Unvegetated Substrate Research Articles

Multivariate analysis of the bentho-demersal ichthyofauna along soft bottoms of the Eastern Atlantic: comparison between unvegetated substrates, seagrass meadows and sandy bottoms beneath sea-cage fish farms

By means of multivariate techniques, we studied: (1) the differences in the structure of bentho-demersal, non-cryptic, fish assemblages associated with unvegetated sandy substrates, vegetated meadows constituted by the seagrass Cymodocea nodosa and the bottoms under the influence of sea-cage fish farms; as well as (2) the persistence of these patterns with regard to different scales of spatial variability, across three islands of the Canarian Archipelago (Central East Atlantic). Our sampling strategy (involving three islands, with five locations per island, and two sites within each location) detected significant changes in the composition and structure of the fish assemblages between the three habitats. Fish assemblages associated with the unvegetated and vegetated bottoms were similar among the surveyed islands. In contrast, we observed a significant inter-island variability in the fish populations associated with the sea-cage fish farms. The presence of the sea-cage fish farms increased the overall fish abundance (184.8±49.8 ind 100 m−2) as compared to both the vegetated (38.8±9.7 ind 100 m−2) and unvegetated habitats (1.1±0.4 ind 100 m−2). Differences within and between the habitats were found to be associated with the relative abundance of a few fish species. The most abundant species were Xyrichthys novacula in the unvegetated bottoms and Diplodus annularis, Spondyliosoma cantharus and Mullus surmuletus on the seagrass meadows. Finally, we recorded an increase in the abundance of Heteroconger longissimus, Trachinus draco and Pagellus acarne in the bottoms beneath the sea-cage fish farms. These species, in addition to a group of large benthic chondrichthyes, were responsible for the differences between islands in the composition and structure of the demersal ichthyofauna beneath the sea-cage fish farms.

Capture of breeding and wintering shorebirds with leg-hold noose-mats

Abstract Development of effective trapping techniques is important for conservation efforts, as marking and subsequent monitoring of individuals is necessary to obtain accurate estimates of demography, movements, and habitat use. We describe a leg-hold noose-mat trap for capturing breeding and nonbreeding shorebirds. Using this method, we trapped 50 Piping Plovers (Charadrius melodus), 2258 Snowy Plovers (C. alexandrinus), 38 Killdeers (C. vociferus), and 64 Dunlins (Calidris alpina) in the western and southern United States. The trap was lightweight, making it easy to transport and set up. It was effective on unvegetated substrates at both coastal and inland sites and could be modified for a variety of habitats. Furthermore, this trap allowed researchers to target specific groups of birds including territorial individuals. Easy removal of birds from traps minimized handling time, stress, and injury.

A Comparison of Macroepifauna Among Vegetated and Unvegetated Habitats in a South Florida Estuary Using a Passive Sampling Gear

We compared abundance, richness, diversity, and community structure of macroepifauna among the seagrasses Halodule wrightii, Thalassia testudinum, and Syringodium filiforme, and unvegetated substrate in Tarpon Bay, Caloosahatchee River estuary, Florida. Sampling was conducted using wire-mesh minnow traps deployed over fifty-six 24-h periods from Jan. 1999 to Jan. 2000. A total of 36, 35, 28, and 28 species were identified from Halodule, Thalassia, Syringodium, and unvegetated samples, respectively. The gastropod Nassarius vibex was the most abundant species from Halodule and unvegetated substrate, whereas the pinfish (Lagodon rhomboides) was the most abundant species from Thalassia and Syringodium. Abundance of these codominant species varied seasonally throughout the study. For all taxa combined and for codominants, each seagrass contained greater averages than unvegetated substrate in each season. Seagrasses typically had higher average species richness and diversity than unvegetated substrate in each season. Results indicate that Tarpon Bay typifies subtropical estuaries in that its epifaunal community is dominated by few species, faunal abundances vary seasonally, and more organisms are found in seagrasses than in unvegetated areas. Our results serve as a foundation to compare against future research in an understudied system.

