Biodiversity in the Mangrove Intertidal and Subtidal: Algal and Seagrass Communities

D. Biodiversity in the Mangrove Intertidal and Subtidal: Algal and Seagrass Communities

By Christopher E. Tanner, Ilka C. Feller, and Aaron M. Ellison

Major Points of Chapter

1. Primary producers in mangrove communities and adjacent areas include microscopic algae, macroscopic algae, and seagrasses.

2. Algae are a phylogenetically diverse assemblage of photosynthetic organisms that are distinguished primarily by pigmentation and other biochemical and cellular characteristics.

3. Microscopic algae are found in the plankton (phytoplankton), on mangrove roots (periphyton) and other substrates (epibenthic), and within the tissues of corals (zooxanthellae), sea anemones, ascidians, sponges, and other animals (endobionts).

4. The distribution of macroscopic algae, commonly called seaweeds, is determined by substrate availability, light and nutrient levels, salinity, competition, and herbivory. Characteristic assemblages of seaweeds occur on mangrove roots (epiphytes), mangrove peats, and surrounding substrates.

5. Algae contribute a substantial portion of the total primary production of mangrove communities. Also, epiphytic and benthic algal communities provide habitat and food for a diverse assemblage of animals.

6. Seagrasses are marine flowering plants that are grass-like in appearance, although they are not closely related to true grasses. Five species of seagrass are common in the coastal waters of Belize.

7. Turtle grass forms extensive and very productive beds that provide food and shelter for marine invertebrates, fish, and manatees. Many species that are commercially important in Belizean fisheries use these beds as nursery grounds.

8. Turtle grass also serves as substrate for a diverse epibenthic community. Although species of mangroves are the most conspicuous photosynthetic organisms along

the coast of Belize and contribute substantial amounts of food energy to coastal waters, algae and seagrasses contribute even more primary production to the marine food web and have important ecological roles within mangrove and coral reef communities (Koltes et al. 1998). For example, Rodriguez & Stoner (1990) estimated that the total algal biomass for a mangrove ecosystem in Puerto Rico was similar to the total annual leaf litterfall from Rhizophora mangle into the surrounding waters. Since algae epiphytic on mangroves turns over its biomass 4-5 times per year, the coast of Belize and contribute substantial amounts of food energy to coastal waters, algae and seagrasses contribute even more primary production to the marine food web and have important ecological roles within mangrove and coral reef communities (Koltes et al. 1998). For example, Rodriguez & Stoner (1990) estimated that the total algal biomass for a mangrove ecosystem in Puerto Rico was similar to the total annual leaf litterfall from Rhizophora mangle into the surrounding waters. Since algae epiphytic on mangroves turns over its biomass 4-5 times per year,

Within mangrove stands, epibenthic microscopic algae can be found on prop roots, pneumatophores, peat banks, mudflats and on virtually every other substrate in the intertidal and subtidal. In sheltered areas, diatoms can produce macroscopic gelatinous mats with high rates of production (Littler et al. 1985). Certain species of benthic dinoflagellates produce toxins that can

be concentrated through the food chain and cause ciguatera in humans. While ciguatera is relatively rare in Belize, species (e.g., Prorocentrum lima) that are suspected of producing ciguatera toxin have been reported from Belizean mangrove communities (Faust 1991). Increases in ciguatera-causing algae in other areas of the world have been associated with natural and human disturbance of reef systems (Hallegraeff 1993).

Seaweed Communities

Different intertidal and subtidal substrates often support distinctive communities of macrophytes (Littler et al. 1985, Taylor et al. 1986). Typical substrates of the mangrove intertidal and shallow subtidal are prop roots, pneumatophores, peat banks, and mudflats. The bottom in subtidal channels and lagoons bordering mangrove stands is usually composed of deep, fine silt which grades toward coarser unconsolidated sediments associated with grassbeds and green algae with stolons and rhizoids.

