Factors affecting the distribution, abundance and diversity of fishes of small, soft-substrata tidal pools within Moreton Bay, Australia

Clearance rates of Limnoperna fortunei (Bivalvia) were investigated in laboratory experiments using monocultures of the alga Chlorella vulgaris. Experimental conditions included two mollusc sizes (15 and 23 mm), and three water temperatures (15, 20 and 25 °C) covering the normal seasonal range in the lower Paraná river and Río de la Plata estuary. Filtration rates obtained were, for the larger mussels: 9.9, 13.1 and 17.7 ml mg tissue dry weight^–1 h^–1 at 15, 20 and 25 °C, respectively; and for the smaller ones: 17.7, 20.8 and 29.5 ml mg^–1 h^–1. Differences between sizes and between temperatures (except 15 vs. 20 °C) were statistically significant. In absolute terms larger animals have higher clearance rates, but as a function of body mass smaller individuals feed more actively. Within the range of experimental values used, filtration rates were positively associated with water temperature. These clearance rates (125–350 ml individual^–1 h^–1) are among the highest reported for suspension feeding bivalves, including the invasive species Dreissena polymorpha, D. bugensis and Corbicula fluminea. High filtration rates, associated with the very high densities of this mollusc in the Paraná watershed (up to over 200,000 ind m^–2) suggest that its environmental impact may be swiftly changing ecological conditions in the areas colonized.     

Growth and age structure of the swan mussel Anodonta cygnea (L.) at different depths in lake Mattsee (Salzburg, Austria)

Unionid clams were collected at 1–2 m, 3–4 m and 6–7 m depth in lake Mattsee, a moderately mesotrophic lake, to investigate the effect of depth on clam growth and age structure. No significant differences in age structure of Anodonta cygnea were found (p=0.65). Three and ten years old clams were present at all depths, but in different percentages. Whereas at 1–2 m 13.3% of the collected clams were <4 years old, this percentage was 4.4% at 6–7 m and 7.1% at 3–4 m. A greater percentage (6.7%) of older mussels (9, 10 years) were collected at 6–7 m than at 1–2 m (2.2%). Growth declined with depth. Total length at a given age of clams at 1–2 m and 3–4 m did not differ (p=0.54), whereas differences were significant between clams at 1–2 m and 6–7 m (p<0.05) as well as between 3–4 m and 6–7 m (p<0.05). The Growth constant k was highest at 1–2 m depth.     

Secondary production of introduced Asiatic clam,Corbicula fluminea,in a central Arizona canal

Asiatic clam, Corbicula fluminea, is often a serious pest where introduced and established outside its native range. This includes many canals of the southwestern U.S.A. Because of its potential role in organic matter processing, nutrient dynamics, and sedimentation, the clam is an important component of the benthic community and of the aquatic ecosystems which it inhabits. To better understand the ecology of Corbicula in canals, secondary production and life history of an introduced population of clams in a small, earthen canal in the Phoenix metropolitan area, central Arizona, were studied over a 12-month period in 1981–1982. Clams had a monthly mean density of 2 255 m^–2, single annual spawning, 2 year life span, and overlapping cohorts. Annual secondary production (size-frequency method) was 25.62 g DM m^–2 and cohort production (composite Allen curve) was 29.32 g DM m^–2, with turnover ratios of 2.70 and 5.81, respectively. These are among the highest known single-species estimates for molluscs, and illustrate the importance of clams in these unique southwestern aquatic systems.     

Impact of the infaunal Manila clam, <Emphasis Type="Italic">Ruditapes philippinarum</Emphasis>, on sediment stability

The aim of this study was to examine the impact of bioturbation by the Manila clam, Ruditapes philippinarum, on sediment stability. A laboratory benthic annular flume system (AFS) was deployed to evaluate the relationship between sediment stability of a subtidal mudflat and density of the infaunal clam under the influence of different current velocities. There was a significant correlation between mean erosion rate and current velocities in all treatments with clams (p < 0.001). There was also a significant correlation between mean erosion rate and R. philippinarum density (p < 0.001), reflecting bioturbation-enhanced sediment erosion. The effects of clam density on sediment erodability were more marked at the lower current velocities. In the control, the critical erosion velocity (Ū_crit) was about 32 cm s⁻¹. With increasing R. philippinarum density, Ū_crit decreased down to the minimum value of about 20 cm s⁻¹ at a density of 206 clams m⁻². This study demonstrated that the burrowing activity of R. philippinarum reduces sediment stability, particularly at relatively low current velocities (25 cm s⁻¹) and at densities below those found in the clam cultivation areas within the Sacca di Goro lagoon.     

