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1.
We examined the effect of nutrients and grazers on Thalassia testudinum in Jobos Bay, Puerto Rico by fertilizing sediment and manipulating grazer abundances. Bottom-up effects were variable: Added
nutrients did not increase seagrass aboveground biomass, but decreased belowground biomass—perhaps as a result of less biomass
being allocated to belowground structures in response to greater nutrient supply in porewater. Experimental fencing of 1.5 × 1.5 m
plots provided shelter that attracted large aggregations of fish, including seagrass herbivores. Seagrass biomass and shoot
density decreased with increasing abundance of herbivorous fish, indicating a significant top-down effect. There were interactions
between nutrient supply, provision of shelter, and grazing pressure. Fertilization enhanced seagrass %N; however, %N also
increased in unfertilized plots that were fenced, most likely due to uptake of N excreted from the large numbers of fish associated
with the fences. Only plots where shelter was provided and fertilizer was applied to sediments exhibited evidence of heavy
grazing, reducing both seagrass cover and aboveground biomass. In the unfertilized fenced plots, signs of grazing were fewer
despite large abundances of fish and enhanced nutritional quality of seagrass leaves. This suggests the possibility that high
nutrient availability in sediments lowered concentrations of chemical defense compounds in the seagrass and that cues other
than %N may have been involved in stimulating grazing. This study highlights the complexity of bottom-up and top-down interactions
in seagrass systems and the important role of refuge availability in shaping the relative strengths of these controls. 相似文献
2.
Seagrass both disappeared and recovered within 4 yr in one region of northern Indian River Lagoon (IRL). For the specific
area referred to as Turnbull Bay, a relatively pristine area of the IRL, over 100 ha of seagrass completely disappeared from
1996 to 1997 and then recovered by 2000. Based on lagoon-wide mapping from aerial photographs taken every 2–3 years since
1986, coverage of seagrass in Turnbull Bay declined from 124 ha in 1989 to 34 ha by 1999 and increased to 58 ha in 2003. Bi-annual
monitoring of fixed seagrass transects tells a more detailed story. Species composition along the Turnbull transect shifted
fromHalodule wrightii toRuppia maritima beginning in 1995, and macroalgal abundance increased. By the summer of 1997, seagrass completely disappeared along the transect,
as well as in most of the surrounding areas in Turnbull Bay; macroalgae covered much of the sediment surface. No significant
water quality changes were detected. Light attenuation and suspended solid values did increase after the seagrass disappeared.
Porewater sulfide concentrations, taken after all the grass was gone in 1997, were high (2,000 μM), but did improve by 1998
(1,200 μM). Seagrass recovery was rapid and occurred in the reverse sequence of species loss. Seedlings ofR. maritima were the first colonizers, then patches ofH. wrightii appeared. In 2000,Halophila engelmannii returned in the deeper water (>0.6m). By the summer of 2000, the beds had completely recovered. We conclude that this demise
was a natural event caused by a long-term buildup of seagrass biomass and a thick (10–15 cm) layer of organic detritus and
ooze. We surmise that such a crash and subsequent recovery may be a natural cycle of decline and recovery within this semirestricted,
poorly-flushed area. The frequency of this cycle remains uncertain. 相似文献
3.
Grazing by small epifauna on live seagrass leaves was formerly viewed as unimportant in controlling plant biomass and growth,
instead researchers focused on the indirect benefits of small invertebrates that crop algal competitors. Recent evidence shows
that the emerald nerite Smaragdia viridis preferentially ingests seagrass leaf tissue. In contrast, the button snail Modulus modulus feeds on epiphytes and periphyton coating the leaves. We conducted laboratory microcosm and field experiments to investigate
how the different feeding preferences of these seagrass-associated snails affect turtlegrass Thalassia testudinum primary production. Data revealed that after 24 h S. viridis reduced foliar biomass (25%) and chlorophyll (30%) and injured the equivalent of 50% of daily seagrass growth per shoot.
