Development of epiphytic communities on eelgrass (Zostera marina) along a nutrient gradient in a Danish estuary

The effect of nutrient enrichment on epiphyte development was examined by following the seasonal development of epiphyte biomass on eelgrass (Zostera marina L.) at four localities along a nutrient gradient in Roskilde Fjord, Denmark between March and December 1982. In the most nutrient-poor area, epiphyte biomass followed a distinct bimodal seasonal pattern with maxima in spring and early fall. Low nutrient availability and a high rate of eelgrass leaf renewal kept epiphyte biomass at a low level throughout the summer period. Unlike phytoplankton, the epiphytic community was not stimulated by nutrient enrichment during spring, however, from May through August, the biomass of both components increased exponentially with increasing concentrations of total N in the water. Along the nutrient gradient, phytoplankton biomass increased 5- to 10-fold, while epiphyte biomass increased 50- to 100-fold. Thus differences in nutrient conditions among study sites were more clearly reflected by epiphytes than phytoplankton.Contribution No. 419 from the Freshwater-Biological Laboratory, University of Copenhagen     

Stoichiometry, growth, and fecundity responses to nutrient enrichment by invertebrate grazers in sub-tropical turtle grass (Thalassia testudinum) meadows

Although the effectiveness of herbivores in mitigating the effects of nutrient enrichment is well documented, few studies have examined the effects of nutrient enrichment on components of consumer fitness. Enclosures were deployed in shallow turtle grass (Thalassia testudinum) beds in Florida Bay, Florida in fall 2003, spring 2004, and fall 2004 to measure the effects of nitrogen and phosphorous enrichment on the growth, fecundity, and stoichiometry of three invertebrate epiphyte grazers commonly associated with T. testudinum. The gastropod Turbo castanea exhibited significantly greater wet weight gain and lower C:P and N:P in enriched than in ambient treatments. Although nutrient enrichment did not have any significant effects on the growth of caridean shrimp (treatment consisted of several different caridean shrimp species), their C:N was significantly lower in enriched treatments. The final size and stoichiometry of the hermit crab Paguristes tortugae was not significantly affected by nutrient enrichment, nor did nutrient enrichment significantly affect the fecundity of P. tortugae, the only grazer in which gravid individuals or egg masses were present. Our study demonstrated that nutrient enrichment of primary producers can positively affect the growth of marine invertebrate grazers and alter their stoichiometry; however, these effects were species-specific and may be dependent upon the life stage, specific diets, and/or compensatory feeding habits of the grazers.     

How do nutrient conditions and species identity influence the impact of mesograzers in eelgrass-epiphyte systems?

Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers, have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations.     

Use of nucleic acids as indicators of growth in juvenile white shrimp,Penaeus vannamei

The use of nucleic acids to estimate crustacean growth is not well documented, and may be complicated by biochemical changes associated with their molt cycle. The objectives of this study were to assess the effects of molt stage on nucleic acid concentrations in abdominal muscle tissue of juvenile white shrimp,Penaeus vannamei, and to examine the relationship between nucleic acid concentrations and growth rates of shrimp exposed to different feeding regimes throughout a 12 d feeding experiment. RNA and DNA concentrations and RNA:DNA ratios were not significantly different among the five major molt stages early postmolt, late postmolt, intermolt, early premolt, and late premolt. In the feeding experiment, RNA concentrations and RNA:DNA ratios accounted for >70% of the variation in shrimp growth on three different sampling days. In addition, RNA concentrations and RNA:DNA ratios were used successfully to discriminate between unfed, moderately-fed, and well-fed shrimp. These variables exhibited significant treatment differences in <24 h after the initiation of the different feeding regimes, whereas significant changes in whole-body weight took longer to detect. Rapid detection of significant treatment effects can be useful in ecological studes, especially those concerned with food-web interactions.     