Comparative bioenergetics of permanent and temporary pond populations of the freshwater clam,Musculium partumeium (Say)

The population energetics of a temporary and a permanent pond population of Musculium partumeium in Southwest Ohio were studied. In the permanent pond (surface area = 396 m², maximum depth = 0.7 m) the population was bivoltine and iteroparous whereas in the temporary pond (surface area = 1042 m², maximum depth = 0.9 m) the population was usually univoltine and semelparous.Growth and biomass were assessed as total organic carbon and total nitrogen to provide estimates of productivity and seasonal changes in C:N for each generation. Productivity (non-respired assimilation = growth + reproduction; N-R.A. = G + R) was 6939 mgC·m^–2·a^–1 (3858 and 3353 mgC·m^–2·a^–1 for each generation) and 1661 mgC·m^–2·a^–1 for the permanent and temporary pond populations respectively. The average standing crop biomass (B) was 606.8 mgC·m^–2 (357.5 and 249.3 mgC·m^–2 for each generation) and 231.9 mgC·m^–2 with overall productivity: biomass ratios of 11.4 and 7.2 for the permanent pond and temporary pond populations respectively.Respiration rates were converted to carbon equivalents (respired assimilation = R.A.) and used to evaluate the components of total assimilation (T.A. = R.A. + N-R.A.) and the efficiency of partitioning this energy to N-R.A. for G and R. When expressed as a percentage, the production efficiencies (100 × N-R.A.:T.A.) were 50.4 and 62%, and the reproductive efficiencies (100 × R:N-R.A.) were 26.4 and 18% for the permanent and temporary pond populations respectively. The reproductive efficiencies for populations of these viviparous clams are greater than those for most oviparous molluscs.The comparative information on the energetics of these populations does not completely fit any theoretical consideration of reproductive effort or life-history strategy. These data are discussed in relation to selection for population success in temporary ponds.Funded in part by grants to Albert J. Burky from the Ohio Biological Survey and the University of Dayton Research CouncilFunded in part by grants to Albert J. Burky from the Ohio Biological Survey and the University of Dayton Research Council     

The Effects of Food Concentration and Quality on the Feeding Rates of Three Size Classes of the Greenshell<Superscript>TM</Superscript> Mussel, <Emphasis Type="Italic">Perna canaliculus</Emphasis>

In order to calibrate carrying capacity models, investigations were conducted into the effects of food concentration and food quality on the feeding rates of small (25–50 mm), medium (60–85 mm) and large (90–115 mm) Greenshell mussels (Perna canaliculus). Experimental diets varying from 3.3 to 6.0 μg l⁻¹ chlorophyll a concentration and 12–25% organic content were fed to mussels housed in individual flow through chambers. Not surprisingly, this study found that the main factor affecting feeding rates is mussel size. Small mussels were observed to maintain a constant filtration rate of approximately 20 mg h⁻¹ irrespective of food concentration or quality, whereas mussels of greater than 60 mm length had more variable filtration rates between 30 and 80 mg h⁻¹. The filtration rates of these large mussels were also observed to increase positively with organic content, and showed no sign of levelling out, even at the highest organic content tested (25%). Highest rejection rates (50–70 mg h⁻¹) were observed when the organic content of the available seston was low, suggesting that P. canaliculus are able to selectively reject organic material, thereby organically enriching their diet. It appears that the organic content of the seston is the primary determinant of the net efficiency with which food is selected from the available seston by the mussel. The present study shows that P. canaliculus of all sizes are capable of adapting their feeding behaviour to compensate for changes in the food supply, which may occur over relatively short time periods, in the culture environment.     