Comparison of predation pressure in temperate and subtropical seagrass habitats based on chronographic tethering

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 224:77-85 (2001) - doi:10.3354/meps224077 Comparison of predation pressure in temperate and subtropical seagrass habitats based on chronographic tethering Bradley J. Peterson1,*, Kip R. Thompson1, James H. Cowan Jr2, Kenneth L. Heck Jr2 1Department of Marine Sciences, University of South Alabama, Mobile, Alabama 36688, USA 2Dauphin Island Sea Laboratory, University of South Alabama, PO Box 369, Dauphin Island, Alabama 36528, USA *Present address: Department of Biological Sciences, Florida International University, University Park, Miami, Florida 33199, USA. E-mail: petersob@fiu.edu ABSTRACT: Chronographic tethering devices were used to investigate the effects of habitat type, prey type, tidal state and time of day on predation in the Damariscotta River, Maine (43°57¹N, 69° 35¹W) and in St. Joseph Bay, Florida (30°00¹N, 85°30¹W). Traditional tethering data (presence/absence) revealed that there were significant differences in the number of predation events between sites (p = 0.037) with more predation events occurring at the St. Joseph Bay site. In addition, there were significant differences in predation between prey types (p < 0.001). Regardless of site, significantly more Œsoft¹ bodied prey types (shrimp) were removed than Œhard¹ bodied prey types (brachyuran crabs). Separating the data by site revealed that there were significant differences between habitat types (seagrass interior, seagrass edge and unvegetated substrate) at the Damariscotta River site (p = 0.027), but not at the St. Joseph Bay site (p = 0.943). Using the time to predation recorded on the chronographic tethering devices, survival times were calculated. Based on prey survival time, comparisons were made between predation intensity in monotypic stands of the seagrasses Zostera marina L. (Damariscotta River) and Thalassia testudinum (St. Joseph Bay), and bare substrate and the grass edge at both study sites. There were no significant differences in survival time between habitats at the Damariscotta River site (p = 0874). However, the average prey survival time was significantly less for the seagrass bed edge at the St. Joseph Bay site than for the T. testudinum bed interior and unvegetated habitats (p = 0.019). In addition, the time to predation allowed the effect of tidal state and light versus dark on predation to be assessed. While neither tidal state nor light versus dark had any effect on predation at the Damariscotta River site (p = 0.096 and p = 0.481 respectively), at the St. Joseph Bay site, there were significantly more predation events during the day than at night (p < 0.001), regardless of tidal state (p = 0.664). The use of the chronographic tethering devices revealed that while there was no significant difference in the number of predation events or rate of predation at the St. Joseph Bay site, the interface (i.e. Œedge¹) between bare or unvegetated substrates and seagrass assemblages was associated with the highest predation risks. KEY WORDS: Chronographic tethering · Edge effects · Predation · Thalassia testudinum · Zostera marina Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 224. Online publication date: December 18, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Pre- and post-settlement factors as determinants of juvenile blue crab Callinectes sapidus abundance: results from the north-central Gulf of Mexico

We estimated juvenile abundance and predation potential for young-of-the-year (YOY) blue crabs Callinectes sapidus in upper- and lower-salinity marsh and submerged aquatic vegetation (SAV) 'nursery habitats' of Mobile Bay, Alabama, USA, and surrounding waters, and combined this information with previously published post-larval abundance data from these habitats to evaluate the relative roles of pre- and post-settlement events in determining the abundance of YOY crabs. We found little evidence for a significant relationship between megalopal supply and juvenile abundance, except shortly after a few very large, episodic pulses of postlarvae. However, even after large settlement events, in less than 14 d densities of YOY crabs had declined in a density-dependent manner to prior 'background' levels. These declines in density were probably due to high predation rates that produced as much as 95% mortality on a daily basis. As many previous studies had found, greater predation risk was recorded for crabs on unvegetated substrate than for those in SAV or marsh grass. Overall, losses to predators were high at our study sites compared to those reported in similar studies along the Atlantic coast of the United States. In both years of the study (1990 and 1991), the largest numbers of juvenile crabs were found in poly- and mesohaline SAV and salt marsh habitats. In Year 1, no significant correlations were observed between either marsh stem density or SAV vegetation biomass and YOY crab abundance, although at one location in the second year there was a significant positive correlation between the number of juveniles and mid-bay stem density. No significant differences in crab size (carapace width, CW) were found among locations in either year. In comparison with other, better-studied mid-Atlantic coast estuaries in the US (e.g., the Chesapeake and Delaware bays), very large numbers of megalopae (frequently 1 to 2 orders of magnitude greater) invade the Mobile Bay system, as well as other Gulf Coast estuaries. However, YOY abundance in marsh and SAV 'nursery habitats' was of the same order of magnitude as that reported for mid-Atlantic estuaries. We conclude that although postlarval supply was very large, post-settlement loss to predators was the dominant factor influencing blue crab population dynamics in our north central Gulf of Mexico study area.

