraging, spawning and nursery habitat. Miss. Dept. of Archives and History

Wicander and Monroe 1993 Sale Cypremont: 4,600 – 7,000 years ago Cocodrie: 3,500 – 4,600 years ago Teche: 2,800 – 3,500 years ago St. Bernard: 1,000 – 2,800 years ago Lafourche: 300 – 1,000 years ago Plaquemine: 500 – 750 years ago Balize: present – 500 years ago

Oxygen Level Controls Photosynthesis produces oxygen: Solar Energy + CO 2 + H 2 0  C 6 H 12 O 6 + O 2 Respiration consumes oxygen: C 6 H 12 O 6 + O 2  CO 2 + H chemical energy(ATP)

Estuaries Swamps and marshes Tropical rain forest Temperate forest Northern coniferous forest (taiga) Savanna Agricultural land Woodland and shrubland Temperate grassland Lakes and streams Continental shelf Open ocean Tundra (arctic and alpine) Desert scrub Extreme desert 8001,6002,4003,2004,0004,8005,6006,4007,2008,0008,8009,600 Average net primary productivity (kcal/m 2 /yr) Net Primary Production (measure of produced energy)

What is Hypoxia Dissolved Oxygen less than 2.0 mg/L Normoxic = DO > 2.0 mg/L Generally, most fish can not tolerate hypoxic conditions for long periods. –Gar, bowfin (choupique), bullheads can

Why Hypoxia? During low water times, the dry lands are extremely fertile and grow a lot of plants. When the spring floods come and temperatures rise, bacteria begin to decompose the vegetation on the floodplain floor. Bacterial respiration is what removes the oxygen (lack of flushing in backwater habitats contributes). Respiration rates exceed photosynthetic rates.

High Water vs. Low Water

When and Where Is Hypoxia? Generally found during high water times when temperatures are warm. Backwater areas (away from the mainstem river). –Low flow

Eventually the swamp drains and backwater areas become very productive.

Three General Types of Water Brown –High flow, lots of sediment, fairly high oxygen levels, riverine Green –Low flow, stratification, very high surface oxygen levels, highly productive, lacustrine Black –Low flow, very low surface oxygen levels, not productive, swamp

BackwaterInterior LakesMainstem December June August

How Do ‘Unproductive’ Areas Support Living Populations? Submerged Aquatic Vegetation –Oxygen Refuge –Productive microhabitats

Fish and Aquatic Vegetation Densities of young fish are often greater in aquatic vegetation than in adjacent open water

NormoxicHypoxic Mean Surface Dissolved Oxygen in Open Water and Plants at Each Site Green BrownBlack

Air-Water Interface Low DO Water Atmospheric oxygen diffuses into water Fish ‘pipe’ at the microsurface layer

How Do ‘Unproductive’ Areas Support Living Populations? Detritus-Based Production –Decomposers (e.g., bacteria) transfer energy stored in old organic matter to consumers Insects, crawfish –Low-oxygen tolerant organisms Gar, bowfin (choupique), bullheads

Energy flow through an aquatic ecosystem. From Cole 1988, Waveland Press

Detritus Based Food Web From Cole 1988, Waveland Press