Cascading effects of hydrodynamics on an epiphyte–grazer system in intertidal seagrass beds of the Wadden Sea

This study examines experimentally how water movement may alter epiphyte-grazer systems in intertidal seagrass beds. Field observations in the Sylt-Rømø Bay (German Wadden Sea, SE North Sea) showed that the biomass of seagrass epiphytes was highest on seagrasses exposed to water movement, whereas at sheltered sites the epiphyte cover was negligible. In contrast, the seagrass shoot density and aboveground biomass was comparably sparse and the abundance of Hydrobia ulvae was extremely low at exposed areas, but showed maximum values at sheltered seagrass beds. Cross transplantation experiments and enclosure experiments between sheltered and exposed seagrass beds showed that adhering snails were washed off from seagrasses soon after transplantation into an exposed seagrass bed, and epiphytes started to grow. After 4 weeks the epiphyte biomass was similar to the that of the adjacent exposed seagrass bed. When heavily epiphytised seagrasses were transplanted from exposed into sheltered areas, the epiphytes were completely grazed down by immigrating snails within a week. Experiments carried out by means of an in situ "three-current-flume", modifying the entire current velocity, showed that snail density was significantly negatively correlated with increasing current velocity, whereas epiphyte biomass showed a significant positive correlation with current speed. These results suggest a cascading impact of hydrodynamics on an epiphyte-grazer system in intertidal seagrass beds, by directly affecting the density of grazers and indirectly leading to enhanced epiphyte growth, thereby inhibiting seagrass development. Additionally it shows that cascading effects within the trophic web cannot only be triggered by biotic interdependencies, but can also be caused by physical factors.     

Consumers affect prey biomass and diversity through resource partitioning

Consumer presence and nutrient availability can have contrasting and interactive effects on plant diversity. In a factorial experiment, we manipulated two levels of nutrient supply and the presence of two moderately specialized grazers in different combinations (no grazers, two species in monoculture, and both in combination). We tested how nutrients and grazers regulated the biomass of marine coastal epiphytes and the diversity of algal assemblages, based on the prediction that the effect of consumers on prey diversity depends on productivity and consumer specialization. Nutrient enrichment increased the epiphytic load, while monocultures of single grazer species partly prevented epiphyte growth. However, only the presence of two species with complementary feeding preferences effectively prevented epiphyte overgrowth. The epiphytes comprised micro- and macroalgal species, and the diversity of these algal assemblages differed, depending on grazer identity. For the microalgae, diversity was reduced by nutrient addition when grazer control was inefficient, but not when specialist microalgal grazers were present. Macroalgal diversity was reduced in ambient water with specialist macroalgal grazers compared to the treatment with inefficient ones. These results indicate that grazer composition and productivity are crucial in determining whether consumer pressure will have a positive or negative effect on algal diversity.     

围封6年对温带典型草原坡地物种多样性及其与地上生物量的关系的影响

以内蒙古太仆寺旗地区典型克氏针茅-羊草草原为研究对象,通过调查围封6年后围栏内外典型草原群落特征,分析围封、放牧处理下典型草原植物群落结构及地上生物量的动态变化,揭示围封放牧对植被群落结构、物种丰富度和地上生物量的影响。结果发现,在围封6年后,各功能群物种丰富度与盖度围栏内外的空间变化趋势基本一致。群落物种丰富度和地上生物量均随坡位下降而显著增加,初步显示了物种多样性与生态系统功能的空间变化的一致性。围栏内物种丰富度显著低于围栏外,而地上生物量却显著大于围栏外。围栏内多年生禾草、一年生植物物种丰富度显著低于围栏外,而半灌木盖度、生物量显著高于围栏外。围栏内外在禁牧和放牧不同处理下,表现出不同的草原退化方式：在禁牧条件下,尽管生物量有所恢复,但灌木和半灌木的增加却代表了草原的退化;放牧、刈割等人类活动增加了耐牧物种和不适口植物生长的可能,表现为草原退化指示物种增加,同样是草原退化的表现。围栏外一年生植物盖度和生物量显著高于围栏内,而一年生植物的频数也远远大于围栏内（围栏内外频数比13：71）,且独行菜、猪毛菜等仅出现于围栏外。同时,地上生物量随着物种丰富度的增加而增加,但只有围栏外的关系达到统计显著程度,说明围封调制了生物多样性与生态系统功能的关系。本研究表明,对于干旱半干旱典型草原的恢复,不能简单地采用长期完全禁牧措施,应当根据区域环境、植被以及社会经济情况,制定季节性放牧或者间歇性禁牧的恢复措施,从而保证草场的可持续性恢复利用。     