SEXUAL SELECTION AGAINST HYBRIDS BETWEEN SYMPATRIC STICKLEBACK SPECIES: EVIDENCE FROM A FIELD EXPERIMENT.

Sexual selection against viable, fertile hybrids may contribute to reproductive isolation between recently diverged species. If so, then sexual selection may be implicated in the speciation process. Laboratory measures of the mating success of hybrids may underestimate the amount of sexual selection against them if selection pressures are habitat specific. Male F1 hybrids between sympatric benthic and limnetic sticklebacks (Gasterosteus aculeatus complex) do not suffer a mating disadvantage when tested in the laboratory. However, in the wild males choose different microhabitats and parental females tend to be found in the same habitats as conspecific males. This sets up the opportunity for sexual selection against male hybrids because they must compete with parental males for access to parental females. To test for sexual selection against adult F1 hybrid males, we examined their mating success in enclosures in their preferred habitat (open, unvegetated substrate) where limnetic males and females also predominate. We found significantly reduced mating success in F1 hybrid males compared with limnetic males. Thus, sexual selection, like other mechanisms of postzygotic isolation between young sister species, may be stronger in a wild setting than in the laboratory because of habitat-specific selection pressures. Our results are consistent with, but do not confirm, a role for sexual selection in stickleback speciation.

Factors Affecting Interannual Movements of Snowy Plovers

-We studied the interannual movements of 361 individually color-banded adult Snowy Plovers (Charadrius alexandrinus nivosus) at Great Salt Lake, Utah from 1990 to 1993. In northern Utah, Snowy Plovers nested in a dynamic environment; suitable breeding habitat declined by 50% at two study areas in four years. Male Snowy Plovers were more site faithful than females; 40% of males exhibited fidelity compared with 26% of females (P < 0.01). However, as the amount of available suitable nesting habitat declined, male site fidelity diminished, whereas female fidelity remained relatively constant. We found strong evidence that female site fidelity was affected by nesting success in the previous year. Females that nested unsuccessfully were less likely than successful females to exhibit site fidelity the following year; males did not exhibit this nest-success bias. In addition, unsuccessful females breeding at sites with high densities of nests tended to disperse the following year, whereas male site fidelity did not appear to be affected by either a study site's overall nesting success the previous year or a study site's nest density the previous year. Female avoidance of areas with high densities of nests may be an antipredator strategy. Snowy Plovers in northern Utah have biparental incubation duties, but only males care for broods. Familiarity with brood-rearing areas was one plausible explanation for male-biased fidelity. However, we could not eliminate an alternative hypothesis that both focal study sites represented scarce breeding areas due to the presence of freshwater, and male Snowy Plovers preferred to use the same areas rather than disperse. We propose that more landscape-level studies are needed to address questions concerning local and regional movement patterns. Received 22 August 1994, accepted 27 January 1995. MIGRATORY BIRDS face a dilemma when returning to their breeding grounds: Should an individual breed at the same site as the previous year, or disperse to a new area? Individuals faithful to their previous breeding grounds presumably have some knowledge concerning the distribution of essential resources, predation risks, and potential mates (Shields 1982, Oring and Lank 1984, Gratto et al. 1985, Wiens and Cuthbert 1988). However, dispersing individuals have the opportunity to find a new territory or another mate, either of which could change reproductive output (Ollason and Dunnet 1978, Cuthbert 1988). Birds typically exhibit female-biased natal and breeding dispersal (i.e. females disperse farther and more often; Greenwood 1980, Greenwood and Harvey 1982; but see Oring and Lank 1984). The proximate and ultimate causes for sex-biased dispersal are still 3 Present address: Department of Natural Resources Science, University of Rhode Island, Kingston, Rhode Island 02881, USA. E-mail: ppaton@uriacc.uri.edu under debate (Bollinger and Gavin 1989, Anderson et al. 1992). Previous studies of avian populations have found a strong tendency for individuals to exhibit a higher degree of site fidelity after a successful nesting attempt than after an unsuccessful one (Greig-Smith 1982, Oring and Lank 1982, Gratto et al. 1985, Cuthbert 1988, Part and Gustafsson 1989, Hepp and Kennamer 1993; but see Haig and Oring 1988a, Schamel and Tracy 1991). In addition, older birds are often more likely to return to the same area than younger birds (Oring et al. 1983, Oring and Lank 1984, Thompson and Hale 1989, Montalvo and Potti 1992). Site quality, measured in terms of the number of young fledged from an area, also can affect return rates, with greater return rates to higher-quality sites regardless of an individual's nesting success the previous year (Bollinger and Gavin 1989, Montalvo and Potti 1992). Charadrius plovers provide an excellent opportunity to investigate proximate causes of dispersal. Birds in this genus often forage on unvegetated substrates (making it easy to see