Combined Effects of Temperature and Seston Concentration on the Physiological Energetics of the Manila Clam Ruditapes philippinarum

The suspension-feeding Manila clam Ruditapes philippinarum is a native species of the western Pacific that is now widely distributed around the globe because of its commercial importance. To determine the adaptive physiological responses to changing thermal and nutritional conditions, clearance, filtration, feces production, ammonium excretion, respiration rates, and scope for growth (SFG) were measured in adult clams. The clams were exposed to 24 treatments involving the combination of four water temperatures (8, 13, 18, and 23°C) and six concentrations of suspended particulate matter (SPM: 9.5 to 350.5 mg L^–1). Physiological rates were standardized by using the mean (480 mg) of tissue dry weights of experimental clams using allometric equations between physiological variables and tissue dry weight. Higher clearance rates were recorded at higher temperatures and lower SPM concentrations, and these rates decreased with increasing SPM concentration at individual temperatures. Consumed energy increased with increasing temperature and SPM concentration, peaking at around 100–200 mg L^–1 at 18–23°C. Whereas fecal energy was largely determined by SPM concentration, ammonia excretion was mainly governed by temperature. Respiration rate studies revealed a predominant quadratic effect of temperature on the metabolism, indicating a lack of acclimatory adjustment of metabolic rate to rising temperature. SFG values were positive under almost all the treatment conditions and were much higher at higher SPM concentrations (> 45 mg L^–1), with the highest level being recorded at 18°C and 100–200 mg L^–1 SPM. Increased filtration rate offset the increased metabolic cost at warm temperatures. Our holistic findings suggest that a high degree of physiological plasticity allows R. philippinarum to tolerate the wide range of temperatures and SPM concentrations that are found in tidal flats, accounting in part for the successful distribution of this species over a wide variety of geographical areas.     

Population status of giant clams (Mollusca: Tridacnidae) in the northern Red Sea,Egypt

Throughout their range, giant clams (family Tridacnidae) are increasingly threatened by anthropogenic impacts and natural disasters, but little is known about their population status. In this first assessment of the tridacnid population at Abu Sauatir in the northern Red Sea, a total of 491 m² were surveyed and >200 clams recorded. Tridacna maxima was the only species found. The population's live:dead ratio was 3:1. Overall clam density was 0.08?±?0.008 live and 0.02?±?0.007 dead individuals per 0.25 m². Greatest densities occurred on the reef flat in 1 m depth (live), and on the northern reef slope in deeper waters (dead). On the slope, live clam density decreased significantly, whereas dead clam density increased significantly with depth. Sizes of live and dead individuals differed significantly. Live clams ranged from 1 to 30 cm (median 5 cm). Juveniles ≤2 cm (8.2% of the population) and individuals >11 cm occurred on the reef slope but not on the reef flat. Live clam sizes did not differ significantly between reef sites. Dead clam size ranged from 2 to 15 cm (median 6.5 cm). 2.1% of the empty shells were ≤2 cm long. Dead clam sizes differed significantly between 5 and 10 m depth on the northern reef slope. The low clam abundance (live and dead) in the shallowest and most easily accessible areas of the reef flat, combined with small sizes, strongly suggest artisanal reef-top gathering for meat and shells.     

Suspension feeding in Bithynia tentaculata (Prosobranchia,Bithyniidae), as affected by body size,food and temperature

The suspension feeding of Bithynia tentaculata was tested in laboratory experiments. The animals were fed in 1-1 aerated glass beakers, and filtration rates were calculated from changes in cell concentrations during the 6-h experiment. Temperature influenced the filtering rate, with minimum values of 5ml · ind^–1 · h^–1 at 5° C and maxima of 17.2 ml · ind^–1 · h^–1 at 18° C. Three food species of different size, motility and cell surface characteristics (Chlamydomonas reinhardii, Chlorella vulgaris and Chlorogonium elongatum) did not affect filtration rates. Suspension feeding increased with increasing food concentrations up to 12 nl · ml^–1, above which feeding rate was kept constant by lowering the filtering rates. Even the smallest animals tested (<4 mm body length) were found to be feeding on suspended food at a rate of 2.7 ml · ind^–1 · h^–1, and increasing rates up to 8.4 ml were found in the 6–7 mm size class. All size classes of Bithynia showed a circannual fluctuation of their filtration rates. The ecological consequences of Bithynia's ability to switch between two feeding modes, grazing and suspension feeding, are discussed.     

