Reproduction in the seagrass Zostera novazelandica on intertidal platforms in southern New Zealand

This study investigates the reproductive periodicity and reproductive output of the seagrass Zostera novazelandica on two intertidal reefs. Peak numbers of flowering shoots occurred during March (late summer) of two years at both sites and no flowering shoots occurred during the winter months of July to September. There were greater numbers of flowering shoots in seagrass patches in the low intertidal zone (up to an average of 55 per 0.1 m²) compared to the middle (up to 20 per 0.1 m²) and upper (up to 9 per 0.1 m²) zones, and about three times greater reproductive output in patches associated with tidepools compared to those not bordering tidepools. The average number of inflorescences per shoot was 3.1 (±0.25) at one site vs 1.2 (±0.08) at the other, and showed a progressive decrease from the lower shore to the upper shore. Patches associated with tidepools had twice the number of inflorescences per shoot (2.8 ± 0.24) than patches not bordering tidepools (1.5 ± 0.16). The number of flowering shoots was highly correlated with leaf width, leaf length, and ramet density, while the leaf-area index decreased from the lower shore to the upper shore. The reproductive effort of plants, as measured by the percent biomass invested in flowering shoots during peak reproduction, was significantly different between sites, tidepool associations, and shore level. For all the variables measured, there was considerable spatial variation, with significant interaction terms between most factors investigated. In laboratory experiments, more inflorescences were produced at light intensities of 30 and 300 μE m⁻² s⁻¹ than at 100 μE m⁻² s⁻¹. At a salinity of 17‰, 1.5 × the number of flowers was produced than at 33‰, while none was formed at 70‰. Plants cultured at 5 °C had about three times the number of inflorescences than those at 15 °C, while none was formed at 25 °C. Received: 25 June 1997 / Accepted: 24 September 1997     

Fish assemblages in seagrass beds are influenced by the proximity of mangrove forests

Mangrove forests and seagrass beds frequently occur as adjacent habitats in the temperate waters of southeastern Australia. At low tide when fish cannot occupy mangroves they might utilise adjacent habitats, including seagrass. We first sampled small fish from seagrass beds close to and far from mangroves in the Pittwater estuary, NSW, Australia. Seagrass beds close to mangroves had a greater density of fish species than beds far from mangroves (close: mean 16.0 species net⁻¹, SE 1.0; far: 13.2, 1.3; P < 0.05). In particular, juvenile fish were in greater densities near to than far from mangroves (close: 5.3, 0.4; far: 3.1, 0.4; P < 0.05). We then sampled the mangrove forests during the high tide and seagrass beds during the low tide, in beds along a continuum of distances from mangroves. Multivariate analysis showed that fish assemblages differed with distance from mangroves, and the differences were attributed to the composition of the fish assemblage (i.e. presence/absence of fish species), not the abundances of individual species. In particular, fish that utilise mangrove forests at high tide were found in greater species densities and species richness in seagrass nearer to mangroves. A negative relationship was found between the density of mangrove-utilising fish species and the distance of the bed from mangroves (R ² = 0.37, P < 0.05). This confirms the important connectivity between mangroves and seagrass for fish in temperate Australian waters.     

Are mangroves and seagrasses sources of organic carbon for penaeid prawns in a tropical Australian estuary? A multiple stable-isotope study

The vertical distribution of the larvae of shelf-dwelling fish species that spawn in the NW Mediterranean Sea in spring was studied in relation to environmental data. Two sampling cycles were carried out at fixed stations on the continental shelf in May and June 1992. Three patterns of larval vertical distribution for the various taxa represented in the samples were observed. The larvae of most species (e.g. Boops boops, Diplodus sargus) were mainly located in the surface layer (10 m), others (e.g. Arnoglossus sp.) had broader distributions in the upper 40 m of the water column, and but a few (e.g. Gobiidae) were present in large concentrations at greater depths. The vertical distribution patterns of the various species showed no variations, despite high hydrographic variability during the study. The vertical distribution of only a few species (e.g. Arnoglossus sp., Crystallogobius linearis and Engraulis encrasicolus) varied over the diel cycle. The possible influence of the vertical distribution of fish larvae on their horizontal distribution patterns is discussed. Received: 10 March 1997 / Accepted: 4 April 1997     

Relative importance of benthic microalgae,phytoplankton, and mangroves as sources of nutrition for penaeid prawns and other coastal invertebrates from Malaysia