Population regulation of blue crabs Callinectes sapidus in the northern Gulf of Mexico:postlarval supply

Author(s): Morgan, SG; Zimmer-Faust, RK; Heck, KL; Coen, LD | Abstract: Larval supply and early postsettlement mortality are crucial to the maintenance of most marine populations. Larval supply may be especially important in regulating populations of blue crabs Callinectes sapidus because many larvae could be lost during long migrations between estuaries and offshore waters. We examined the relative contributions of differential transport and habitat preferences of megalopae (postlarvae) to 5 potential nursery habitats in Mobile Bay and Mississippi Sound, Alabama, USA. Settlement in each habitat was determined daily during summer and fall for 2 yr and was related to sea surface temperature, wind stress, tides, current velocity and the lunar and tidal amplitude cycles. In 1990, most megalopae settled when winds blew megalopae onshore and tidal amplitudes were minimal. In 1991, onshore winds were light and infrequent, and megalopae settled even more densely during minimum amplitude tides. Semidaily collections of megalopae at dusk and dawn revealed that most megalopae recruit to estuaries during nocturnal flood tides, which only occur during summer and fall in this diurnal tidal regime. Thus, onshore winds facilitated transport of megalopae into estuaries episodically, but megalopae recruited regularly and abundantly during nocturnal minimum amplitude flood tides even when onshore winds were light or absent. Most megalopae were collected in the lower bay although some megalopae settled at the head of the estuary 50 km away from the baymouth. Megalopae required about 2 d to travel this distance as they became increasingly competent to settle from the plankton. Physiological tolerances may explain why most larvae settled in high salinity waters, but differences in current regimes may best explain differences in settlement among sites in the lower bay. Several types of substrate were transplanted from nursery habitats to a sandy area at the baymouth to determine whether megalopae prefer to settle in seagrass beds, marshes or unvegetated substrate once they arrive at a site. Megalopae strongly preferred to settle on vegetation, but it remains unclear whether or not they discriminate among plant types. Thus, passive delivery and habitat preferences of megalopae both may determine the initial distributions of juvenile blue crabs.