Conversely, M. modulus did not affect these variables. Our results emphasize that in subtropical seagrass communities not all small epifauna browse
off leaf surfaces and some can have important direct negative impacts on their seagrass host. 相似文献
4.
We evaluated the distribution of waterfowl in relation to a seagrass (Ruppia maritima) patch, to infauna, and on its relationship with substrate characteristics. An experiment performed in the Mar Chiquita coastal lagoon (37°46′S, 57°27′W; Argentina) was used to evaluate the effect of herbivory on widgeon grass,R. maritima. Control plots of equal size, located between bird exclosures, were exposed to herbivory. The experiment ran for 31 d, starting on December 1, 1994. Censuses showed that black-necked swan ( $\bar x = 50$ birds ha?1, SD = 37, n = 10) and coots ( $\bar x = 42$ birds ha?1, SD = 28, n = 10) were the most important (always present) of the waterfowl seen feeding onR. maritima. The results of the experiment showed greater leaf lengths, lower belowground (rhizomes and roots) biomass, greater aboveground (leaves and shoots) biomass, and greater abundance of the polychaeteHeteromastus similis in exclosure plots. There were no exclosure effects on total biomass (belowground plus aboveground), reproductive parts (fruits and pre- and postpollination flowers), or abundance of most benthic infauna. Topographic surveys showed that sediment surface was higher within theR. maritima patches, but there were no significant differences in granulometric composition. Surveys also showed that bird distribution was strongly associated with the presence ofR. maritima. 相似文献
5.
In November 2004, we evaluated the effect of Hurricane Ivan on seagrass meadows in Alabama by surveying all coastal locations
known to support seagrass prior to Hurricane Ivan's Iandfall in September 2004. We found that 82% of the sites containing
seagrass in 2002 still supported seagrass, and that, as in 2002, the most abundant species wasHalodule wrightii (shoalgrass). We also found more sites containingRuppia maritima (widgeongrass) than previously recorded. We confirmed the existence ofThalassia testudinum (turtlegrass) in Little Lagoon, Alabama, whose first record in the state had been noted in 2002. A resurvey of the western
half of Alabama's coastal waters in October 2005 after Hurricane Katrina found no loss of seagrass, with all sites that supported
seagrass in 2004 still containing seagrass in 2005. There was no major loss of Alabama's seagrass resources due to Hurricanes
Ivan or Katrina, even though both category 3 hurricanes severely affected the northern Gulf Coast. 相似文献
6.
Margaret O. Hall Michael J. Durako James W. Fourqurean Joseph C. Zieman 《Estuaries and Coasts》1999,22(2):445-459
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. 相似文献
7.
Lucinid bivalves dominate the infauna of tropical seagrass sediments. While the effect of seagrass on lucinids has been studied,
the reverse effect has largely been ignored. Lucinids can alter porewater chemistry (i.e., increase porewater nutrients by
suspension feeding and decrease porewater sulfides by oxygen introduction and bacterial oxidation), which can potentially
change seagrass productivity and growth morphology. To observe correlations between porewater chemistry and lucinid presence,
a field survey and laboratory microcosm experiment were conducted. Survey sampling sites with lucinids had significantly lower
sulfide and higher ammonium concentrations than sampling sites without lucinids. There was no difference in phosphate concentration
among sampling sites. Both lucinid species used in the microcosm experiment (Ctena orbiculata andLucinesca nassula) significantly lowered sulfide concentrations in the sediment porewater. Microcosm and field survey results were incorporated
into a sulfide budget. In seagrass sediments, lucinids remove 2–16% of the total sulfide produced. Sulfide is a major stressor
to both plants and animals in Florida Bay sediments; this removal may be important to maintaining seagrass productivity and
health. Oxygen introduction into sediments byC. orbiculata was estimated in a dye experiment.C. orbiculata were added to small tubes containing sieved mud and incubated in a bath of seawater with a Rhodamine WT. Rhodamine WT accumulation
in the sediment was measured. A first order estimate showed that oxygen introduction can account for less than 5% ofC. orbiculata sulfide removal. 相似文献
8.