Growth,biomass, mortality,production and potential yield of the West African clam,Egeria radiata (Lamarck) (Lamellibranchia,Donacidae) in the Cross River system,Nigeria

An important fishery for the West African hemi-estuarine clam, Egeria radiata exists in the lower reaches of the Cross River (CR) system. The stock is heavily exploited but its productivity and potential yield have not been studied. This paper discusses the distribution and ecology of, and presents data on growth, abundance, mortality, biological production and potential yield of the CR stock of E. radiata based principally on length structured relative age. Growth was exponential and conformed to the von Bertalanffy's growth model (VBGM), with the parameters L = 9.3 cm, K = 0.36 and T _o = –0.10 year. The instantaneous total mortality coefficient (Z) by the catch curve method was 0.82 yr^–1; natural mortality coefficient (M) was estimated at 0.32 yr^–1. Total mean biomass computed from data obtained in two month's grab sample surveys in the area of occurrence of E. radiata was 1923 kg ha^–1 for total (i.e. flesh + shell) and 628 kg ha^–1 for flesh (i.e. shell-free) production units. The production-to-mean-biomass (or P/B ratio, R) were 0.31 and 0.52 for total and shell-free production units respectively. The maximum sustainable yield (Y _max) was estimated at 398.4 kg ha^–1 for total and 112.2 kg ha^–1 for shell-free production units. With an exploitation ratio (E) of 0.61 and an estimated present catch rate of 1970 tonnes(t) yr^–1 total weight (or about 82% of Y _max) the Egeria stock of the CR is currently being fished above the level of its optimum yield. Much of the fishing pressure is on the very young clams, thus lowering the age at first capture (T _c) to 1 year. This is an unhealthy situation for a species with limited area of distribution and low (< 1.0) P/B ratio; it can lead to rapid extinction of the stock if for some reason there is a substantial reduction in annual recruitment. It is recommended that in order to conserve this valuable resource and keep it at a level of sustained optimum yield, there should be a regulation prohibiting the landing of young clams (< 6.0 cm shell length). Furthermore the fishermen should learn to cultivate the animal by transplanting the young clams to new beds upstream to give them time to grow larger before harvesting. This type of management has been practised in Ghana for a long time and has helped to conserve the Egeria stock of the Volta River. The need for a continuous monitoring of the effluents from the Nigerian Newsprint Manufacturing Company's factory located at the bank of the CR in the centre of the clam industry is stressed; such monitoring would ensure that dangerous toxic chemicals are not discharged into the river to damage the Egeria industry.     

文蛤的生物沉积和呼吸排泄过程及其在双台子河口水层-底栖系统中的耦合作用

为揭示河口埋栖性双壳贝类优势种类在水层——底栖系统中的生态耦合作用,利用生物沉积物捕集器和封闭式代谢瓶,于双台子河口现场研究了文蛤主要生理生态过程如生物沉积速率、耗氧率、排氨率和排磷率的季节变化。结果表明,文蛤的生物沉积速率、耗氧率、排氨率及排磷率均具有明显的季节变化:夏季最高,冬季最低。二龄及三龄文蛤个体的生物沉积速率周年变化分别为0.02—0.30 g-1个-1d-1、0.06—0.60 g-1个-1d-1;耗氧率变化分别为0.45—16.64 mg-1个-1d-1、1.03—30.51 mg-1个-1d-1;排氨率季节变化分别为0.001—0.14 mg-1个-1d-1、0.002—0.28 mg-1个-1d-1;排磷率季节变化分别为0.002—0.069 mg-1个-1d-1、0.003—0.16 mg-1个-1d-1。文蛤的生物沉积速率及呼吸排泄速率均受龄期制约:在同一季节,文蛤的单位个体生物沉积速率及呼吸和排泄速率均表现为二龄三龄。方差分析显示,季节、龄期及两者交互作用对文蛤生物沉积速率、耗氧率、排氨率及排磷率均有显著影响。基于不同季节双台子河口文蛤生物量(0.67个/m2、2.4 g/m2),估算出文蛤种群每年向该河口排放大约5321.90 t生物沉积物(干重)、1.43 t NH+4-N和0.93 t PO3-4-P,并且消耗大约221.59 t O2。研究结果表明,文蛤通过生物沉积及呼吸排泄作用,大大加强了双台子河口沉积物-水界面的物质交换通量,在双台子河口水层-底栖系统耦合作用中扮演着重要生态角色。     

Collection of the clam Anodontia edentula in mangrove habitats in Panay and Guimaras,central Philippines