Variability in growth rates of juvenile fishes in different estuarine habitats

Estuaries are vital nursery grounds for many marine fishes. During initial estuarine residence, juvenile fishes presumably benefit greatly from rapid growth, which can diminish susceptibility to size-selective predators. To measure the degree of variability in growth rates across different estuarine sites and habitats. I conducted caging experiments with 3 species (Pseudopleuronectes americanus, Tautoga onitis, and Gobiosoma bosci) at 4 sites in New Jersey, USA, estuaries. Two sites were in an estuary with dense eelgrass Zostera marina beds (Little Egg Harbor) and 2 were in an estuary lacking eelgrass but supporting patchy accumulations of the macroalgae Ulva lactuca (Great Bay). Experiments were conducted in vegetated (either Zostera or Ulva) and unvegetated habitats at each site. Relative differences in growth rates among the 4 sites and between the 2 habitats were generally maintained across experiments withln species, suggesting a consistency in foraging value. For all 3 species, growth .was highest at a Great Bay site and generally lowest at a httle Egg Harbor site. The presence of vegetation (either Zostera or Ulva) was beneficial to growth only for T onitis. Observed patterns in water temperature, sediment structure, and food availability were potential factors effecting the observed differences in growth rates. Natural densities of the 3 fish species were quantified and compared to the results of growth experiments to determine if there were trade-offs between rapid growth and other attributes of habitat quality. P. americanus and T onitis were more abundant in areas supporting faster growth, but the complete absence of tautog from unvegetated substrates suggested some degree of habitat avoidance. A clear trade-off between habitat selection and foraging quality occurred for G. bosci; gobies were most abundant inside eelgrass beds, which supported the poorest growth rates.

A comparison of eelgrass, sea lettuce macroalgae, and marsh creeks as habitats for epibenthic fishes and decapods

Densities of epibenthic fishes and decapod crustaceans (excluding xanthids and pagurids) were quantified with daytime throw trap sampling in shallow water habitats of New Jersey estuaries. We compared eelgrass ( Zostera marina), sea lettuce macroalgae ( Ulva lactuca), unvegetated sand/mud substrates adjacent to these vegetation types, and saltmarsh creeks. The highest total density of fishes occurred in marsh creeks, due primarily to high abundances of Menidia menidia. The highest total decapod density was also in a marsh creek, but only slightly surpassed the density in Zostera. Results of apriori comparisons tests for individual species demonstrated that vegetation (either Zostera or Ulva) was superior in quality (based on fish and decapod densities) to adjacent unvegetated substrates. Sites with Zostera as the dominant vegetation had higher densities of most fish species than sites with Ulva as the dominant vegetation, but only one decapod, Hippolyte pleuracanthus, was more abundant at eelgrass sites. Ulva lactuca, therefore, was an important habitat in areas lacking Zostera marina; for the decapods the two vegetation types were comparable in habitat quality, but for fishes Ulva did not provide an equivalent substitute for Zostera. Marsh creeks supported very high densities, but only for a few species that were also common in other habitats. Comparison of recruitment patterns suggested many species do not begin exploiting these estuarine habitats until relatively late in the summer, perhaps as result of peak spawning in mid-summer.

Fishes and Decapod Crustaceans of Cape Cod Eelgrass Meadows: Species Composition, Seasonal Abundance Patterns and Comparison with Unvegetated Substrates

Bimonthly trawl samples from eelgrass and nearby unvegetated areas on Cape Cod, Massachusetts, showed greater species richness in eelgrass meadows relative to unvegetated areas, and greater summer abundance in vegetation for decapod crustaceans and fishes. The composition of eelgrass-associated decapods and fishes was dominated by cold-water taxa and was strikingly different from that of the better studied eelgrass meadows of the mid-Atlantic coast. Four of the eight decapod species collected, including the second and third most abundant taxa, do not even appear in collections reported from Chesapeake Bay eelgrass meadows. Similarly, 10 of the 22 fish species taken, including the first and sixth most abundant species, are not reported from Chesapeake Bay eelgrass samples. Cape Cod eelgrass beds seem to play a nursery role for several commercially important fish species, although the nursery function is less obvious than in previously studied mid-Atlantic eelgrass meadows.

