E.

The Biological Environment and Communities of the Gulf of Mexico

Because there is little available data concerning the

phytoplankton of the Texas shelf waters, the information must be inferred from the few studies carried out in the eastern Gulf. These include Balech (1967a,b), Curl (1959), El Sayed (1967), Kabanova (1966), Marshall (1956), Odum and Hoskin (1956), Riley (1937), Simmons and Thomas (1962), and Steele (1964).

It may be presumed from these studies that the marine phytoplankton reaches an annual maximum in the early spring months, followed by an annual low in late summer and early fall. Average annual primary productivity should fall around 20-30 milligrams of carbon per cubic meter per day.

The diatoms and dinoflagellates are the major source of primary production in the sea. The important genera of neritic phytoplankton are somewhat ubiquitous in neritic waters of the world's oceans, and the waters over the continental shelf of the northwestern Gulf contain

the usual populations of Skeletonema, Biddulphia, Chaetoceros, Rhizosolenia, Ceratium, Gonyaulax, and others.

Red Tides

Red tides are fairly common throughout the world and are associated with areas of heavy land runoff or upwelling. In the Gulf, these blooms or increases in population refer to discolored patches of seawater usually accompanied by fish kills. The phytoplanktonic

organisms causing these natural phenomena are dinoflagellates which produce a neurotoxin that, when in high concentrations, is capable of paralyzing and killing a variety of fishes but relatively few invertebrates. The most widespread damage is caused by the species Gymnodinum breve, a coastal species and one of four which can bloom and produce the red tide. Red tides appear to coincide with increased iron or other trace metals discharged via land runoff

after heavy rains.

2.

Zooplankton

Again, the data concerning zooplankton is very scanty

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for the western Gulf. Only one quantitative study has been carried out and that was by Khromov (1965). He studied the zooplankton biomass and reported a low density of 0.5 g/m east of Galveston. Southwest of this area, he found the density higher than 0.5 in the shallow waters but lower going seaward.

8/m3

Therefore considering the eastern Gulf's taxonomic studies and the few collections of zooplankton, it can be concluded that the shelf waters are quite rich in the same species found in other areas of the Gulf. The most important are larval shrimp, copepods, euphausiids, mysids, pteropod and heteropod molluscs, and the salps.

Zooplankton populations normally fluctuate in size according to the abundance of phytoplankton after a short lag in time.

The open Gulf waters become increasingly less productive with increasing distance from land. This is mainly in response to lower concen

trations of nutrients delivered by river runoff from the continent.

Lower concentrations of nutrients means less primary productivity, which in turn means less zooplankton biomass and so on.

Some plankton called holoplankton, spend their entire life cycle adrift in the oceans. Others, called meroplankton, are pelagic as larvae, eventually growing to larger sessile or swimming forms.

3. Benthic Biota

Benthic or bottom species constitute an important part to

the food chain. The significance of the algae and seagrasses as primary producers is often overlooked. They serve not only as a food source but as a habitat for fishes and invertebrates in the Gulf.

Even the pelagic or drifting species should not be overlooked.

The algal and seagrass abundance and distribution is: limited along the Texas coast due to the absence of a hard-bottom or rocky shores (except jetties). Within the last 75 years, the U.S. Army Corps of Engineers has constructed five pairs of rubblestone jetties in Sabine, Galveston, Freeport, Aransas, and Brazo Santiago to maintain navigability into these passes. These jetties provide an ideal substratum for algae and seagrasses.

Some of the seagrasses and algae present in the proposed area are Thalassia, Ruppia, Halodule, Entophysalis, Oscillatoria, Calothrix, Enteromorpha, Ulva, Chaetomorpha, Cladophora, Bryopsis, Giffordia Dictyota, Sargassum (pelagic), and Laurencia.

The benthic invertebrates comprise a variety of species with representatives from the cnidaria, anellida, mollusca, bivalves, crustacea, echinodermata, and chordata.

There is a special interest in the commercially important shrimp. There are three species harvested for commercial purposes. These are

the white shrimp, Penaeus setiferus; the brown shrimp, Penaeus aztecus; and the pink shrimp, Penaeus duorarum.