Severe droughts reduce estuarine primary productivity with cascading effects on higher trophic levels

Using a 10-yr time-series data set, we analyzed the effects of two severe droughts on water-quality and ecosystem processes in a temperate, eutrophic estuary (Neuse River Estuary, North Carolina). During the droughts, dissolved inorganic nitrogen concentrations were on average 46–68% lower than the long-term mean due to reduced riverine input. Phytoplankton productivity and biomass were slightly below average for most of the estuary during a spring–autumn drought in 2002, but were dramatically lower than average throughout the estuary during an autumn–winter drought in 2007–2008. Droughts affected upper trophic levels through alteration of both habitat condition (i.e., bottom-water dissolved oxygen levels) and food availability. Bottomwater dissolved oxygen levels were near or slightly above average during the 2002 drought and during summer 2007. Concomitant with these modest improvements in bottom-water oxygen condition, fish kills were greatly reduced relative to the long-term average. Low-oxygen bottom-water conditions were more pronounced during summer 2008 in the latter stages of the 2007–2008 drought, and mesozooplankton abundances were eight-fold lower in summer 2008 than during nondrought years. Below-average mesozooplankton abundances persisted for well over 1 yr beyond cessation of the drought. Significant fish kills were observed in summer 2008 and 2009, perhaps due to the synergistic effects of hypoxia and reduced food availability. These results indicate that droughts can exert both ephemeral and prolonged multiyear influence on estuarine ecosystem processes and provide a glimpse into the future, when many regions of the world are predicted to face increased drought frequency and severity due to climate change.

Nocturnal fish movement and trophic flow across habitat boundaries in a coral reef ecosystem (SW Puerto Rico)

Few studies have quantified the extent of nocturnal cross-habitat movements for fish, or the influence of habitat adjacencies on nutrient flows and trophodynamics. To investigate the patterns of nocturnal cross-boundary movements of fish and quantify trophic connectivity, fish were sampled at night with gillnets set along the boundaries between dominant habitat types (coral reef/seagrass and mangrove/seagrass) in southwestern Puerto Rico. Fish movement across adjacent boundary patches were equivalent at both coral reefs and mangroves. Prey biomass transfer was greater from seagrass to coral reefs (0.016 kg/km) and from mangroves to seagrass (0.006 kg/km) but not statistically significant, indicating a balance of flow between adjacent habitats. Pelagic species (jacks, sharks, rays) accounted for 37% of prey biomass transport at coral reef/seagrass and 46% at mangrove/seagrass while grunts and snappers accounted for 7% and 15%, respectively. This study indicated that coral reefs and mangroves serve as a feeding area for a wide range of multi-habitat fish species. Crabs were the most frequent prey item in fish leaving coral reefs while molluscs were observed slightly more frequently than crabs in fish entering coral reefs. For most prey types, biomass exported from mangroves was greater than biomass imported. The information on direction of fish movement together with analysis of prey data provided strong evidence of ecological linkages between distinct adjacent habitat types and highlighted the need for greater inclusion of a mosaic of multiple habitats when attempting to understand ecosystem function including the spatial transfer of energy across the seascape.

Collective Cell Migration: Leadership, Invasion and Segregation

A number of biological processes, such as embryo development, cancer metastasis or wound healing, rely on cells moving in concert. The mechanisms leading to the emergence of coordinated motion remain however largely unexplored. Although biomolecular signalling is known to be involved in most occurrences of collective migration, the role of physical and mechanical interactions has only been recently investigated. In this paper, a versatile framework for cell motility is implemented in-silico in order to study the minimal requirements for the coordination of a group of epithelial cells. We find that cell motility and cell-cell mechanical interactions are sufficient to generate a broad array of behaviours commonly observed in vitro and in vivo. Cell streaming, sheet migration and susceptibility to leader cells are examples of behaviours spontaneously emerging from these simple assumptions, which might explain why collective effects are so ubiquitous in nature. This analysis provides also new insights into cancer metastasis and cell sorting, suggesting in particular that collective invasion might result from an emerging coordination in a system where single cells are mechanically unable to invade.

Structure of a fig wasp community: Temporal segregation of oviposition and larval diets

Observations were made on six fig wasp species on Ficus racemosa growing in the Xishuangbanna Tropical Botanic Garden, Yunnan Province, China. The oviposition sequence was determined for Apocryptophagus testacea, Apocrypta sp2, Apocryptophagus mayri, Cera

Trophic State Index, morphoedaphic index and fish yield prediction in a sub-tropical lake, Manchar (Sindh), Pakistan

Limnological factors of a sub-tropical lake Manchar were studied on seasonal basis. The mean values of various parameters were: transparency, (secchi disc reading): 90.5 cm, Orthophosphate: 0.257 mg/l, TDS: 3310,5 mg/l, Conductivity: 5232 µs/l, Total Chlorophyll (Planktonic): 31.3 µg/l Planktonic biomass: 5466 µg/l. Trophic state index (TSI) was calculated by using Carlson's (1977) equations. Mean TSI for transparency was 61, while for orthophosphate and chlorophyll, it was 82 and 64 respectively. TSI values indicate advanced eutrophic state of Manchar Lake. Morphoedaphic index (MEI) was also calculated on seasonal basis. The mean values were, TDS: 1103, conductivity: 1744, alkalinity: 60, transparency: 29 and biomass (plankton dry weight): 1746. Fish yield prediction for Manchar Lake (Z =3m, mean area=100 km²) was calculated by using MEI values. The results were quite different among various parameters. Conductivity (89.1mt/y), biomass (67.6 mt/y) and TDS (44.6 mt/y) were found to be good predictors of fish yield. Chlorophyll, transparency and alkalinity values gave very low estimate.

