Colonization and Vertical Zonation of Macrophyte Algae on Introduced Granite Substrates in the Ship Channel at Port Aransas, Texas




Caskey, Bobby Joe

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Patterns of algal colonization and vertical zonation of six algal species; Ectocarpus siliculosus, Bryopsis plumosa, Petaloniafascia, Ulva lactuca, Ulvafasciata and Enteromorpha sp. were studied in the ship channel at Port Aransas, Texas using introduced granite substrates from October 1997 through May 1998. A deep and shallow rig were built to contain the granite substrates (tiles) at two different locations. The deep rig was placed under the pier at the University of Texas Marine Science Institute pier while the shallow rig was placed approximately 3 m off the southwest jetty in Port Aransas. The shallow rig was exposed to wave action and reduced current and the deep rig experienced no direct wave action and strong current. Herbivory exclusion cages and partial cages were used to determine the effects of herbivory on abundance and vertical zonation of algal species. Comparisons of caged and control treatments found a significant difference in colonization of algal organisms along a depth gradient. Algal colonization occurred on the shallow rig from mean sea level to 0.90 m below mean sea level and from 0.30 m to 0.90 m below mean sea level on the deep rig. This upward deflection in algal colonization on the shallow rig was attributed to wave action. A trend of decreased percent cover on tiles (all species combined) with increased depth was evident in all treatments. This was attributed to large loads of particulates carried into the channel from the bays, thus increasing turbidity and reducing incident light in the water column. Species composition with respect to depth seemed to be controlled by herbivory. The number of species was highest in caged treatments at shallow water depths and decreased as a function of depth and decreased protection from cages. The green algae Ulva lactuca, Enteromorpha and Bryopsis plumosa showed the strongest response to herbivory with respect to depth. Herbivory was highest from 0.60 to 0.90 m below mean sea level on unprotected substrates. The algal species Bryopsis plumosa and Petaloniafascia showed affinities for cooler temperatures. Ulvafasciata appeared in March 1998 collection and persisted throughout the remainder of the study.



marine algae, marine plants, Port Aransas


Caskey, B.J. (1988). Colonization and vertical zonation of macrophyte algae on introduced granite substrates in the ship channel at Port Aransas, Texas (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.


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