J. M. Castillo S. Redondo C. Wharmby M. E. Figueroa T. Luque E. M. Castellanos A. J. Davy 《Estuaries and Coasts》2005,28(5):761-766
Spartina species tend to exhibit a range of phenotypes, often with short and tall growth forms. Such differences have been attributed
variously to environmentally induced phenotypic plasticity and genetic diferentiation between populations. This work examines
the basis of height variation inSpartina maritima (Curtis) Fernald at Odiel salt marshes, southwest Spain. Populations from sites with lower sediment redox potentials tended
to have significantly taller shoots. Thirty-four natural populations with an 8-fold range of shoot height were transplanted
to a common environment on an unvegetated, intertidal plain and shoot height was measured annually for 3 yr. There was a striking
convergence in height across populations after transplantation and the change in height in each year of a population was linearly
related to its initial height. Most populations grew taller after transplantation, suggesting environmental limitation in
their natural habitats. Populations that were originally tall tended to become shorter. The change in shoot height was negatively
related to the difference in surface sediment redox potential between their natural sites and the common transplant site.
Hypoxic sediments may stimulate stem growth, resulting in improved photosynthetic gas exchange and internal aeration of roots
and rhizomes. Although height variation inS. maritima appears mainly to be a result of phenotypic plasticity, a genetic component cannot be ruled out. This study emphasizes the
importance of long-term studies, preferably longer than turnover time of shoot populations. The highly plastic growth form
ofS. maritima allows it to colonize a wide range of habitats in environmentally heterogeneous salt marshes. 相似文献
9.
Changes in seagrass food-web structure can shift the competitive balance between seagrass and algae, and may alter the flow
of energy from lower trophic levels to commercially important fish and crustaceans. Yet, trophic relationships in many seagrass
systems remain poorly resolved. We estimated the food web linkages among small predators, invertebrate mesograzers, and primary
producers in a Chesapeake Bay eelgrass (Zostera marina) bed by analyzing gut contents and stable C and N isotope ratios. Though trophic levels were relatively distinct, predators
varied in the proportion of mesograzers consumed relative to alternative prey, and some mesograzers consumed macrophytes or
exhibited intra-guild predation in addition to feeding on periphyton and detritus. These findings corroborate conclusions
from lab and mesocosm studies that the ecological impacts of mesograzers vary widely among species, and they emphasize the
need for taxonomic resolution and ecological information within seagrass epifaunal communities. 相似文献
10.
D. Elaine Evers Charles E. Sasser James G. Gosselink Deborah A. Fuller Jenneke M. Visser 《Estuaries and Coasts》1998,21(1):1-13
Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River. 相似文献
11.
Although epibiotic protozoans are commonly observed on the chitinous exoskeleton of aquatic crustaceans, relatively little
is known about their ecological significance. The significance of protozoan epibionts on benthic copepods has never been examined.Coullana sp., a meiobenthic harpacticoid copepod, is abundant in Louisiana salt marshes and has high incidence (∼50%) of ciliate epibionts.
Field and laboratory grazing experiments indicated that ciliate epibionts did not hinderCoullana feeding on benthic or planktonic algae. Contrary to expectations,Coullana with high levels of ciliate epibionts (>8 ind−1) grazed at a significantly higher rate on14C-labeled benthic diatoms added to intact sediment cores than didCoullana with no epibionts.Coullana neutral lipids (examined using Nile Red, a hydrophobic fluorophore) were not significantly influenced by the presence of
ciliate epibionts, suggesting that copepods are able to compensate for any additional energetic demands imposed by epibionts.
Epibiont effects onCoullana susceptibility to hydrocarbon contaminants were measured by examining survivorship in diesel-spiked sediments. The presence
of ciliate epibionts significantly decreased survivorship at relatively low PAH concentrations (12.8 ppm). While ciliate epibionts
onCoullana do not dramatically alter total food acquisition or energy storage, they may cause stress, which in turn makesCoullana more susceptible to contaminants and possibly other natural stressors such as food limitation. 相似文献
12.