Feeding habits and trophic levels of some demersal fish species in the Persian Gulf (Bushehr Province) using Ecopath model

A trophic study was carried out in February of 2012 to January 2013 on the ecosystem in the Persian Gulf, Bushehr provience. A total of 2,948 samples of stomach contents were analyzed based on the weight and number of food items and were identified about 40 preys. Crustacean and bony fish were as a main prey in most of the stomach contents . The mean average trophic level was estimated at 3.6 by Ecopath software. In this research, the mean level were studied between eight species varied from 3.47 to 4.61, Saurida tumbil occupy the highest and the Argyrops spinifer was the lowest level. The ranges of total mortality varied from 0.7 to 3.05 per years. The food consumption rate was estimated about 69.82 per year. The overlap index showed that the prey items such as fish, crustacean, bivalve and echinoderm were shared about 65, 15, 15 and 6 percent in all stomach of individual in respectively. Mixed trophic analysis indicates that benthos have a positive effect on most of the fish species. Most species have a negative impact on themselves, interpreted here as reflecting increased with in group competition for resources. This preliminary model can be helpful to determine the gaps in the present knowledge about demersal system of the Persian Gulf.

Triosephosphate isomerase genes in two trophic modes of euglenoids (Euglenophyceae) and their phylogenetic analysis

Two different length cDNAs encoding triosephosphate isomerase (TIM) were identified in the two trophic modes of euglenoids, the phototrophic Euglena gracilis and Euglena intermedia and the saprotrophic Astasia longa. Sequence analyses and presequence pred

Implications of changes in trophic diversity and food webs on fisheries and the environment

Most of the earth's ecosystems are experiencing slight to catastrophic losses of biodiversity, caused by habitat destruction, alien species introduction, climate change and pollution (Wilcove et al., 1998). These human effects have led to the extinction of native fish species, the collapse of their populations and the loss of ecological integrity and ecosystem functioning (Ogutu-Ohwayo & Hecky, 1991; Witte et al. , 1992a; Mills et al., 1994; Vitousek et al., 1996). Food webs are macro-descriptors of community feeding interactions that can be used to map the flow of materials and nutrients in ecosystems (Jepsen & Winemiller, 2002). Comparative food web studies have been used to address theoretical questions such as 'does greater trophic connectivity increase stability?' (Cohen et al., 1990), and 'does the number of trophic levels increase with productivity?' (Briand & Cohen, 1987). Answers to such questions have obvious applications for natural resources management. From a multi-species fisheries standpoint, there is a need to understand consumer-resource dynamics within complex trophic networks.

The role of Yssichromis species (Pisces: Cichlidae) in the trophic ecology and foodwebs of Lake Victoria

Many haplochromine cichlids coexisted in Lake Victoria before the upsurge of Nile perch. The introduction of the Nile perch led to depletion of many haplochromines and other fish species in Lake Victoria. The impact of Nile perch predation on haplochromines differed for different haplochromine trophic groups. Yssichromis fusiformis (G) and Yssichromis laparogramma (G) are among the species that have survived in the lake. Yssichromis spp. was studied with the aim of determining their trophic role, food and feeding habits. Samples were collected from Bugaia, Buvuma channel and Napoleon Gulf in the northern part of Lake Victoria. The food of Yssichromis spp. varied with size of fish. Both Y fusiformis and Y laparogramma fed on Copepods, Cladocerans, Chaoborus and Chironomids. Juvenile Yssichromis spp. fed exclusively on zooplankton comprising Cyclopoid copepods, Calanoid copepods and Cladocera. The relative importance of Chironomid larvae and Calanoid copepods was higher in Bugaia than in Buvuma channel while Cyclopoid copepods and Chironomid pupae were relatively less important in Bugaia. The main food items that Yssichromis spp. fed on in Buvuma channel were Chironomid larvae Cyclopoid copepods, Cladocerans and Calanoid copepods. In Napoleon Gulf, fish caught from commercial fishery of Rastrineobola argentea (P) had fed on Chaoborus and Chironomids. Overall, Yssichromis spp. fed on more zooplankton in Buvuma than in Bugaia. Yssichromis spp. and R. argentea are presently the most abundant zooplanktivores in the northern part of Lake Victoria and are playing an important trophic role as major consumers of zooplankton and insect larvae in the foodweb of the lake ecosystem. Yssichromis spp. are bridging the transfer of energy from the lower to the higher trophic levels as secondary consumers. The fishery is still not contributing to the direct conversion of the primary products, the phytoplankton and detritus that were efficiently utilised by the diverse haplochromine trophic groups that existed before the Nile perch boom.