Young mangrove stands produce a large and high quality litter input to aquatic systems

Mangrove swamps are key ecosystems along the Vietnam coast. Although mangrove litter is thought to represent an important input of organic matter and nutrients to the coastal aquatic systems, the factors determining the quality and size of this litter flux have not been studied so far. We monitored leaf, stipule, twig, and reproductive litter monthly in monocultures of Rhizophora apiculata mangrove forests of 7, 11, 17 and 24 years old in the Camau province, Mekong Delta, Vietnam. Litter traps were used to measure litter fall production from June 2001 till May 2002. Total litter fall was in the range of 8.86-14.16 t DW ha-1 year-1. Leaves were the main component, and represented 70% of litter fall production in all stands. Total litter fall was lower in the older stands but the amount of reproductive litter was significantly higher in these stands (17 and 24 years). Biomass of leaf litter was highest between the end of the wet season and the beginning of the dry season. Phosphorus and nitrogen levels in leaf litter were significantly higher in younger than in older stands. Overall, our study indicated that young stands produced the highest input of litter and particularly of nitrogen and phosphorus to the surrounding aquatic system. Consequently, these stands contribute significantly to the fisherie

Penaeus monodon post-larvae and their interaction with Rhizophora apiculata

In recent years, expansion of shrimp aquaculture in Vietnam has brought considerable financial benefits to farmers and local communities. In the coastal provinces in the Mekong Delta, brackish shrimp aquaculture is the major economy activity. Extensive shrimp-mangrove culture systems are popularly practiced here. Although the average shrimp production is low, due to over-exploitation and destruction of mangrove forests and salt marshes, these systems are of special interest in view of the problems of sustainability of intensive aquaculture (Naylor et al. 2000 Nature 405: 1017-1024). Several studies demonstrated that mangrove swamps are highly productive ecosystems providing food, shelter and nurseries for various aquatic organisms, many of which are commercially important. The tiger shrimp, Penaeus monodon , is a clear example in this case. Natural shrimp production in these areas is believed to depend to a large extend on the presence of mangroves. However, the complex of mechanisms through which mangroves affect shrimp production is still poorly understood. The work in this thesis is an attempt to unravel some of the key-processes involved. It confirms the picture that mangrove litter represents a formidable input of organic material and nutrients into the aquatic system, and reveals how this input may have positive as well as negative effects on growth and survival of post-larval shrimp.Mangrove stands of different age have been studied for one year with respect to their litter fall and nutrient input (chapter 2). Litter fall consisted for 70% of leaf litter and organic matter accounted for 90% of the dry weight. Litter fall declined with the age of the mangrove stands, and also nitrogen and phosphorus levels were considerably higher in the leaf litter of younger stands (7 and 11 years) as compared to the older stands (up to 24 years). Thus, both the amount and the quality of litter input to the aquatic systems are highest in younger mangrove stands.As a next step key factors affecting the decomposition of mangrove leaves were analyzed (chapter 3). Decomposition rates tended to be highest at lower salinities, and reached an optimum at 5 ‰. The decomposition rates were also highest in the wet season, and this may well be due torelatively low salinities in this period. Wet season salinity in the Camau area was in the range of 4 - 9 ‰, close to the optimum for decomposition derived from laboratory experiments. Our studies also indicated an effect of humidity per se. We found that the decomposition rate was higher for leaves submerged in the ditches, than for leaves incubated near the roots of mangrove stands in the open air, where decomposition rates were higher in the wet than in the dry season. We also analyzed the dynamics of nutrient concentrations in decomposing litter. Nitrogen and phosphorus levels in decomposing leaves increased during the decomposition period. This enrichment indicates an increase of food quality over the first period of decomposition .The following chapters show that the effects of decomposing mangrove leaves on shrimps can be positive but also negative (Chapter 4 and 5). The amount of decomposing leaves appeared key. At high concentrations of leaves negative effects prevailed. These effects were probably due tothe release of nitrite and sulphide, and a decrease in dissolved oxygen concentration. On the positive side, mangrove moderate concentrations of leaves promoted growth of Penaeus monodon post-larvae, and apparently served as a shelter and as a food source.The fact that micro-organisms growing on the leaves, rather than the leaf material itself may be important as food was illustrated by the result that shrimps feeding on mangrove leaves grew better when a periphyton layer covered these leaves (chapter 5). A somehow surprising positive effect of leaves was the apparent prevention of excessive concentrations of ammonium and nitrite. The results suggest that adding conditioned mangrove leaves might ameliorate negative effects of high protein pellets on the water quality. The high C/N-ratio of leaves tends to balance the stochiometry of the system which may otherwise be dominated by the excessive N-input through CP pellets.In the final chapters the interaction among the shrimp larvae themselves, i.e. the effects of stocking density and the release of crowding chemicals and possible alarm pheromones on the shrimp populations are addressed (chapter 6, 7). A strong effect of crowding on shrimp growth and survival was shown. Physical interference stress and cannibalism could be excluded as causal factors. It was thus clear that the effects were caused by other water quality variables. Temperature, pH, salinity, dissolved oxygen, chlorine, nitrite and nitrate appeared of minor influence. However, ammonia toxicity could not be excluded as the causal factor for the observed mortality and reduced growth of P. monodon post-larvae in our experiments.On the other hand, alarm cues, as released by crushed conspecifics had negative effects on post-larval survival at high concentrations (100, 70, 50 and 30 crushed shrimps.l -1 ). Surprisingly, low concentrations of crushed conspecifics (1 crushed shrimp.l -1 ) were shown to have rather stimulatory effects on body size and dry weight.Put in an applied perspective, this study suggests simple ways to improve the management of mangrove-shrimp systems. Clearly, mangrove leaves can promote the survival and growth of shrimp post-larvae. However, at high leaf concentrations negative effects may prevail related to a drop in dissolved oxygen and the release of sulphide. A straightforward way to ameliorate such negative effects may be to increase the water flow. This will reduce the risk of local anoxia, and may help spreading the litter over the area, thus avoiding accumulation of these leaves at some sites. The reduction of potentially toxic nitrite and ammonium concentrations by decomposing leaves suggests that mangrove leaves may serve as a useful complement to CP pellets in semi-natural production systems