In many areas of the North American mid-Atlantic coast, seagrass beds are either in decline or have disappeared due, in part, to high turbidity that reduces the light reaching the plant surface. Because of this reduction in the areal extent of seagrass beds there has been a concomitant diminishment in dampening of water movement (waves and currents) and sediment stabilization. Due to ongoing declines in stocks of suspension-feeding eastern oysters (Crassostrea virginica) in the same region, their feeding activity, which normally serves to improve water clarity, has been sharply reduced. We developed and parameterized a simple model to calculate how changes in the balance between sediment sources (wave-induced resuspension) and sinks (bivalve filtration, sedimentation within seagrass beds) regulate turbidity. Changes in turbidity were used to predict the light available for seagrass photosynthesis and the amount of carbon available for shoot growth. We parameterized this model using published observations and data collected specifically for this purpose. The model predicted that when sediments were resuspended, the presence of even quite modest levels of eastern oysters (25 g dry tissue weight m?2) distributed uniformly throughout the modeled domain, reduced suspended sediment concentrations by nearly an order of magnitude. This increased water clarity, the depth to which seagrasses were predicted to grow. Because hard clams (Mercenaria mercenaria) had a much lower weight-specific filtration rate than eastern oysters; their influence on reducing turbidity was much less than oysters. Seagrasses, once established with sufficiently high densities (>1,000 shoots m?2), damped waves, thereby reducing sediment resuspension and improving light conditions. This stabilizing effect was minor compared to the influence of uniformly distributed eastern oysters on water clarity. Our model predicted that restoration of eastern oysters has the potential to reduce turbidity in shallow estuaries, such as Chesapeake Bay, and facilitate ongoing efforts to restore seagrasses. This model included several simplifiying assumptions, including that oysters were uniformly distributed rather than aggregated into offshore reefs and that oyster feces were not resuspended. 相似文献
13.
Subsidence and erosion of intertidal salt marsh at Galveston Island State Park, Texas, created new areas of subtidal habitat
that were colonized by seagrasses begining in 1999. We quantified and compared habitat characteristics and nekton densities
in monospecific beds of stargrassHalophila engelmanni and shoalgrassHalodule wrightii as well as adjacent nonvegetated substrates. We collected 10 replicates per habitat type during April, July, October, and
December 2001. Most habitat characteristics varied with season. Water temperature, salinity, and dissolved oxygen were similar
among habitat types. Turbidity and depth were greatest inH. engelmanni beds and least inH. wrightii beds.H. engelmanni exhibited shorter leaves and higher shoot density and biomass core−1 thanH. wrightii. Densities of almost all dominant species of nekton (fishes and decapods) were seasonally variable, all were higher in seagrass
habitats than in nonvegetated habitats, and most were higher in one seagrass species than the other. Naked gobyGobiosoma bosc, code gobyGobiosoma robustum, bigclaw snapping shrimpAlpheus heterochaelis, and blue crabCallinectes sapidus, were most abundant inH. engelmanni. Brown shrimpFarfantepenaeus aztecus, brackish grass shrimpPalaemonetes intermedius, and daggerblade grass shrimpPalaemonetes pugio were most abundant inH. wrightii. PinfishLagodon rhomboides and pink shrimFarfantepenaeus duorarum were equally abundant in either seagrass. Most dominant nekton varied in size by month, but only two (L. rhomboides andC. sapidus) exhibited habitat-related differences in size. Nekton densities in these new seagrass habitats equaled or exceeded densities
associated with historical and current intertidal smooth cordgrassSpartina alterniflora marsh. Continued seagrass expansion and persistence should ensure ecosystem productivity in spite of habitat change. 相似文献
14.
