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

Miss. Dept. of Archives and History

http://www.lmrcc.org/ARMP%20folio.pdf

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 + CO2 + H20 C6H12O6 + O2

Respiration consumes oxygen:C6H12O6 + O2 CO2 + H20 + 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

800 1,600 2,400 3,200 4,000 4,800 5,600 6,400 7,200 8,000 8,800 9,600

Average net primary productivity (kcal/m2/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

Backwater Interior Lakes Mainstem

December

June

August

6

7

8 9

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

0

2

4

6

8

10

12

14

16

GLN GLS INT FL BL SOC

Diss

olve

d O

xyge

n (m

g/L)

Open

Plant

Normoxic Hypoxic

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

Green

Brown Black

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