Calvin McMillan

Phenolic acids in seagrasses

Abstract Six phenolic acids were found in leaves of over 50% of the seagrasses surveyed. p -Hydroxybenzoic acid was the most ubiquitous and was recorded in each of the 12 genera and in each of the 25 species that included both Gulf-Caribbean and Indo-Pacific seagrasses. p -Coumaric, caffeic, ferulic, vanillic and protocatechuic acids were also of frequent occurrence. The six phenolic acids were detected in rhizomes and roots as well as in leaves of four Texas seagrasses, Thalassia testudinum Banks ex Konig, Halodule wrightii Aschers., Syringodium filiforme Kutz., and Halophila engelmannii Aschers. The phenolic acids that predominate in the seagrasses also occur widely in land plants, but gallic acid was detected in a greater percentage of seagrasses than had been observed in a comparable number of land plants of Central Texas origin. Although trhe phenolic acids have been cited in the allelopathic literature of land plants as major water-borne inhibitors, their role in seagrass ecosystems is under investigation.

SULPHATED PHENOLIC COMPOUNDS IN SEAGRASSES

Abstract A survey of 43 species of seagrass showed that all contained flavone and/or phenolic acid sulphates. Five of the 12 genera had sulphated flavones; only Zostera and Enhalus lacked sulphated phenolic acids. In Zosteroideae of the Potamogetonaceae, sulphated flavones were shown in all species of two genera, Zostera and Phyllospadix , but were absent from a third genus, Heterozostera . In the other two subfamilies, Cymodoceoideae and Posidonioideae , sulphated flavones were not present but each of the six genera had two or three sulphated phenolic acids. Sulphated flavones occurred in each of three genera representing three subfamilies of the Hydrocharitaceae. The occurrence of the sulphated phenolic compounds indicated subgeneric differences in Zostera and interspecific differences in Halophila . The compounds may play a role in adjustment to the seawater habitat as well as in the allelochemical relations to seagrasses.

Salinity Tolerances of Five Marine Spermatophytes of Redfish Bay, Texas

Thalassia testudinum Konig and Sims and Halophila engelmanni Aschers. (Hydrocharitaceae); Diplanthera wrightii (Aschers.) Aschers., Ruppia maritima L., and Syringodium filiforme Kutzing (Potamogetonaceae) are the major plants of the highly productive estuaries along the Texas coast. When transplanted to outdoor ponds and to controlled growth rooms, the greatest tolerance to increasing salinity was shown by Diplanthera. Lesser salinity tolerance was shown by Thalassia and Ruppia. Although survival was not complete under all transplant conditions for Syringodium, it showed least tolerance of high salinity. Halophila survived sporadically in the study but showed active growth at high salinity. The distributional patterns of these species in Redfish Bay are partially correlated with their salinity tolerances. See full-text article at JSTOR

Phenology of eelgrass, Zostera marina L., along latitudinal gradients in North America

Eelgrass, Zostera marina L., was collected monthly from 1976 to 1979 and its phenology compared in the distributional range on both coastlines of North America. Each of three reproductive phenophases (initial appearance of macroscopically visible flower buds, initial anthesis, initial appearance of visible fruits) differed significantly among the sites in their dates of occurrence. Among the sites, flowering at the same latitude was later in the Atlantic than in the Pacific and flowering along a latitudinal gradient occurred increasingly later at more northern latitudes in the Atlantic than in the Pacific. Although the reproductive periodicity probably is controlled primarily by water temperature at the different sites, the variation in timing at the same latitude among Pacific and Atlantic sites indicates that Z. marina may include genotypes with different temperature requirements that are selectively adapted to different habitats along the latitudinal gradients.

Environmental factors affecting seedling establishment of the Black Mangrove on the central Texas Coast.

The effects of salinity, water turbulence, water depth, and temperature on the establishment of seedlings of black mangrove (Avicennia germinans L.) on the central Texas coast were evaluated in the laboratory. Salinity was not the chief factor limiting seedling establishment, because seedlings rooted in distilled water and in salinities approaching twice the concentration of sea water. Water turbulence, either of distilled or sea water, inhibited root and seedling development. Seedlings tumbled for as much as 12 weeks showed rapid root development when subsequently stabilized. Various water depths promoted extensive root systems, but seedlings did not become established until water depth was reduced to 5 cm or less. High temperature treatment, exposure to 39 degrees —40 degrees C for 48 hr, was lethal to stemless seedlings, but not to seedlings with stems and roots. The results suggest that vivaparous development in this A. germinans population phenologically timed by environmental relations during winter and early spring as a protection against the lethal effects of the high temperatures that prevail on beaches and in shallow water during late spring and summer.