Monica Goigel Turner 《Estuaries and Coasts》1987,10(1):54-60
Responses ofSpartina alterniflora marsh to combinations of feral horse grazing, clipping, simulated trampling, and a late winter burn were studied on Cumberland Island National Seashore, Georgia. Replicated 200-m2 plots were established and sampled bimonthly from July 1983 to November 1984. Clipping and trampling each reduced peak aboveground biomass by 20% in 1983 and 50% (clipping) and 55% (trampling) in 1984. A March burn reduced peak aboveground biomass by 35% in 1984. Trampling and burning earch reduced net aboveground primary production (NAPP) by 35%, but clipping did not reduce NAPP. Standing stocks of live rhizomes were correlated with aboveground biomass and were reduced with experimental treatments. Abundance of the periwinkle snail (Littorina irrorata) was also reduced. Horse grazing had a substantial impact on standing stocks and NAPP ofSpartina, but grazing was not uniform throughout the marsh. Moderately grazed plots had NAPP reduced by 25% compared to ungrazed plots. Heavily grazed plots had extremely low NAPP, and abovegroundSpartina never exceeded 40 g m?2 dry mass compared to 360 g m?2 within exclosures. 相似文献
15.
Geoffrey R. Hosack Brett R. Dumbauld Jennifer L. Ruesink David A. Armstrong 《Estuaries and Coasts》2006,29(6):1150-1160
The complexity of habitat structure created by aquatic vegetation is an important factor determining the diversity and composition
of soft-sediment coastal communities. The introduction of estuarine organisms, such as oysters or other forms of aquaculture,
that compete with existing forms of habitat structure, such as seagrass, may affect the availability of important habitat
refugia and foraging resources for mobile estuarine fish and decapods. Fish and invertebrate communities were compared between
adjacent patches of native seagrass (Zostera marina), nonnative cultured oyster (Crassostrea gigas), and unvegetated mudflat within a northeastern Pacific estuary. The composition of epibenthic meiofauna and small macrofaunal
organisms, including known prey of fish and decapods, was significantly related to habitat type. Densities of these epifauna
were significantly higher in structured habitat compared to unstructured mudflat. Benthic invertebrate densities were highest
in seagrass. Since oyster aquaculture may provide a structural substitute for seagrass being associated with increased density
and altered composition of fish and decapod prey resources relative to mudflat, it was hypothesized that this habitat might
also alter habitat preferences of foraging fish and decapods. The species composition of fish and decapods was more strongly
related to location within the estuary than to habitat, and fish and decapod species composition responded on a larger landscape
scale than invertebrate assemblages. Fish and decapod species richness and the size of ecologically and commercially important
species, such as Dungeness crab (Cancer magister), English sole (Parophrys vetulus), or lingcod (Ophiodon elongatus), were not significantly related to habitat type. 相似文献
16.
Coastal ecosystems worldwide face increased nutrient enrichment from shoreline and watershed development and atmospheric pollution.
We investigated the response of the faunal community of a small microtidal estuary dominated byRuppia maritima (widgeon grass) in Maine, United States, to increased nitrogen loading using an in situ mesocosm enrichment experiment. Community
response was characterized by assessing quantitative shifts in macroinvertebrate community composition and identifying changes
in food web structure using stable carbon and nitrogen isotope ratios of producers and consumers. The community was dominated
by brackish water invertebrates including midge larvae, oligochaetes, damselfly larvae, amphipods, and ostracods. Experimental
nutrient additions resulted in significantly lower densities of herbivorous chironomids and predatory damselflies and greater
densities of deposit feeding oligochaetes. Grazing midge larvae (Chironomidae:Dicrotendipes, Cricotopus) consumed epiphytic algae under both natural and enriched conditions. Deposit feedingChironomus was dependent on allochthonous sources of detritus under natural conditions and exhibited a shift to autochthonous sources
of detritus under enriched conditions. PredatoryEnallagma primarily consumed grazing chironomids under all but the highest loading conditions. Experimental nutrient loading resulted
in an increase in generalist deposit feeders dependent on autochthonous sources of detritus. 相似文献
17.
