Joseph C. Zieman

Influence of the seagrass, Zostera marina L., on current flow☆

Abstract A salt-water flume was used to describe the mechanics of current flow around an articial Zostera marina meadow. Shear velocity and roughness height were positively correlated with seagrass surface area, and were positively/negatively correlated with current velocity. Current velocity intrusion into the meadow before diminution and maximum reduction (both at the 2 cm height line) proceed by factors of 1·25 and 2·07 cm into the meadow per cm s −1 of current velocity, respectively. Froude number was correlated with mean bending angle of the canopy as a whole. Maximum bending had occurred with Froude = 1, but most bending had taken place by Froude = 0·4, a velocity of 40–50 cm s −1 in this experiment. The meadow edge is the most dynamic zone of a seagrass meadow in regard to current flow. Bending of the shoot canopy is a mechanism for re-direction of current flow and in-canopy reduction of current velocity. Meadow dimensions may be regulated by scouring processes in different hydraulic regimes. Shoot bending and subsequent in-meadow current velocity reduction are mechanisms that affect self-shading and photosynthetic capabilities as well as providing habitat stability.

Methods for the study of the growth and production of turtle grass, Thalassia testudinum König

Research has shown that the measurement of the productivity of vascular hydrophytes by gas exchange methods is inaccurate due to the storage of gases within the leaves. Methods were developed for the study of turtle grass, Thalassia testudinum Konig, which allow: (a) the monitoring of the blade populations without disturbing the plant, and (b) the determination of leaf growth and net production of the blades, in addition to other biotic variables associated with the growth and development of the plant. The technique involves the marking of individual blades with a modified stapler, and the retrieval of the marked blades after a 2–3 week interval. The production measured is that which is readily available to the consumers of the Thalassia community.

Role of larger herbivores in seagrass communities

The nutritional ecology of macroherbivores in seagrass meadows and the roles of grazing by urchins, fishes and green turtles in tropical systems and waterfowl in temperate systems are discussed in this review. Only a few species of animals graze on living seagrasses, and apparently only a small portion of the energy and nutrients in seagrasses is usually channeled through these herbivores. The general paucity of direct seagrass grazers may be a function of several factors in the composition of seagrasses, including availability of nitrogen compounds, presence of relatively high amounts of structural cell walls, and presence of toxic or inhibitory substances. The macroherbivores, however, can have a profound effect on the seagrass plants, on other grazers and fauna associated with the meadow, and on chemical and decompositional processes occurring within the meadow. Grazing can alter the nutrient content and digestibility of the plant, as well as its productivity. Removal of leaf material can influence interrelations among permanent and transient faunal residents. Grazing also interrupts the detritus cycle. Possible consequences of this disruption, either through acceleration or through decreased source input, and the enhancement of intersystem coupling by increased export and offsite fecal production, are discussed. The extent and magnitude of these effects and their ecological significance in the overall functioning of seagrass meadows only can be speculated, and probably are not uniform or of similar importance in both tropical and temperate seagrass systems. However, areas grazed by large herbivores provide natural experiments in which to test hypotheses on many functional relations in seagrass meadows.

The role of current velocity in structuring eelgrass (Zostera marina L.) meadows

Measurements of velocity profiles, bathymetry, and surface sediment characteristics across eelgrass (Zostera marina L.) meadows yielded information on community development processes and functional attributes of this ecosystem. Height/length ratios of the meadows were positively correlated with tidal current velocity. Low, medium, and high current regimes were separated by surface current velocities of approximately 50 and 90 cm s−1. Z. marina can tolerate approximately 120–150 cm/sec current velocities in the areas studied. Per cent silt-clay and organic matter content of the surface sediments are negatively associated with shear velocity, suggesting that meadows in high current areas are sources while meadows in low current areas are sinks of autochthonous detritus. Current velocity maintains seagrass meadows at different equilibrium levels (relative climaxes). We theorize these different equilibrium levels provide unequal habitat utilization potentials for the associated faunal community.

Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment

The primary source of nutrients for seagrass growth is considered to be the sediment porewater. Porewater nutrient concentrations were measured in 18 seagrass beds across Florida Bay, USA, a shallow, seagrass-dominated subtropical embayment, during the summers of 1987 and 1988. Concentrations of nutrients in porewater varied widely, with median values of 0.34 μM for soluble reactive phosphorus (SRP) and 78.6 μM for NH4+. SRP and NH4+concentrations were positively correlated. Due to spatial heterogeneity, there were no apparent trends with sediment depth (down to 40 cm) in the porewater nutrient concentrations. The SRP concentration of the porewater was highest in areas supporting Halodule wrightii, intermediate in areas of Thalassia testudinum, and lowest in sediments without seagrasses. There was no similar relationship with NH4+. Porewater SRP, but not NH4+, was significantly correlated with total seagrass standing crop. Elemental content (both N and P) of green leaves of T. testudinum was a function of the concentration of the nutrients in the porewater. Standing crop of T. testudinum was correlated with phosphorus content, but not with N content, of the seagrass leaves. The results support the hypothesis that sediment porewaters are the most important source of nutrients for seagrass growth. In this subtropical carbonate environment, the availability of phosphorus in the porewater limits the development, and controls the species composition, of seagrass beds.

Seagrass die-off in Florida bay : Long-term trends in abundance and growth of turtle grass, Thalassia testudinum

Beginning in late 1987 Florida Bay experienced a large and unprecedented die-off ofThalassia testudinum. The die-off occurred only in stands of denseT. testudinum. We initiated an experimental monitoring effort in 1989 to attempt to ascertain the cause of this die-off phenomenon. From 1989 to 1995 the abundance and productivity ofT. testudinum was measured at five stations associated with the seagrass die-off and three stations where no die-off had occurred (including one on the seaside of Key Largo, outside of Florida Bay). Early in the study the salinity was very high, exceeding 46 psu, but it has decreased to 29–38 psu in recent years. Seagrass standing crop and either short-shoot density or mass per short shoot declined at nearly all stations, including the stations without die-off (unaffected stations). Over the course of the study, areal productivity declined at three die-off stations; but mass-specific productivity increased at all die-off stations and one unaffected station. Seasonality was pronounced; detrended standardized residuals showed responses for all of the seagrass parameters to be greater than the yearly mean in spring and summer and less than the mean in fall and winter. Detrended residuals also showed decreased productivity to be correlated with increased salinities in the summer despite a long-term record of declining salinities. We propose a conceptual model of the seagrass die-off phenomenon. We document that salinity does contribute to stress onT. testudinum in Florida Bay, but salinity is believed to be only one contributing factor to the loss of seagrasses. The documented increase in the mass-specific productivity ofT. testudinum over the period 1989–1995 suggests seagrasses are growing rapidly in Florida Bay by 1995; we predict that the loss ofT. testudinum may be slowing down and that recovery is possible.

Decadal changes in seagrass distribution and abundance in Florida Bay

The Florida Bay ecosystem has changed substantially in the past decade, and alterations in the seagrass communities have been particularly conspicuous. In 1987 large areas ofThalassia testudinum (turtlegrass) began dying rapidly in western Florida Bay. Although the rate has slowed considerably, die-off continues in many parts of the bay. Since 1991, seagrasses in Florida Bay have been subjected to decreased light availability due to widespread, persistent microalgal blooms and resuspended sediments. In light of these recent impacts, we determined the current status of Florida Bay seagrass communities. During the summer of 1994, seagrass species composition, shoot density, shoot morphometrics, and standing crop were measured at 107 stations. Seagrasses had been quantified at these same stations 10 yr earlier by Zieman et al. (1989).T. testudinum was the most widespread and abundant seagrass species in Florida Bay in both 1984 and 1994, and turtlegrass distribution changed little over the decade. On a baywide basis,T. testudinum density and biomass declined significantly between surveys; mean short-shoot density ofT. testudinum dropped by 22% and standing crop by 28% over the decade.T. testudinum decline was not homogeneous throughout Florida Bay; largest reductions in shoot density and biomass were located principally in the central and western bay. Percent loss ofT. testudinum standing crop in western Florida Bay in 1994 was considerably greater at the stations with the highest levels of standing crop in 1984 (126–215 g dry wt m−2) than at the stations with lower levels of biomass. While turtlegrass distribution remained consistent over time, both the distribution and abundance of two other seagrasses,Halodule wrightii andSyringodium filiforme, declined substantially between 1984 and 1994. Baywide,H. wrightii shoot density and standing crop declined by 92%, andS. filiforme density and standing crop declined by 93% and 88%, respectively, between surveys. Patterns of seagrass loss in Florida Bay between 1984 and 1994 suggest die-off and chronic light reductions were the most likely causes for decline. If die-off and persistent water-column turbidity continue in Florida Bay, the long-term future of seagrasses in the bay is uncertain.