Reef-basel herbivores and the distribution of two seagrasses (Syringodium filiforme and Thalassia testudinum) in the San Blas Islands (Western Caribbean)

Two seagrasses, manatee grass (Syringodium filiforme) and turtle grass (Thalassia testudinum), predominated in the areas bordering Ukkup Tupo, San Blas Islands, Western Caribbean. These seagrasses occupied the following three concentric zones extending outward from the reef: a near-reef turtle grass zone, an intermediate manatee grass zone and an off-reef turtle grass zone. Feeding experiments between January and March 1980 indicate that the absence of manatee grass close to the reef resulted from grazing by reef-associated herbivores, mainly day-active fishes and night-active sea urchins (Diadema antillarum). Grazing on manatee grass by fishes was approximately six times greater than grazing by sea urchins; thus, it appears that herbivorous fishes restrict the near-reef distribution of manatee grass at the study area. Where grazing was heaviest, the inner boundary of the manatee grass zone was farthest from the reef. The volume of manatee grass grazed during experiments was five times the volume of turtle grass consumed, strongly suggesting that the former species is a preferred food item. This is the first evidence for selective grazing on seagrasses.     

Cars, Cows, and Checkerspot Butterflies: Nitrogen Deposition and Management of Nutrient-Poor Grasslands for a Threatened Species

Abstract: Nutrient-poor, serpentinitic soils in the San Francisco Bay area sustain a native grassland that supports many rare species, including the Bay checkerspot butterfly ( Euphydryas editha bayensis ). Nitrogen (N) deposition from air pollution threatens biodiversity in these grasslands because N is the primary limiting nutrient for plant growth on serpentinitic soils. I investigated the role of N deposition through surveys of butterfly and plant populations across different grazing regimes, by literature review, and with estimates of N deposition in the region. Several populations of the butterfly in south San Jose crashed following the cessation of cattle grazing. Nearby populations under continued grazing did not suffer similar declines. The immediate cause of the population crashes was rapid invasion by introduced annual grasses that crowded out the larval host plants of the butterfly. Ungrazed serpentinitic grasslands on the San Francisco Peninsula have largely resisted grass invasions for nearly four decades. Several lines of evidence indicate that dry N deposition from smog is responsible for the observed grass invasion. Fertilization experiments have shown that soil N limits grass invasion in serpentinitic soils. Estimated N deposition rates in south San Jose grasslands are 10–15 kg  N/ha/year; Peninsula sites have lower deposition, 4–6 kg N/ha/year. Grazing cattle select grasses over forbs, and grazing leads to a net export of N as cattle are removed for slaughter. Although poorly managed cattle grazing can significantly disrupt native ecosystems, in this case moderate, well-managed grazing is essential for maintaining native biodiversity in the face of invasive species and exogenous inputs of N from nearby urban areas.     