Richard E. Matheson David K. Camp Susan M. Sogard Kimberly A. Bjorgo 《Estuaries and Coasts》1999,22(2):534-551
The fauna of seagrass-covered mud banks in Florida Bay, documented in the mid 1980s prior to recent seagrass die-off, phytoplankton blooms, and other ecosystem changes, was reexamined in the mid 1990s for faunal changes that might be associated with environmental perturbations. During both decades, decapod crustaceans and fishes were collected with 1-m2 throw traps from seagrass beds at six sites that differ in the amount of freshwater and/or marine influence and in seagrass community metrics. The most common faunal changes were declines in seagrass-canopydwelling forms and increases in benthic forms. At three sites with relatively lush seagrass meadows, above-ground seagrass standing crop declined and abundance of the benthic predatory fishOpsanus beta increased. The degree of faunal change among these sites appeared to be related either to salinity variability or to the degree of exposure to the ecosystem changes that have taken place in Florida Bay. At two sites with poorly developed seagrass meadows, seagrass standing crop and canopy height did not change significantly between decades, but there was an increase in shoot density and total leaf area. The animal communities at these sites were characterized by significant increases in the abundance of benthic crustaceans. At the site on the edge of Rankin Lake, the basin where seagrass die-off was first observed in Florida Bay during 1987, seagrass standing crop, canopy height, shoot density, and leaf area declined significantly between decades, but species richness of both crustaceans and fishes increased. The abundance of canopy-dwelling crustaceans and fishes declined markedly at this site, whereas the abundance of benthic forms less dependent on seagrass cover generally increased. In retrospect, we believe the fauma at this site during the 1980s, characterized by high productivity but few species, was already showing signs of the stresses that led to the seagrass die-off that began in 1987. 相似文献
18.
Gordon W. Thayer Karen A. Bjorndal John C. Ogden Susan L. Williams Joseph C. Zieman 《Estuaries and Coasts》1984,7(4):351-376
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. 相似文献
19.
Iris E. Hendriks Miguel Cabanellas-Reboredo Tjeerd J. Bouma Salud Deudero Carlos M. Duarte 《Estuaries and Coasts》2011,34(1):60-67
We assess the sheltering effect of Posidonia oceanica meadows on drag forces exerted on shells of the fan mussel Pinna nobilis. We examine a range of shell sizes under four unidirectional flow speeds (0.05–0.34 m s−1) and two oscillating regimes. Three meadow densities are evaluated and a control without vegetation. We found that the attenuating
effect of the meadow on drag forces experienced by bivalves is determined by the form of the hydrodynamic energy, e.g., as
unidirectional flow or wave action. In tidal currents, the meadow protects most sizes of bivalves, with a higher efficiency
for dense meadows, while in wave dominant zones the meadow reduces drag forces for bivalves with shell areas below a threshold
of 0.019 m2, whereas larger animals experience increased drag forces within the meadow independent of meadow density. Reduction of shoot
density in seagrass meadows might therefore not affect the effectiveness of the canopy to reduce drag forces on associated
species like the fan mussel in wave-dominated areas while increased storm frequency could result into losses of larger individuals
during periods of high wave action. 相似文献
20.
A broad-scale survey of seagrass species composition and distribution along Florida's central Gulf Coast (known as the Big
Bend region) was conducted in the summer of 2000 to address growing concerns over the potential effects of increased nutrient
loading from adjacent coastal rivers. Iverson and Bittaker (1986) originally surveyed seagrass distribution in this region
between 1974–1980. We revisited 188 stations from the original survey, recording the presence or absence of all seagrass species.
Although factors such as accuracy of station relocation, differences in sampling effort among studies, and length of time
between surveys preclude statistical comparisons, several interesting patterns emerged. While the total number of stations
occupied by the three most common seagrass species,Thalassia testudinum, Syringodium filiforme, andHalodule wrightii, was similar between the two time periods, we observed a change in the number of records of each species as well as changes
in distribution with depth.T. testudinum andHalophila engelmanni occurrence declined in the deepest areas of the region, while the number of stations occupied byS. filiforme andH. wrightii increased in nearby areas. We observed several localized areas of seagrass loss, frequently associated with the mouths of
coastal rivers. These results suggest that increased nutrient loading to coastal rivers that discharge into the Big Bend area
may be affecting seagrasses by increasing phytoplankton abundance in the water column, thus changing water clarity characteristics
of the region. 相似文献