Experimental studies on flowering and reproduction in seagrasses

Abstract Flowering and reproduction of seagrasses in laboratory cultures were compared with responses of the same clones in Redfish Bay, Texas. Halophila engelmanni Aschers. produced flowers continuously from January to September under controlled conditions. Flower production in the bay was confined to the period from April to mid-June. The possible effects of salinity, temperature and photoperiod were studied in the laboratory and monitored in the bay. Of these, temperature seemed to be the chief control of the flowering period of Halophila. Under laboratory conditions, no flowering was recorded in Thalassia testudinum Banks ex Konig, Syringodium filiforme Kutz., Ruppia maritima L., or Halodule wrightii Aschers., but the flowering of Halodule in the bay from May to August suggested a response to higher temperatures than indicated for Halophila. Fruit development of Halodule in the laboratory also indicated a higher temperature response.

13C12C ratios in seagrasses

Seagrasses have a wide range of δ 13C values. For 47 species from 12 genera, the values were within the range of −3.0 to −19.0%0. Only two species, Halophila tricostata Greenway and Halophila beccarii Aschers., had lower values, of −20.8 and −23.8%0, respectively. Among the 12 genera, Syringodium and Enhalus had the highest mean values and Phyllospadix and Amphibolis had the lowest mean values. The δ 13C values for most seagrasses are within the range usually associated with C4 metabolism, but the status of seagrasses as members of this photosynthetic group has not been confirmed by morphological and physiological studies. The high variability in δ 13C values may reflect a variable photosynthetic metabolism.

Chlorophyll composition under controlled light conditions as related to the distribution of seagrasses in Texas and the U.S. Virgin Islands

Seagrasses from various depths in St. Croix, U.S. Virgin Islands, and from shallow beds in Redfish Bay, Texas, were grown in the laboratory under three light conditions. For all collections, light absorption readings of extracted pigments showed that total chlorophyll content is inversely related to reduced light over the range from 35 to 200 μM m−2 s−1. The ratio of chlorophyll a to chlorophyll b decreases in response to reduced light for Caribbean collections of Halodule wrightii Aschers., Syringodium filiforme Kutz., and Halophila decipiens Ostenfeld but not for Thalassia testudinum Banks ex Konig or the Texas collections of Halodule, Syringodium, Thalassia, and Halophila engelmannii Aschers. There is a correlation of the maximum depth of the St. Croix seagrasses and the ratio of chlorophyll a to chlorophyll b: H. decipiens, with the greatest depth range, to −42 m, has the lowest ratio; T. testudinum, with the least depth range, to −12 m, has the highest ratio; H. wrightii and S. filiforme have intermediate depth ranges and rations. Although light quality and sea bottom characteristics may play roles in the ultimate depth to which a seagrass may occur, photon flux density is suggested as a primary environmental determinant.

Morphogeographic variation under controlled conditions in five seagrasses, Thalassia testudinum, Halodule wrightii, Syringodium filiforme, Halophila engelmannii, and Zostera marina

Abstract The morphogeographic evidence udner controlled conditions demonstrated variation in leaf width that correlated with the leaf patterns in the indigenous habitats. In Thalassia testudinum Banks ex Konig, Halodule wrightii Aschers., and Syringodium filiforme Kutz., the narrow-leaved variants from shallow bays in the northern Gulf of Mexico continued to produce narrow leaves in laboratory culture. Broader-leaved variants in the same three species that originated from seagrass beds associated with coral reefs in the southern Gulf of Mexico and the Carribbean continued to produce broad leaves. The turbid water and variable salinity and temperature of the shallow northern bays correlate with the presence of narrow-leaved plants, and the clear water and relatively constant salinity and temperature of the southern regions correlate with the broader-leaved populations. Although an ecotypic status for the narrow- and wide-leaved populations was suggested, the selective role of these habitat conditions needs further investigation. Clonal variation in leaf width was demonstrated within Halodule populations under uniform conditions. Plants of Halophila engelmannii Aschers. from deep seagrass beds in the Gulf of Mexico along the western coast of Florida continued to produce narrower leaves than those produced by plants from shallow Texas bays. Three collections of Zostera marina L. from Washington and Alaska produced leaves of significantly different widths under each of three temperature regimes. The experimental evidence suggests that the width of a seagrass leaf is dependent on its immediate environmental surroundings but that the limits of its ecoplasticity vary geographically depending on the genotype.

Differentiation in habitat response among populations of New World seagrass

Abstract Seagrass populations in diverse ecosystems show the selective influence of the local habitat conditions. The patterns of differentiation in the Gulf of Mexico and Caribbean seagrasses, Thalassia testudinum Banks ex Konig, Syringodium filiforme Kutz., and Halodule wrightii Aschers., and in the circumboreal seagrass, Zostera marina L., have been investigated by a variety of manipulative techniques in the laboratory and in the field. Although seagrasses may be vegetatively moved for long distances either by oceanic transport or by experimental procedures, their survival patterns reflect the selective influence of their indigenous habitats.