Carbon, sulfur, and nitrogen stable isotope ratio techniques were used in 1989 and 1990 to evaluate the relative importance of algae and of mangrove detritus in the nutrition of two penaeid prawn species on the west coast of Peninsular Malaysia. Mangrove detritus was found to contribute to the nutrition of juvenile Penaeus merguiensis de Man living within tidal creeks, but not to adult P. merguiensis and juvenile and adult Parapenaeopsis sculptilis (Heller) captured offshore. Results from radiotracer feeding studies, with refractory ¹⁴C mangrove lignocellulose as the food source, indicated that juvenile P. merguiensis from tidal creeks assimilated mangrove carbon with an efficiency of 13.4%. This did not differ significantly (P=0.05) from the assimilation efficiencies of juvenile and adult P. sculptilis living offshore (10.0 and 10.9%, respectively); these values were significantly higher (P<0.001) than for adult P. merguiensis (2.1%). Thus, the differential contribution of mangrove material to the nutrition of prawns in the tidal creeks and offshore was not related to differences in the prawn's ability to utilize detritus. Instead, our analysis of C stable isotopes in sediments indicated that mangrove detritus was generally more abundant within the tidal creeks than offshore. Juvenile prawns in the tidal creeks may also utilize mangrove material indirectly by feeding on small detritivorous invertebrates. Stable isotope analyses suggest that benthic microalgae constitute the other major dietary component for prawns living in tidal creeks. Prawns offshore were utilizing mainly phytoplankton-based material.     

Effects of conversion of mangroves into gei wai ponds on accumulation,speciation and risk of heavy metals in intertidal sediments

Environmental Geochemistry and Health - Mangroves are often converted into gei wai ponds for aquaculture, but how such conversion affects the accumulation and behavior of heavy metals in sediments...     

Annual cycle of biomass of a threatened population of the intertidal seagrass Zostera japonica in Hong Kong

The phenology and primary productivity of a population of Zostera japonica (Aschers. & Graebn.) threatened by the construction of Hong Kong's new international airport were studied over a 12-month period. The need to conserve the population, and the small leaf size of Z. japonica rendered traditional destructive or marking techniques inapplicable for percentage cover and biomass estimation. A nondestructive method based on image analysis techniques was therefore devised for repeated estimation of percentage cover, biomass and leaf area index. This technique, which involved random quadrat sampling, photographic recording and image analysis, was able to provide data on the three parameters with acceptable precision and was cost-effective in the field. Z. japonica demonstrated a strongly seasonal cycle of vegetative growth, with different patterns for leaf density (peak in March) and overall bed area (peak in June). Total (above- and below-ground) net primary productivity was estimated at between 344 and 688 g AFDW m⁻² yr⁻¹. Percentage cover of Z. japonica was negatively correlated with total suspended solids (TSS) in the water column while total bed area was negatively correlated with water salinity. Increased sedimentation associated with the new airport project was identified as one important factor affecting the growth of the seagrass, as TSS reached the high level of ≈1 g DW l⁻¹ during the first half of the study period. Sediment traps set in the beds also recorded potential sedimentation rates at between 2.89 and 14.5 mg cm⁻² d⁻¹. This high turbidity resulted in a sharp decrease in the density of Clithon spp., the dominant grazers of epiphytic algae on Z. japonica. Effects of sedimentation and shading on growth of Z. japonica were investigated by field manipulative experiments. Experimental increase of sedimentation rate and shade both resulted in larger decreases in percentage cover and above-ground AFDW compared with the control. Received: 3 March 1997 / Accepted: 14 March 1997     

Acute toxicity of copper sulphate to fresh water prawns

Fresh water prawns, Macrobrachium lamarrei and Macrobrachium dayanum (Crustacea-Decapoda) were subjected to static bioassay tests to ascertain the LC50 values of copper sulphate. The 24, 48, 72 and 96 hr LC50 values of copper sulphate for M. lamarrei were 0.38, 0.361, 0.343 and 0.300 mg/l and for M. dayanum were 1.634, 0.988, 0.532 and 0.418 mg/l respectively. Behavioral responses and LC50 values indicate that M. lamarrei were more sensitive to copper sulphate than M. dayanum.     

The value of small size: loss of forest patches and ecological thresholds in southern Madagascar.