In some mangrove stands the most abundant and characteristic intertidal mangrove community is D-1), and is frequently observed on prop roots and pneumatophores. In the Caribbean, other seaweeds

Shallow subtidal mangrove peat banks frequently have well-developed seaweed communities of fleshy algae (e.g., Caulerpa spp., Dictyota spp., Lobophora spp., Spyridia spp.) and/or calcified algae (e.g., Halimeda spp., Jania spp., Neogoniolithum spp.). Littler et al. (1985) found that species richness and biomass differed for wave-exposed and sheltered sites at Twin Cays, Belize, related to sea urchin herbivory and wave turbulence. In areas with abundant herbivorous sea urchins, fleshy algae such as Acanthophora spp., Spyridia spp., and Caulerpa spp. dominate on hanging roots of the red mangrove, whereas calcified algae such as Halimeda spp. and Lithophyllum spp. are more common on attached roots (Taylor et al. 1986). Apparently, the hanging root dominants are 6-20 times more susceptible to herbivory than the calcified algae and are removed from attached roots by urchins that move from the peats up the roots. In areas with no or low urchin densities, fleshy algae will out compete calcareous algae on both hanging and attached roots.

In shallow subtidal areas enriched by guano from islands with bird rookeries, entangled mats of the hair-like green alga Chaetomorpha linum can be found. These mats can reach diameters of a meter or more and are generally not attached to the substrate. Other indicators of nutrient-rich waters are the sheet-like green algae Entomorpha spp. and Ulva spp., which are found attached to shell and coral fragments or free-floating in sheltered bays. On most mud and sand bottoms seaweeds are relatively rare; the dominant species are green algae with well-developed basal systems (e.g., stolons, rhizoids) for anchoring in unconsolidated sediments.

Seagrass Communities

Marine algae are plants that lack true roots, vascular tissue (xylem and phloem), and flowers. The seagrasses, however, are angiosperms or flowering plants with true stems, leaves, and roots with vascular tissue. There are only a handful of species of marine vascular plants, whereas there are numerous species of marine algae, a pattern of diversity which is the reverse of that found on land, where angiosperms are the dominant plant group in terms of species diversity, abundance, and biomass production.

In Belize, as in the rest of the Caribbean, the dominant seagrass is Thalassia testudinum, or turtle grass. Manatee grass (Syringodium filiforme) often grows with turtle grass. These two species form extensive beds that are highly productive and serve as important habitat for numerous species of invertebrates and fish. Other species found in Belize include Halophila decipiens, Halophila engelmannii, and Halodule beaudettei. Tropical seagrasses usually grow on sandy sediments to which the plants are anchored by an extensive system of belowground rhizomes and roots. Several species of marine algae grow in the sediment among the turtle-grass beds in mangrove channels. The most common of these are species of the green algal genera Caulerpa, Halimeda, Rhipocephalus,

(approx. 350 g m -2 ). Although turtle grass plants in the mangrove channels grow much faster than

those in the back reef and lagoon, the productivity (g m -1 yr ) is similar in all three habitats because of the higher density of turtle grass in the lagoon and back reef.

-2

Sea urchins such as Lytechinus variegatus and herbivorous fish such as the bucktooth parrotfish (Sparisoma radians) graze on living and detrital turtle grass in shallow mangrove channels. The sea urchin population in mangrove channels is maintained by immigration of juveniles from the shallow waters surrounding the mangroves. Sea urchin density decreases with depth and distance from the mangroves; they are rarely found in the turtle-grass beds adjacent to patch reefs or shoals. In these other habitats, sea urchin abundance and distribution appears to be limited by reef-based predators. Manatees also graze on seagrasses. The path of their feeding can be tracked by following the uprooted turtle-grass fragments floating on the surface of mangrove channels.

Like subtidal mangrove prop roots, blades of T. testudinum serve as substrate for an incredibly diverse array of microscopic and macroscopic epibionts that give the leaves a fuzzy appearance (Fig. D-2). Common epiphytes include green and red filamentous algal genera (e.g., Cladophora, Ceramium, Polysyphonia, Champia) as well as calcareous algae. Epibiotic animals include the stinging anemone Bunodeopsis antillarium, the bivalve Spirorbis spp., numerous bryozoans, hydroids, sponges, ascidians, and setpulid worms.

Seagrass beds are important nursery grounds for juvenile shrimp and fishes of commercial importance, such as prawns, snapper, grouper, and barracuda. The juvenile fish are dependent on the plants, their epiphytes, or the benthos associated with the seagrass beds for food and refuge. Thalassia testudinum beds also act as a refuge habitat for seaweeds as grazing rates are considerably lower than in back reef and fore reef habitats (Lewis & Wainwright 1985). In mangrove channels, the prop roots provide additional shelter, and the high accumulation of organic detritus supports a rich benthic fauna Together, mangroves, microalgae, seaweeds and seagrass beds form the basis for some of the richest fisheries in the world.