Using a fisheries ecosystem model with a water quality model to explore trophic and habitat impacts on a fisheries stock: A case study of the blue crab population in the Chesapeake Bay

Recent calls for the development of ecosystem-based fisheries management compel the development of resource management tools and linkages between existing fisheries management tools and other resource tools to enable assessment and management of multiple impacts on fisheries resources. In this paper, we describe the use of the Chesapeake Bay Fisheries Ecosystem Model (CBFEM), developed using the Ecopath with Ecosim (EwE) software, and the Chesapeake Bay Water Quality Model (WQM) to demonstrate how linkages between available modeling tools can be used to inform ecosystem-based natural resource management. The CBFEM was developed to provide strategic ecosystem information in support of fisheries management. The WQM was developed to assess impacts on water quality. The CBFEM was indirectly coupled with the WQM to assess the effects of water quality and submerged aquatic vegetation (SAV) on blue crabs. The output from two WQM scenarios (1985-1994), a baseline scenario representing actual nutrient inputs and another with reduced inputs based on a tributary management strategy, was incorporated into the CBFEM. The results suggested that blue crab biomass could be enhanced under management strategies (reduced nutrient input) when the effective search rate of blue crab young-of-the-year's (YOY's) predators or the vulnerability of blue crab YOY to its predators was adjusted by SAV. Such model linkages are important for incorporating physical and biological components of ecosystems in order to explore ecosystem-based fisheries management options.     

Functional diversity of mesograzers in an eelgrass–epiphyte system

Historically, small invertebrate grazers in marine plant communities have been considered to be a relatively homogeneous group in their impact on ecosystem processes. However, recent studies propose that species composition is an important agent in determining grazer effects. We used four mesocosm experiments to test the biomass-specific and density-dependent effects of common mesograzers in temperate regions (Littorina littorea, Rissoa membranacea, Idotea baltica and Gammarus oceanicus) on epiphyte and eelgrass biomass and productivity. Mesograzer species identity strongly influenced epiphyte accumulation and eelgrass growth, where Rissoa was the most efficient mesograzer (per biomass) and Gammarus had the weakest impact. Density-dependent effects varied considerably among species. Both gastropod species reduced epiphyte accumulation in direct proportion to their density, and Littorina had the strongest negative effect on epiphyte biomass. The impact of Idotea seemed to level off to a threshold value and Gammarus had no density-dependent effect on epiphyte accumulation at all. Rissoa and Idotea increased eelgrass productivity in accordance with their effect on epiphyte accumulation, whereas Littorina showed a less positive effect than could be expected by its strong impact on epiphyte biomass. Gammarus had no significant impact on eelgrass growth. Our results show that the different functional traits of superficially similar mesograzers can have important consequences for ecosystem processes in macrophyte systems.     

Influence of increased cod abundance and temperature on recruitment of northern shrimp (Pandalus borealis)

Despite an increase in northern shrimp (Pandalus borealis) female biomass in the past years, the recruitment of the offshore population north and northeast of Iceland has remained very low. In this study, the influence of abiotic and biotic factors was studied in relation to shrimp recruitment. Two factors, cod (Gadus morhua) abundance and summer sea surface temperature (SST), were found to have a negative effect on offshore shrimp recruitment, explaining 71 % of the observed variation. Both cod abundance and temperature on the offshore shrimp grounds have increased in the past years, while recruitment has decreased and been at historically low levels since 2005. No significant relationship was found between recruitment and spawning biomass, indicating that recruitment variability is mainly driven by other factors. Cod abundance and summer SST are likely to affect different life stages of shrimp, as SST influences shrimp during its planktonic phase while cod abundance influences the demersal stage.     

Demographic performance predicts invasiveness of species in the Commelinaceae under high-nutrient conditions.

