Algae as Bioindicators - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 121

3.3 Wetlands

wetlands is normally only 1-2 m in depth, it is not
stratified and the photic zone (light penetration)
extends to the sediments - promoting growth of
benthic and other attached algae.
Wetlands are typically dominated by free float-
ing and rooted macrophytes, which are the major
source of carbon fixation. Although growth of algae
may be limited due to light interception by macro-
phyte leaves, leaf and stem surfaces frequently pro-
vide a substratum for epiphytic algae - and extensive
growths of periphyton may occur. Wetlands tend to
be very fragile environments, liable to disturbance by
flooding, desiccation (human drainage), eutrophica-
tion (agriculture and waste disposal) and increased
salinity (coastal wetlands). Algal bioindicators of
water quality are particularly important in relation
to eutrophication and changes in salinity - such as
those occurring in Florida (USA) coastal wetlands.

Wetlands are habitats that contain shallow water and
can be divided into two main groups - marshes and
peatlands.

3.3.1 Marshes

These typically contain areas of open water that
is static or flowing and may be fresh, brackish or
saline (Boon and Pringle, 2009). Active decompo-
sition means there is no excessive accumulation of
plant material. Marsh wetlands form an ecological
continuum with shallow lakes (Sigee, 2004), having
standing water present for the entire annual cycle
(permanent wetlands) or just part of the annual cycle
(seasonal wetlands). Because the water column of

Case study 3.1 Salinity changes in Florida Wetlands

In recent decades, wetlands in Florida (USA) have been under particular threat due to extensive drainage, with
many of the interior marshlands lost to agricultural and urban development. This has resulted in a shrinkage of
wetland areas to the coastline periphery. In addition to their reduced area, coastal marshes in SE Florida have
also suffered a rapid rise in saltwater encroachment due partly to freshwater drainage, but also to rising sea levels
resulting from global warming.
Studies by Gaiser et al. (2005) have been carried out on an area of remnant coastal wetland to quantify algal
communities in three major wetland ecosystems - open freshwater marshland, forested freshwater marshland
and mangrove saltwater swamps. The study looked particularly at periphyton (present as an epiphyte and on soil
sediments) and the use of diatom bioindicator species to monitor changes in salinity within the wetland system.

Effectsofsalinity The major microbial community throughout the wetland area occurred as a cohesive peri-
phyton mat, composed of filaments of blue-green algae containing coccoid blue-greens and diatoms. Periphyton
biomass, determined as ash-free dry weight, was particularly high (317 g m − 2 ) in open freshwater marshes, falling
to values of 5-20 g m − 2 in mangrove saltwater swamps. Salinity had an over-riding effect on algal community
composition throughout the wetlands. The filamentous blue-greens Scytonema and Schizothrixwere most abun-
dant in freshwater, while Lyngbya and Microcoleus dominated saline areas. The most diverse algal component
within the periphyton mats were the diatoms, with individual species typically confined to either freshwater or
saline regions. Dominant species within the separate ecosystems are listed in Table 3.8, with freshwater diatoms
predictably having lower salinity optima (2.06-4.20 ppt) compared to saltwater species (11.79-18.38). Salinity
tolerance range is also important, and it is interesting to note that dominant diatoms in freshwater swamps had
higher salinity optima and tolerance ranges compared to other freshwater diatoms - suggesting that the ability
to tolerate limited saltwater contamination may be important. The converse is true for the saltwater swamps,
whe r e do mi nant spec i es were no t t hose wi t h the highest sa li nity op t i m a.