The mangrove clam Anodontia edentula is highly prized in thePhilippines for its flavor and large size. Because this infaunal species isfound down to one meter deep in mangrove areas, harvesting the clam reportedlydamages mangrove stands. To evaluate such reports, a survey of collectionmethods was undertaken in Panay and Guimaras, central Philippines in August1997–December 1999. Host to chemosynthetic bacterial symbionts thatutilize sulfide as energy source, A. edentula are strategically situated insulfide-rich anoxic substrates but also gain access to oxygenated seawaterthrough a ventilation burrow or tube. By locating the opening of this burrow,collectors can detect the presence of a buried clam and harvest itnondestructively with a blade or bare hands. In contrast, the indiscriminatetilling of wide mangrove areas can damage mangrove plants. Most collectors were40–45 years old with 22–30 years collection experience, married with5–7 children, and had low educational attainment. They sold clams directlyin the local markets or through middlemen (to restaurants and beach resorts);sales provided from 10% to 100% of daily family income. Collectors complained ofdecreasing clam sizes and numbers and the physically strenuous work ofcollecting.     

Potential impacts of brown tide, Aureococcus anophagefferens, on juvenile hard clams, Mercenaria mercenaria, in the Coastal Bays of Maryland, USA

The rate of growth of juvenile hard clams, Mercenaria mercenaria, was studied in the Coastal Bays of Maryland during an outbreak of the brown tide, Aureococcus anophagefferens. Brown tide dominated the plankton community during the month of June 2002, with cell densities at several sites reaching category 3 (>200,000 cells ml⁻¹) levels. Temperatures during the bloom were 18.6–27.5 °C. Nutrient conditions preceding and during the bloom were conducive for the proliferation of A. anophagefferens: while inorganic nitrogen and phosphorus were <1 μg at N or P l⁻¹, urea was elevated during bloom development. Organic nitrogen, phosphorus and carbon were in the range of levels observed in previous brown tide blooms and increased following the collapse of the bloom. Growth rates of juvenile clams were significantly lower during the period of the brown tide bloom than following its collapse. Growth rates of M. mercenaria were found to be negatively impacted at brown tide densities as low as 20,000 cells ml⁻¹, or category 1 levels. The low growth rates of M. mercenaria could not be explained by temperature, as the lowest growth rates were found when water temperatures were at levels previously found to be optimal for growth.     

Food Sources for Ruditapes philippinarum in a Coastal Lagoon Determined by Mass Balance and Stable Isotope Approaches

The relationship between the food demand of a clam population (Ruditapes philippinarum (Adams & Reeve 1850)) and the isotopic contributions of potential food sources (phytoplankton, benthic diatoms, and organic matter derived from the sediment surface, seagrass, and seaweeds) to the clam diet were investigated. In particular, we investigated the manner in which dense patches of clams with high secondary productivity are sustained in a coastal lagoon ecosystem (Hichirippu Lagoon) in Hokkaido, Japan. Clam feeding behavior should affect material circulation in this lagoon owing to their high secondary productivity (ca. 130 g C m⁻² yr⁻¹). Phytoplankton were initially found to constitute 14–77% of the clam diet, although phytoplankton nitrogen content (1.79–4.48 kmol N) and the food demand of the clam (16.2 kmol N d^–1) suggest that phytoplankton can constitute only up to 28% of clam dietary demands. However, use of isotopic signatures alone may be misleading. For example, the contribution of microphytobenthos (MPB) were estimated to be 0–68% on the basis of isotopic signatures but was subsequently shown to be 35±13% (mean ± S.D.) and 64±4% (mean ± S.D.) on the basis of phytoplankton biomass and clam food demand respectively, suggesting that MPB are the primary food source for clams. Thus, in the present study, the abundant MPB in the subtidal area appear to be a key food source for clams, suggesting that these MPB may sustain the high secondary production of the clam.     