Demographic models are powerful tools for making predictions about the relative importance of transitions from one life stage (e.g., seeds) to another (e.g., nonreproductives); however, they have never been used to compare the relative performance of invasive and noninvasive taxa. I use demographic models parameterized from common garden experiments to develop hypotheses about the role of different life stage transitions in determining differences in performance in invasive and noninvasive congeners in the Commelinaceae. I also extended nested life table response experiment (LTRE) analyses to accommodate interactions between nested and unnested factors. Invasive species outperformed their noninvasive congeners, especially under high-nutrient conditions. This difference in performance did not appear to be due to differences in elasticities of vital rates, but rather to differences in the magnitude of stage transitions. Self-compatible invasive species had greater fecundity in high-nutrient environments and a shorter time to first reproduction, and all invasive species had greater vegetative reproduction than their noninvasive congeners. Thus greater opportunism in sexual and asexual reproduction explained the greater performance of invasive species under high-nutrient conditions. Similar common garden experiments could become a useful tool to predict potential invaders from pools of potential introductions. I show that short-term and controlled experiments considering multiple nutrient environments may accurately predict invasiveness of nonnative plant species.     

Dissolved iron supply limits early growth of estuarine mangroves

Three mesocosm experiments were performed in an outdoor facility to quantify the responses of five mangrove species grown from seedling to sapling stage to increasing rates of dissolved iron supply. Stem extension and biomass of mangroves were measured in the first two experiments, and in the third experiment, rates of microbial iron reduction were measured in relation to stem extension of two mangrove species. In all experiments, mangrove growth was enhanced by increasing iron supply, although some species showed iron toxicity at the higher supply rates. In the first two experiments, stem extension rates of Rhizophora apiculata, Bruguiera gymnorrhiza, and Xylocarpus moluccensis best fit Gaussian curves with maximal growth at supply rates of 50-60 mmol Fe x m(-2) x d(-1), whereas growth of Avicennia marina and Ceriops tagal increased to the highest rate (100 mmol Fe x m(-2) x d(-1)) of iron supply. Changes in leaf chlorophyll concentrations and iron content of roots mirrored the growth responses. In the third experiment, rates of microbial iron reduction were greater with R. apiculata and A. marina than in controls without plants; for both species, there was a positive relationship between stem extension and iron reduction. The rates of iron reduction and rates of iron supplied to the plants were well within the range of interstitial iron concentrations and rates of iron reduction found in the natural mangrove soils from which the seedlings were obtained. The responses of these species show that mangroves growing from seedling to sapling stage have a strong nutritional requirement for iron, and that there is a close relationship between plant roots and the activities of iron-reducing bacteria. These results suggest that mangrove growth may be limited in some natural forests by the rate at which iron is solubilized by iron-reducing bacteria. Such biogeochemical conditions have significant implications for successful recruitment, establishment, and early growth of mangroves.     

Modeling the role of macroalgae in a shallow sub-estuary of Narragansett Bay, RI (USA)

Proliferation of macroalgal mats is a frequent consequence of nutrient-driven eutrophication in shallow, photic coastal marine ecosystems. These macroalgae have the potential to significantly modify water quality, plankton productivity, nutrient cycling, and dissolved oxygen dynamics. We developed a model for Ulva lactuca and Gracilaria tikvahiae in Greenwich Bay, RI (USA), a shallow sub-estuary of Narragansett Bay, as part of a larger estuarine ecosystem model. The model predicts the biomass of both species in units of carbon, nitrogen, and phosphorus as a function of primary production, respiration, grazing, decay, and physical exchange, with particular attention to the effects of biomass layering on light attenuation and suppression of metabolic rates. The model successfully reproduced the magnitude and seasonal cycle of area-weighted and peak biomass in Greenwich Bay along with tissue C:N ratios, and highlighted the importance of grazing and inclusion of self-limitation primarily in the form of self-shading to overcome an order of magnitude difference in rates of production and respiration. Inclusion of luxury nutrient uptake demonstrated the importance of internal nutrient storage in fueling production when nutrients are limiting. Macroalgae were predicted to contribute a small fraction of total system primary production and their removal had little effect on predicted water quality. Despite a lack of data for calibration and a fair amount of sensitivity to individual parameter values, which highlights the need for further autecological studies to constrain formulations, the model successfully predicted macroalgal biomass dynamics and their role in ecosystem functioning. Our formulations should be exportable to other temperate systems where macroalgae occur in abundance.     