Many services generated by forest ecosystems provide essential support for human well-being. However, the vulnerability of these services to environmental change such as forest fragmentation are still poorly understood. We present spatial modeling of the generation of ecosystem services in a human-dominated landscape where forest habitat patches, protected by local taboos, are located in a matrix of cultivated land in southern Madagascar. Two ecosystem services dependent on the forest habitats were addressed: (1) crop pollination services by wild and semidomesticated bees (Apoidea), essential for local crop production of, for example, beans, and (2) seed dispersal services based on the presence of ring-tailed lemurs (Lemur catta). We studied the vulnerability of these ecosystem services to a plausible scenario of successive destruction of the smallest habitat patches. Our results indicate that, in spite of the fragmented nature of the landscape, the fraction of the landscape presently covered by both crop pollination and seed dispersal services is surprisingly high. It seems that the taboo system, though indirectly and unintentionally, contributes to upholding the generation of these services by protecting the forest patches. Both services are, however, predicted to be very vulnerable to the successive removal of small patches. For crop pollination, the rate of decrease in cover was significant even when only the smallest habitat patches were removed. The capacity for seed dispersal across the landscape displayed several thresholds with habitat patch removal. Our results suggest that, in order to maintain capacity for seed dispersal across the landscape and crop pollination cover in southern Androy, the geographical location of the remaining forest patches is more crucial than their size. We argue that in heavily fragmented production landscapes, small forest patches should increasingly be viewed as essential for maintaining ecosystem services, such as agricultural production, and also should be considered in the ongoing process of tripling the area of protected habitats in Madagascar.     

Intercontinental tests of the effects of habitat patch type on the distribution of chitons within and among patches in intertidal boulder field landscapes

Patchy distributions within landscapes may be caused by migration in response to different types of habitat patches. Intertidal boulder fields are landscapes in which boulders are discrete habitat patches, often with chitons attached to their under-surfaces. Chiton densities and associations with patch edges differed between boulders overlying coarse- versus fine-sediment types, with greater densities occurring over fine sediments. We tested whether adult migration caused between-boulder distributions by measuring immigration and emigration following experimental replacement of coarse sediments with fine sediment under boulders. We also assessed whether the manipulations altered chiton positions relative to patch edges, and large-scale generality was tested by including data from two continents. The manipulations did not influence the association of chitons with edges or amounts of emigration, but chitons did display positive density-dependent immigration that mirrored their distributional patterns, indicating the importance of immigration. Strikingly, all results were consistent between continents despite involving different species and even genera of chitons. By using boulder fields as a small-scale, easily manipulated landscape, we show that, even in sedentary organisms, patchy distributions within landscapes can be caused by migration alone, without the need to invoke mortality or larval recruitment.     

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns (Penaeus semisulcatus): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m² in basal area, 1 m high, with 100 µm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m^-2) on a high biomass seagrass bed (about 70 g m^-2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m^-2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week^-1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week^-1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week^-1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.     

Electrophoretic studies on muscle myogens of some penaeid prawns

The muscle myogen patterns of some penaeid prawns studied by electrophoresis in cellulose acetate membrane are presented. Each pattern was species specific and appears to be independent of sex. The results suggest that muscle myogen patterns could be useful in taxonomic studies of penaeid prawns. Specimens preserved in phenoxyethanol were compared with fresh specimens. Preservation resulted in some losses of protein zones and, in cases where sufficient zones are preserved, the method could be used to advantage for solving taxonomic problems. Mr. Lim unfortunately passed away in November 1969.     

The nitrogen supply to intertidal eelgrass (Zostera marina)

The uptake of ammonium and nitrate by eelgrass (Zostera marina L.) was studied in two-compartment chambers. The plants were collected in 1992 from a population growing on a tidal flat in the S.W. Netherlands. They were incubated under conditions which reflected field conditions; this implied the use of natural seawater and sediment porewater as incubation media. In all six experiments, carried out over the course of a major part of the growing period (from July to the end of September), ammonium appeared to be much more important as a source of nitrogen than nitrate. The largest part was taken up by the leaves: uptake of ammonium by the leaves accounted for 68 to 92% of total plant nitrogen uptake. The uptake of nitrogen compounds by the root-rhizome system represented only 4 to 30% of total plant uptake. Thus, at least during flood tide, the leaves play the major role in nitrogen uptake in this intertidal population. During ebb tide, most of the plants are submerged in very shallow tidepools. It is suggested that during this phase of the tidal cycle, influx of porewater ammonium into the tide-pool water may enable the leaves to exploit local sediment resources.     