Sedimentation in a salt marsh-tidal channel system, southern New Jersey

The tidal channels in a low-mesotidal back-barrier salt marsh of southern New Jersey are comprised of large through-flowing (TF) channels which separate marsh islands and smaller dead-end (DE) channel networks on the marsh islands. TF channels have at-a-station hydraulic geometries similar to other tidal marsh channels and DE channels have hydraulic geometries similar to rivers. The silt-clay (mud) content of DE channel perimeters is high (averaging 78%) and width:depth ratios are low (5–21) in contrast to TF channels with higher width:depth ratios (34–129) and lower contents of silt-clay at the perimeters (averaging 9%). Three distinct environments are associated with the channels: (1) a subtidal thalweg region, (2) an intertidal channel-margin flat (vegetated and unvegetated), and (3) a channel-margin marsh (levee and back-levee). Interchannel sediment trends for each environment proceeding from large TF to small DE channels are: (1) a decrease in grain size of the sediments from sands to silts and clays, (2) increase in total organic matter with increasing mud content, and (3) a change from predominantly physical to biological sedimentary structures. Intrachannel trends at individual channel cross sections comprise a decrease in grain size of the sediments with distance from the thalweg region and increasing total organic matter content from the thalweg to the marsh. TF and DE channels have different origins. TF channels evolved from flood tidal delta channels that became fixed in position with the stabilization of delta shoals by vegetation and cohesive bank-margin sediments. DE channel networks, on the other hand, originated as ebb drainage patterns on the unvegetated substrates of marsh islands. They also became fixed in position by vegetation and cohesive bank margins. The two mechanisms of channel formation have resulted in little to no migration through time after this stabilization.

Influences of aquatic macrophytes on invertebrate community structure, guild structure, and microdistribution in streams

The aquatic macrophytes Ranunculus aquatilis and Rorippa nasturtium-aquaticum were transplanted into substrate trays and placed in a stream alongside unvegetated substrate. Macrophytes were observed to have significant effects on 1) invertebrate community structure, 2) guild structure, and 3) microdistribution. 1) Significantly higher taxa richness and community abundances were associated with macrophytes. 2) Significantly higher abundances of shredder, scraper, and predator guilds were associated with macrophytes in fall, and all guilds had higher abundances in macrophytes in spring. However, guild frequency distributions did not differ among habitats except in spring. 3) Enallagma, Gammarus, Gyraulus, Physa, and Pisidium exhibited a strong association with macrophytes, while Hydropsyche, Simulium, Baetis tricaudatus, Glossosoma velona, and Helicopsyche borealis appeared to avoid them. A strong correlation appeared to exist between current velocity preferences of these taxa and their selection or avoidance of vegetated habitat. Thus, the effect of macrophytes in reducing current velocities appeared to be the most important influence on invertebrate microdistribution. However, macrophytes also increase physical heterogeneity and their large surface areas benefit invertebrate community abundances by creating additional living spaces in the water column where none exist above unvegetated substrate.

The Nursery Role of Seagrass Meadows in the Upper and Lower Reaches of the Chesapeake Bay

A two-year trawling and gill-netting study of vegetated and unvegetated bottoms near Parson’s Island, Maryland and near the mouth of the York River, Virginia was carried out to assess the nursery function of submerged vegetation for populations of fishes and decapod crustaceans in the Chesapeake Bay. Results revealed that vegetated bottoms supported substantially larger numbers of decapods, but not fishes, than unvegetated substrates. The lower Bay grassbed was an important nursery area for juvenile blue crabs, although neither of the grassbeds functioned as a nursery for commercially or recreationally valuable fishes. Our results suggest that: (1) further decreases in lower Bay Seagrass biomass would result in reduced numbers of adult blue crabs, but should not substantially affect populations of valuable fish species; (2) additional decreases in Upper Bay submerged vegetation should not produce dramatic change in the population sizes of either adult blue crabs or fishes.

The effects of aquatic macrophytes on the stream microenvironment

The effects of aquatic macrophytes on various aspects of the stream microenvironment were observed and compared to conditions found in unvegetated substrate. Specifically, factors of micro-current velocity, substrate surface area, fine particle sediment deposition, detritus deposition and attached diatom availability were examined. Macrophytes were transplanted into substrate trays and microenvironment conditions found within them were compared to conditions found in adjacent trays containing unvegetated substrate. Macrophytes were found to alter flow characteristics and create reduced current velocities near the substrate. They also provided significantly more substrate surface area, more fine particle sediments and more detritus than unvegetated substrates. Macrophytes supported significantly lower attached diatom biomass per m 2 in moderate current regions than stones, but in slow water they may support higher biomasses, because sedimentation adversely affects diatom populations on unvegetated substrate. The possible ecological significance of these effects is discussed.