Cattle Grazing Mediates Climate Change Impacts on Ephemeral Wetlands

Abstract:  Climate change impacts depend in large part on land-management decisions; interactions between global changes and local resource management, however, rarely have been quantified. We used a combination of experimental manipulations and simulation modeling to investigate the effects of interactions between cattle grazing and regional climate change on vernal pool communities. Data from a grazing exclosure study indicated that 3 years after the removal of grazing, ungrazed vernal pools dried an average of 50 days per year earlier than grazed control pools. Modeling showed that regional climate change could also alter vernal pool hydrology. Increased temperatures and winter precipitation were predicted to increase periods of inundation. We evaluated the ecological implications of interactions between grazing and climate change for branchiopods and the California tiger salamander (  Ambystoma californiense ) at four sites spanning a latitudinal climate gradient. Grazing played an important role in maintaining the suitability of vernal pool hydrological conditions for fairy shrimp and salamander reproduction. The ecological importance of the interaction varied nonlinearly across the region. Our results show that grazing can confound hydrologic changes driven by climate change and play a critical role in maintaining the hydrologic suitability of vernal pools for endangered aquatic invertebrates and amphibians. These observations suggest an important limitation of impact assessments of climate change based on experiments in unmanaged ecosystems. The biophysical impacts of land management may be critical for understanding the vulnerability of ecological systems to climate change.     

Provenance, life span, and phylogeny do not affect grass species' responses to nitrogen and phosphorus

Successful conservation management requires an understanding of how species respond to intervention. Native and exotic species may respond differently to management interventions due to differences arising directly from their origin (i.e., provenance) or indirectly due to biased representations of different life history types (e.g., annual vs. perennial life span) or phylogenetic lineages among provenance (i.e., native or exotic origin) groups. Thus, selection of a successful management regime requires knowledge of the life history and provenance-bias in the local flora and an understanding of the interplay between species characteristics across existing environmental gradients in the landscape. Here we tested whether provenance, phylogeny, and life span interact to determine species distributions along natural gradients of soil chemistry (e.g., soil nitrogen and phosphorus) in 10 upland prairie sites along a 600-km latitudinal transect running from southern Vancouver Island in British Columbia, Canada, to the Willamette Valley in Oregon, USA. We found that soil nitrate, phosphorus, and pH exerted strong control over community composition. However, species distributions along environmental gradients were unrelated to provenance, life span, or phylogenetic groupings. We then used a greenhouse experiment to more precisely measure the response of common grass species to nitrogen and phosphorus supply. As with the field data, species responses to nutrient additions did not vary as a function of provenance, life span, or phylogeny. Native and exotic species differed strongly in the relationship between greenhouse-measured tolerance of low nutrients and field abundance. Native species with the greatest ability to maintain biomass production at low nutrient supply rates were most abundant in field surveys, as predicted by resource competition theory. In contrast, there was no relationship between exotic-species biomass at low nutrient levels and field abundance. The implications of these findings for management of invasive species are substantial in that they overturn a general belief that reduction of nutrient supplies favors native species. The idiosyncratic nature of species response to nutrients in this study suggests that manipulation of nutrient supplies is unlikely to alter the overall balance between native and exotic species, although it may well be useful to control specific exotic species.     

Preliminary Evaluation of the Use of Phosphogypsum for Reef Substrate. I. A Laboratory Study of Bioaccumulation of Radium and Six Heavy Metals in an Aquatic Food Chain