Chemical and physical effects of crowding on growth and survival of Penaeus monodon Fabricius post-larvae

The hypothesis that crowding effects through physical and/or chemical interference may be an important factor in lowering the chance of survival and reducing growth of Penaeus monodon post-larvae under high stocking densities was tested. To separate physical interference from chemically-exerted effects, two-stage systems were used in which shrimps were cultured at different densities (stage 1) and water from these being supplied to individually kept P. monodon (stage 2). Stocking density significantly affected P. monodon survival, body-size and dry-weights over a 4-week experimental period. At high densities of 50 and 100 shrimps/l, strong negative effects traveled into stage 2. Hence, physical interference stress and cannibalism could be excluded as causal factors meaning that the negative impact of crowding (at 50 and 100 shrimps/l) on shrimp growth and survival was due to some chemical compounds or other water quality variable. Among these pH, temperature, salinity, dissolved oxygen, chlorine, nitrite and nitrate appeared of minor influence. In contrast, ammonium toxicity could not be excluded as the causal factor for the observed mortality and reduced growth of P. monodon post-larvae in our experiment

Effects of crushed conspecifics on growth and survival of Penaeus monodon Fabricius post larvae

In the present study, the hypothesis tested was that Penaeus monodon post-larvae (PL) experience lower growth when exposed to crushed conspecifics, which was achieved by exposing individual P. monodon PL with abundant food for 4 weeks to a gradient from 0 to 100 crushed conspecific PL L1. Both dry weight (48.5±7.2 mg) and body size (28.0±1.3 mm) of animals exposed to 1 macerated PL L1 were significantly (P¿0.011) higher than those of animals in treatments with 0, 5 and 10 crushed PL L1 (average over treatments: 25.6±3.2 mg; 23.4±0.5 mm). All animals died within 1 week when exposed to 70 and 100 crushed PL L1, and within 3¿4 weeks when exposed to 50 and 30 crushed PL L1. Exposure time affected mortality and it appeared that LC50 values decreased from 60 to 13 crushed PL L1 from 1 to 4 weeks' exposure. Survival of P. monodon PL was negatively correlated to pH, biological oxygen demand, ammonia and nitrate. In conclusion, low dose of crushed conspecifics has a stimulatory effect on P. monodon PL, as larvae were heavier and larger, while high doses cause high mortality

Immunocompromised patients with acute respiratory distress syndrome : Secondary analysis of the LUNG SAFE database

The aim of this study was to describe data on epidemiology, ventilatory management, and outcome of acute respiratory distress syndrome (ARDS) in immunocompromised patients. Methods: We performed a post hoc analysis on the cohort of immunocompromised patients enrolled in the Large Observational Study to Understand the Global Impact of Severe Acute Respiratory Failure (LUNG SAFE) study. The LUNG SAFE study was an international, prospective study including hypoxemic patients in 459 ICUs from 50 countries across 5 continents. Results: Of 2813 patients with ARDS, 584 (20.8%) were immunocompromised, 38.9% of whom had an unspecified cause. Pneumonia, nonpulmonary sepsis, and noncardiogenic shock were their most common risk factors for ARDS. Hospital mortality was higher in immunocompromised than in immunocompetent patients (52.4% vs 36.2%; p < 0.0001), despite similar severity of ARDS. Decisions regarding limiting life-sustaining measures were significantly more frequent in immunocompromised patients (27.1% vs 18.6%; p < 0.0001). Use of noninvasive ventilation (NIV) as first-line treatment was higher in immunocompromised patients (20.9% vs 15.9%; p = 0.0048), and immunodeficiency remained independently associated with the use of NIV after adjustment for confounders. Forty-eight percent of the patients treated with NIV were intubated, and their mortality was not different from that of the patients invasively ventilated ab initio. Conclusions: Immunosuppression is frequent in patients with ARDS, and infections are the main risk factors for ARDS in these immunocompromised patients. Their management differs from that of immunocompetent patients, particularly the greater use of NIV as first-line ventilation strategy. Compared with immunocompetent subjects, they have higher mortality regardless of ARDS severity as well as a higher frequency of limitation of life-sustaining measures. Nonetheless, nearly half of these patients survive to hospital discharge. Trial registration: ClinicalTrials.gov, NCT02010073. Registered on 12 December 2013