Ronald C. Phillips

REPRODUCTIVE STRATEGIES OF EELGRASS (ZOSTERA MARINA L.)

Abstract Geographic variation in the reproductive strategy of eelgrass on the Pacific coast of North America is reflected in differences in flowering frequency, seed production and the effects of salinity on germination. The analysis of variance of temperature and salinity effects on seed germination detected significant salinity effects, but not temperature effects. Low salinities enhanced germination except in seeds from the Gulf of California, which germinated readily in normal strength sea water. Populations at the margins of the geographic range of eelgrass are characterized by high incidence of density-independent mortality, and by an increase in the incidence of sexual reproduction. At the southern margin of the range in the Gulf of California, where high water temperatures kill all plants in summer, there is a 100% flowering response and a high rate of germination success. These populations grow subtidally and arise anew each year from seedlings. At the northern margin of the range in the Bering Sea where ice scouring in winter is an important source of mortality, eelgrass is limited to subtidal areas or intertidal pools. In these perennial populations the sexual reproductive effort is greater than that of subtidal populations in the central portion of the range. In the central portion of the range, eelgrass exhibits two reproductive strategies. In subtidal areas where salinity fluctuation is minimal, dense stands of perennial plants reproduce vegetatively. In intertidal areas where seasonally low salinities enhance seed germination, there is a higher incidence of flowering.

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.

Phenology and reproductive biology of eelgrass (Zostera marina L.) at Bahia Kino, sea of Cortez, Mexico

Abstract At Bahia Kino, Mexico, seeds of Zostera marina L. (eelgrass) germinate in October and early November. By January seedlings are widespread throughout the area. A very rapid vegetative growth ensues. By March all eelgrass at the depth limit (7 m) is flowering; and most is flowering in shallow water (low tide to 4 m depth). In April all plants are in flower; a few plants display mature seeds. Windrows of detached plants occur on the beaches. In May all reproductive plants have mature seeds, and few attached plants remain. In July all eelgrass in the area has disappeared. All reproductive activities and plant detachment are completed before the lethal upper limit for the species is reached (30°C). Thus, eelgrass at Bahia Kino is a true annual and represents an ultimate response to high water temperatures.

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.

Transplantation of seagrasses, with special emphasis on eelgrass, Zostera marina L.

Abstract A background, listing the history of transplanting seagrasses is given. There is a need for seagrass field planting methods owing to the extremely high productivity of seagrasses and the perturbations they are subjected to. I have planted vegetative stocks and seeds of Zostera marina L. (eelgrass) across tidal zones, across latitudinal zones (from Alaska into Puget Sound), and into water deeper than indigenous growth. Data from these plantings indicate: (1) varietal distinctions based on leaf dimensions are invalid; (2) the possibility of local physiological race distinction in eelgrass; (3) the depth limit of eelgrass is due to a lack of suitable light deeper than a certain minimum depth; and (4) eelgrass seedlings require a higher light intensity than is present deeper than a certain minimum depth. Thus, transplanting experiments yield much more data concerning seagrass biology than just planting method information.

The occurrence of fossil seagrasses in the Avon Park formation (late middle eocene), Levy County, Florida (U.S.A.)

Within the Avon Park Formation of west-central Florida, U.S.A., fossil seagrasses are preserved as carbonized imprints within the bedding planes of a micritic dolomitized limestone. Six species could be recognized, but only for three of them was the material sufficiently complete to allow a formal description. These three species are Thalassodendron auricula - leporis den Hartog n.sp., Cymodocea floridana den Hartog n.sp. and Thalassia testudinum Banks ex Konig. The genera Thalassodendron and Cymodocea have not been recorded for America before.

Occurrence of seeds and seedlings of Thalassia testudinum banks ex König in the Florida keys (U.S.A.)

Established Thalassia seedlings were found in a shallow subtidal site in the Florida Keys (x = 1 seedling 12 m−2). The 1979 seed crop was probably large since one beach strand site of 0.5 km long and 5 m wide contained on average, 14.8 seedlings m−2. Seeds in these quantities could be used for habitat restoration purposes. The seed crop is probably highly variable on an annual basis.

Common Structures and Properties of Seagrass Beds Fringing the Coasts of the World

Seagrasses are aquatic angiosperms which are completely confined to the marine environment. The name refers to a superficial resemblance to grasses, because of the linear leaves of most of the approximate 60 species (Den Hartog 1970). They are of paramount importance in the coastal environment as, when they occur, they generally form dense beds that may cover extensive areas. One of the most conspicuous ecological functions of the seagrass beds is their capacity to stabilize and to modify the unconsolidated substrates in which they are rooted. By their dense growth, they protect the substrate to considerable extent from erosion, but they also trap floating coarse and suspended fine materials. Further, they produce litter. The accompanying organisms contribute to the alteration of the substrate by contributing fecal material, shells or other protective structures. Part of this material leaves the system, and may be washed up onto the beach, or may be deposited on the bottom of adjacent systems in deeper waters. There are even records of seagrass remains from depths of thousands of metres (Wolff 1976). Another function is that the seagrass beds offer a substrate to many algae and animals which otherwise could not establish themselves on the sandy and muddy bottoms. Furthermore, the seagrass beds serve as a nursery for many fish, crustaceans, and other invertebrates, providing food not only for these animals, but also for a considerable number of migrating birds, sirenians, turtles, and many organisms from neighbouring habitats.

The marine algae and seagrasses of the miskito bank, Nigaragua

Abstract A total of 4 seagrass species and 99 benthic algal species were found (34 Chlorophyta; 17 Phaeophyta; 46 Rhodophyta; 2 Cyanophyta). The highest species diversity occurred on shallow coral reefs and ridges (0–3 m deep: 4 seagrass species and 77 algal species). Intermediate levels of species diversity existed in depths from 3–12 m (4 seagrass species and 55 algal species). The lowest species diversity was found in depths from 12–40 m (1 seagrass and 37 algal species). Thalassia was the dominant seagrass up to 2 m deep; Thalassia and Syringodium were mixed from 2–8 m deep; while Syringodium was the dominant seagrass over 8 m deep. Most species found are widely distributed throughout the western tropical Atlantic area. There was no evidence of endemic species.

Heterozostera tasmanica (Martens ex aschers.) den Hartog in Chile

A 1.2 km2 meadow of Heterozostera tasmanica (Martens ex Aschers.) den Hartog was found at Pto. Aldea, Bahia Tongoy, 60 km south of Coquimbo, 30°S on the coast of Chile. This provides documented evidence of seagrass occurrence on the west coast of South America. Since H. tasmanica is widespread throughout southern Australia, the Chilean stock represents a highly disjunct population. It appears from several growth parameters (density, leaf biomass, leaf area index, specific leaf area, δ 13C) that the H. tasmanica meadow at Pto. Aldea is very similar to those found in other parts of the world.