A study of spatial variability of domoic acid in razor clams: recommendations for resource management on the Washington coast

Domoic acid (DA) levels in Washington State razor clams (Siliqua patula) have been extremely variable and unpredictable, resulting in emergency closures of harvest areas in 1991, 1998, and 1999. Information concerning toxin variability relative to sampling location is important in developing a more reliable plan for managing DA outbreaks. In November 1998, record levels of DA in razor clams (up to 295 ppm) were reported at Kalaloch beach, Washington. A razor clam resource survey conducted there during the summer of 1999, along with the long retention time of DA in the clams, permitted a study of DA levels as a function of tidal elevation and north-south beach location. During the summer low tides (28–31 July 1999) razor clams were collected from six east-west transects, approximately 1.6 km apart. Each clam was individually analyzed for DA to determine the distribution of toxin between (interspecific variability) and within (intraspecific variability) transects. While average DA levels were similar between transects, toxin levels varied substantially among individual clams. The coefficient of variation among all samples (n=445) was 122%, indicating that harvest closures based upon composite analyses of only six clams could be in error. Here we recommend a sampling strategy of razor clams for regulatory purposes that will result in DA measurements more representative of total population toxin values.     

Effects of food and silt on filtration,respiration and condition of the freshwater mussel Hyridella menziesi (Unionacea: Hyriidae): implications for bioaccumulation

The effect of exposure to different concentrations of food and suspended silt on filtration, respiration and condition were studied in the freshwater mussel Hyridella menziesi. Using a milk solids-based food and kaolin to simulate silt, mussels were maintained at different combinations of food and silt concentrations for 3 weeks. Between treatments mean filtration rates ranged from 0.97–1.66 l g^–1 h^–1, and respiration from 0.50–1.35 mg O₂ g^–1 h^–1. Silt (non-volatile suspended solids up to 35 mg l^–1) failed to have a significant effect on filtration rate or condition, but with increasing food levels (volatile suspended solids up to 35 mg l^–1) filtration rate was reduced, and condition was reduced at the lowest food concentration (<5 mg l^–1). Respiration showed a food × silt interaction between treatment blocks. When food was low respiration increased with increasing silt concentrations, and when silt was low (<5 mg l^–1) respiration increased with increasing food concentrations. The observed effects of food and silt on filtration, respiration and condition are discussed in terms of their potential for affecting contaminant bioaccumulation. In low-food situations (i.e., <5 mg l^–1), if mussels are pumping large volumes of water, contaminant uptake rates could be enhanced, whereas abundant food would result in lower pumping rates and lower uptake rates. Changes in metabolism with food concentration have implications for contaminant elimination, and changes in biochemical composition associated with changing condition could affect the tissue distribution and retention of contaminants.     

Morphology and defensive structures in the predator-prey interaction: an experimental study of Parabroteas sarsi (Copepoda,Calanoida) with different cladoceran prey

Parabroteas sarsi is a predaceous calanoid copepod commonly found in South Andes lakes. Feeding experiments were carried out in order to estimate the predation rates and attack patterns on different cladoceran prey. Predation rates were related with prey sizes. The smallest prey, Bosmina longirostris, was ingested up to 5 prey pred^–1 h^–1 while the largest, Daphnia middendorffiana, only at 0.12 prey pred^–1 h^–1. The functional response of P. sarsi differed when confronted with different prey although in all cases, the number of kills increased with prey density. A saturation of ingestion rates at high prey densities was only observed for B. longirostris and Ceriodaphnia dubia juveniles. Remains seldom appeared at the end of the experiments, implying that the predator consumed prey totally. Yet, in all experiments carried out with Daphnia ambigua and D. middendorffiana, remains, including soft parts, were found. Direct observations of attack and of number and types of remains showed interference of handling by tail, helmet, and size of the prey.     

Fishing mortality rates of giant clams (Family Tridacnidae) from the Sulu Archipelago and Southern Palawan,Philippines

Average size frequency distributions of Tridacna squamosa, T. gigas, Hippopus hippopus and H. porcellanus harvested from the Sulu Archipelago and Southern Palawan areas from 1978–1985 were derived from export records and a warehouse inventory of giant clam shells. Average species mortality rates (Z) were estimated and were used to approximate average fishing mortality rates (F) over the period 1978–1985. Crude estimates of exploitation rates (F/Z) indicate that populations of these species are already overexploited. These findings have serious implications in view of the fact that the Sulu Archipelago and Southern Palawan are thought to be the last strongholds of giant clams in Philippine waters.     