Phosphogypsum (PG), a solid by-product of phosphoric acid production, contains radionuclides and trace metals in concentrations which may pose a potential hazard to human health and the environment. to investigate the possibility of bioaccumulation of radium and six heavy metals over time when aquatic organisms experience both trophic and environmental exposure to PG, we designed a laboratory experiment representing three levels of an aquatic food chain. During the 135 day experiment, a meiobenthic copepod species (Amphiascoides atopus) was cultured in the presence of PG. the copepods were subsequently fed to grass shrimp (Palaemonetes vulgaris and P. pugio) which were in turn fed to gulf killifish (Fundulus grandis); both the grass shrimp and the killifish also experienced an environmental PG exposure. Other than elevated radium levels in the experimental grass shrimp, the experiment demonstrated little effect of environmental or trophic exposure to PG on microinvertebrates, macroinvertebrates, or fishes that could be attributed to PG. in all cases where increased concentrations were indicated within the experimental group, roughly equivalent increases in metal concentrations also occurred in the control group.     

The effects of seston food quality on planktonic food web patterns

In planktonic food webs, the conversion rate of plant material to herbivore biomass is determined by a variety of factors such as seston biochemical/elemental composition, phytoplankton cell morphology, and colony architecture. Despite the overwhelming heterogeneity characterizing the plant–animal interface, plankton population models usually misrepresent the food quality constraints imposed on zooplankton growth. In this study, we reformulate the zooplankton grazing term to include seston food quality effects on zooplankton assimilation efficiency and examine its ramifications on system stability. Using different phytoplankton parameterizations with regards to growth strategies, light requirements, sinking rates, and food quality, we examined the dynamics induced in planktonic systems under varying zooplankton mortality/fish predation, light conditions, nutrient availability, and detritus food quality levels. In general, our analysis suggests that high food quality tends to stabilize the planktonic systems, whereas unforced oscillations (limit cycles) emerge with lower seston food quality. For a given phytoplankton specification and resource availability, the amplitude of the plankton oscillations is primarily modulated from zooplankton mortality and secondarily from the nutritional quality of the alternative food source (i.e., detritus). When the phytoplankton community is parameterized as a cyanobacterium-like species, conditions of high nutrient availability combined with high zooplankton mortality led to phytoplankton biomass accumulation, whereas a diatom-like parameterization resulted in relatively low phytoplankton to zooplankton biomass ratios highlighting the notion that high phytoplankton food quality allows the zooplankton community to sustain relatively high biomass and to suppress phytoplankton biomass to low levels. During nutrient and light enrichment conditions, both phytoplankton and detritus food quality determine the extent of the limit cycle region, whereas high algal food quality increases system resilience by shifting the oscillatory region towards lower light attenuation levels. Detritus food quality seems to regulate the amplitude of the dynamic oscillations following enrichment, when algal food quality is low. These results highlight the profitability of the alternative food sources for the grazer as an important predictor for the dynamic behavior of primary producer–grazer interactions in nature.     

Reclamation of abandoned shrimp pond soils in southern Thailand for cultivation of Mauritius grass (Brachiaria mutica)

A study on soil reclamation for cultivation of Mauritius grass was conducted on soils obtained from abandoned shrimp ponds at Ranote District, Songkhla Province, southern Thailand. A glass house experiment on the reclamation of the soils included desalination by leaching soils using various amounts of deionised water, rice husk, plant nutrients and gypsum as well as an omission pot trial experiment. The result showed that Mauritius grass survived in the treatment with 15 L of water, 2% of rice husk with gypsum added or 8% of rice husk without gypsum added. The yield of Mauritius grass increased with increases in the amounts of water for desalination and rice husk. Thus, the highest yield of grass with a height of 148.3 cm, 12.7 tillers/pot and dry weight of 46.43 g/pot was observed in the gypsum added treatment with the highest amount of water and rice husk (25 L of water and 8% by weight of rice husk). Therefore, salinity and unfavourable structure of the abandoned pond soils were major factors governing the survival ability and growth of the grass. The omission pot trial experiment revealed that growth of the grass responded to the application of P, Ca, Mg and S, though existing amounts of such plant nutrient elements in the soils were adequate for plant growth. The anomalous characteristics were probably explained by soil pH, salinity and imbalance of plant nutrient elements.