Use of a seagrass residency index to apportion commercial fishery landing values and recreation fisheries expenditure to seagrass habitat service

Where they dominate coastlines, seagrass beds are thought to have a fundamental role in maintaining populations of exploited species. Thus, Mediterranean seagrass beds are afforded protection, yet no attempt to determine the contribution of these areas to both commercial fisheries landings and recreational fisheries expenditure has been made. There is evidence that seagrass extent continues to decline, but there is little understanding of the potential impacts of this decline. We used a seagrass residency index, that was trait and evidence based, to estimate the proportion of Mediterranean commercial fishery landings values and recreation fisheries total expenditure that can be attributed to seagrass during different life stages. The index was calculated as a weighted sum of the averages of the estimated residence time in seagrass (compared with other habitats) at each life stage of the fishery species found in seagrass. Seagrass‐associated species were estimated to contribute 30%–40% to the value of commercial fisheries landings and approximately 29% to recreational fisheries expenditure. These species predominantly rely on seagrass to survive juvenile stages. Seagrass beds had an estimated direct annual contribution during residency of €58–91 million (4% of commercial landing values) and €112 million (6% of recreation expenditure) to commercial and recreational fisheries, respectively, despite covering <2% of the area. These results suggest there is a clear cost of seagrass degradation associated with ineffective management of seagrass beds and that policy to manage both fisheries and seagrass beds should take into account the socioeconomic implications of seagrass loss to recreational and commercial fisheries.     

Effects of proximity to an offshore hard-bottom reef on infaunal abundances

Rock outcrops represent an extensive, commercially important habitat along the southeast coast of North America, supporting large abundances of algae, invertebrates and fish. These rock outcrops have often been viewed as largely self-supporting systems, but some studies have suggested that the fish and invertebrate predators that use the ledges for shelter are strongly dependent on food from adjacent, soft-bottom habitats. We examined benthic macrofaunal abundances along two 75-m transects away from a rock ledge near Wrightsville Beach. North Carolina (depth ca. 30 m) in July 1990 and April 1991. The July 1990 samples indicated significantly higher abundances of total infauna, and of polychaetes, bivalves, isopods, and scaphopods, at a distance of 75 m from the rock ledge. In April 1991, when macrofaunal abundances were generally higher than in the previous summer, distributions of major taxa with distance from the rock ledge were more variable. Caging studies on macrofaunal abundances 10 m from the ledge, and video observations of fish abundances at 10 and 75 m from the ledge, provide support for a trophic link between the rock ledge and the adjacent soft-bottom communities. Our studies suggest that there are potentially important indirect effects of predator-prey interactions among the rock ledge-associated predators and soft-bottom prey.     

基于景观连接度的斑块分级的尺度效应

认识到尺度对景观格局分析结果的影响,生态学家们越来越多地进行尺度效应的相关研究.已有的尺度效应研究多注重描述景脱格局指数对尺度变化的响应,这只能对景观的状态进行描述.无法辨别那些对景观功能具有重要意义的关键景观元素.文章选取8个景观连接度指数.通过改变最小制图单元和物种的搜索范围,研究尺度对斑块分等定级的影响,并据此确定各连接度指数的尺度敏感性.结果表明:景观中斑块的相对重要性随尺度变化而变化,其变化程度因指数而异.其中,AWF为尺度效应最不明显的指数,PC和IIC次之,其余指数(NL,NC,LCP,H和F)的尺度效应均十分显著;在衡量景观的连通性时,除IIC外的其他所有指数的尺度效应均十分显著.由此说明,根据连接度指数确定的斑块相对重要性的尺度敏感性与该指数本身的尺度敏感性无关.     

The use of clear-water non-estuarine mangroves by reef fishes on the Great Barrier Reef

Within the tropics, mangroves and coral reefs represent highly productive biomes. Although these habitats are often within close proximity, the role and importance of mangrove habitats for reef fish species remains unclear. Throughout the Indo-Pacific, reef fish species appear to have few links with estuarine mangrove habitats. In contrast, clear-water non-estuarine mangrove habitats throughout the Caribbean support many reef fish species and may be fundamental for sustaining reef fish populations. But how important are clear-water non-estuarine mangroves for reef fishes within the Indo-Pacific? Using visual surveys during diurnal high tide, the fish assemblages inhabiting clear-water mangrove and adjacent reef habitats of Orpheus Island, Great Barrier Reef, were recorded. Of the 188 species of fishes that were recorded, only 38 were observed to inhabit both habitats. Of these, only eight were observed more than five times within each habitat. These observations provide little indication that the clear-water mangroves are an important habitat for reef fish species. In addition, although based on just a 3-month survey period, we found little evidence to suggest that these areas are important nurseries for reef fish species. The clear-water mangroves of Orpheus Island may, however, provide an additional foraging area for the few reef fish species that were observed to utilize these habitats during high tide. The difference in the importance of clear-water mangroves for reef fishes within this study compared with clear-water mangrove counterparts within the Caribbean is surprising. Although only preliminary, our observations would support suggestions that the patterns reflect the different hydrological characteristics and evolutionary histories of these two biogeographic regions.