Large- and small-scale effects of habitat structure on rates of predation: how percent coverage of seagrass affects rates of predation and siphon nipping on an infaunal bivalve

Landscape ecology, predominantly a terrestrial discipline, considers the effect of large-scale (tens of meters to kilometers) spatial patterns of habitats on ecological processes such as competition, predation, and flow of energy. In this study, a landscape-ecology approach was applied to a marine soft-sediment environment to examine rates of predation and transfer of secondary production in and around vegetated habitats. Seagrass beds naturally occur in a variety of spatial configurations from patches 1–10s of meters across with interspersed unvegetated sediments (i.e., patchy coverage) to more continuous coverage with little or no bare sediment. I designed experiments to address how percent coverage of seagrass in a 100-m² area of seafloor, and the spatial arrangement (degree of patchiness or fragmentation) of an equal area (100 m²) of vegetation affected predation (lethal) and siphon nipping (sublethal) intensity on an infaunal bivalve, Mercenaria mercenaria (hard clam). Measures of seagrass density and biomass with different percent coverage of seagrass were also made. When clams were placed in both the vegetated and unvegetated portions of the seafloor nearly twice as many clams were recovered live with 99% seagrass cover than with 23% seagrass cover, while survivorship was intermediate with 70% cover. Cropping of clam siphons from both the vegetated and unvegetated sediments was also affected by the amount of seagrass cover in a 100-m² area of seafloor: mean adjusted siphon weights were approximately 76% heavier from the 99% seagrass cover treatment than from the 70% or 23% cover treatments. Survivorship of clams placed within an equal area of seagrass in very patchy, patchy, and continuous spatial configurations was 40% higher in the continuous seagrass treatment than in either of the two patchy treatments. This study demonstrates that transfer of secondary production in the form of predation and cropping on an infaunal organism is altered as the percent cover of seagrass changes. While large-scale changes in the amount and spatial patterning of vegetation may affect habitat utilization patterns and foraging HGLoopbehavior, increased seagrass density and biomass with increased percent coverage of seagrass limit any conclusions concerning predator foraging behavior and feeding success in response to patch shapes and sizes. Instead, local changes in seagrass characteristics provide the most compelling explanation for the observed results.     

Nitrate induction in spruce: an approach using compartmental analysis

Using ¹³NO ₃ ^– -efflux analysis, the induction of nitrate uptake by externally supplied nitrate was monitored in roots of intact Picea glauca (Moench) Voss. seedlings over a 5-d period. In agreement with our earlier studies, efflux analysis revealed three compartments, which have been identified as surface adsorption, apparent free space, and cytoplasm. While induction of nitrate uptake was pronounced, NO ₃ ^– fluxes in induced plants were decidedly lower and the induction response was slower than in other species. Influx rose from 0.1 mol·g^–1·h^–1 (measured at 100 M [NO ₃ ^– _o) in uninduced plants to a maximum of 0.5 mol·g^–1h^–1 after 3 d of exposure to 100 M [NO ₃ ^– _o and declined to 0.3–0.4 mol·g^–1h^–1 at the end of the 5-d period. Efflux remained relatively constant around 0.02-0.04 mol·g^–1h^–1, but its percentage with respect to influx declined from initially high values (around 30%) to steady-state values of 4–7%. Cytoplasmic [NO ₃ ^– ] ranged from the low micromolar in uninduced plants to a maximum of 2 mM in plants fully induced at 100 M [NO ₃ ^– ]_o. In-vivo root nitrate reductase activity (NRA) was measured over the same time period, and was found to follow a similar pattern of induction as influx. The maximum response in NRA slightly preceded that of influx. It increased from 25 nmol·g^–1·h^–1 without prior exposure to NO ₃ ^– to peak values around 150 nmol· g^–1h^–1 after 2 d of exposure to 100 M [NO ₃ ^– ]_o. Subsequently, NRA declined by about 50%. The dynamics of flux partitioning to reduction, to the vacuole, the xylem, and to efflux during the induction process are discussed.The research was supported by an Natural Sciences and Engineering Research Council, Canada, grant to Dr. A.D.M. Glass and by a University of British Columbia Graduate Fellowship to Herbert J. Kronzucker. Our thanks go to Dr. M. Adam and Mr. P. Culbert at the particle accelerator facility TRIUMF on the University of British Columbia campus for providing ¹³N, to Drs. R.D. Guy and S. Silim for providing plant material, and to Dr. M.Y. Wang, Mr. J. Bailey, Mr. J. Mehroke and Mr. J. Vidmar for essential assistance in experiments.