The Effects of Long-Term Manipulation of Nutrient Supply on Competition between the Seagrasses Thalassia testudinum and Halodule wrightii in Florida Bay

Long term (8 yr) continuous fertilization (via application of bird feces) of established seagrass beds in Florida Bay, FL, USA caused a change in the dominant seagrass species. Before fertilization, the seagrass beds were a Thalassia testudinum monoculture; after 8 yr of fertilization the seagrass Halodule wrightii made up 97% of the aboveground biomass. Fertilization had a positive effect on the standing crop of T. testudinum for the first two years of the experiment. The transition from T. testudinum-dominated to H. wrightii-dominated was dependent on the timing of colonization of the sites by H. wrightii; the decrease in T. testudinum standing crop and density at the fertilized sites occurred only after the colonization of the sites by H. wrightii. There were no trends in the standing crop or density of T. testudinum at control sites, and none of the control sites were colonized by H. wrightii. The effects of fertilization on these seagrass beds persisted at least 8 yr after the cessation of nutrient addition, suggesting that these systems retain and recycle acquired nutrients efficiently. Results of these experiments suggest that Halodule wrightii, the normal early-successional seagrass during secondary succession in Caribbean seagrass communities, has a higher nutrient demand than Thalassia testudinum, the normal late successional species, and that the replacement of H. wrightii by T. testudinum during secondary succession is due to the ability of T. testudinum to draw nutrient availability below the requirements of H. wrightii

The ecological effects of physical damage from motor boats on turtle grass beds in Southern Florida

Abstract Observation has shown that beds of turtle grass, Thalassia testudinum , although highly productive, do not recover rapidly following physical disturbance of the rhizome system. In shallow waters the most common form of rhizome disturbance is from the propellers of motor boats. In turtle grass beds which are otherwise thriving, tracks resulting from propellers have been observed to persist from 2 to 5 years. The proportion of fine sediment components is reduced in the sediments from the boat tracks, and the pH and EH are reduced in comparison to the surrounding grass bed. Damage of this type is most likely to occur in the shallow passes between islands and keys. These areas are also the slowest to recover due to the rapid tidal currents present in the shallow passes.

Evidence for the Influence of Seagrasses on the Benthic Nitrogen Cycle in a Coastal Plain Estuary Near Beaufort, North Carolina (USA)

A study was undertaken to evaluate the interrelationship between the presence of seagrasses, Zostera marina and Halodule wrightii, and the physical and chemical properties of sediments in a coastal plain estuary near Beaufort, North Carolina. In sediments underlying a cover of seagrass, silt-clay, organic matter, exchangeable ammonium, ammonium dissolved in pore waters and total nitrogen were larger than in unvegetated profiles. The magnitude of the physical and chemical properties of sediments varied according to the location of the station in relation to the vegetation, as well as the continuity in the distribution of the seagrass. The largest pools of nitrogen, the finest sediment texture, and the greatest organic matter content were in sediments associated with the mid bed regions of seagrass meadows, intermediate at the edges of the bed and small isolated patches of grass, and least in unvegetated substrate.General conclusions from this study are: 1) once established, seagrasses appear capable of modifying the sediment texture as well as the organic matter and nitrogen content; 2) nitrogen accumulates beneath the vegetation suggesting that vegetated sediments are sinks; however, functional recycling mechanisms seem to be operating as suggested by the larger magnitude of remineralized nitrogen in the vegetated profiles; and 3) the establishment of seagrasses in this geographical region are not necessarily restricted by the sediment properties measured in this study. These data and conclusions are discussed in regard to an application of contemporary theories of ecosystem development to seagrass systems.