Pilar Drake

Spatial and temporal variation of the nekton and hyperbenthos from a temperate european estuary with regulated freshwater inflow

The aquatic macrofauna of the Guadalquivir estuary were sampled (1 mm mesh persiana net) at 5 sampling sites located along the entire (except the tidal freshwater region) estuarine gradient of salinity (outer 50 km). A total of 134 fish and macroinvertebrate species was collected but only 62 were considered common or regularly present in the estuary. Univariate measures of the community structure showed statistically significant differences among sampling sites: species richness, abundance, and biomass decreased in the upstream direction, being positively correlated with the salinity. Temporal differences of these three variables were also statistically significant. While a clear seasonal pattern (minimum densities in winter and maximum in spring-summer) was observed for abundance and biomass, no such pattern existed for the number of species. Mysids was the most dominant group throughout the estuary (96% to 99% of abundance; 49% to 85% of biomass), although fish biomass was also important at the outer estuary (36% to 38%). Multivariate analyses indicated highly significant spatial variation in the macrofaunal communities observed along the salinity gradient. These analyses suggest that the underlying structure was a continuum with more or less overlapping distributions of the species dependent on their ability to tolerate different physicochemical conditions. There were also significant temporal (intermonthly + interannual) variation of the estuarine community; the relative multivariate dispersion indicated that monthly variation was more considerable (relative multivariate dispersion >1) at the outer part of the estuary during the wet year (last 20 km) and was higher in the inner stations during the dry year (32 to 50 km from the river mouth). Since a clear negative exponential relationship was observed between the freshwater input (from a dam located 110 km upstream) and water salinity at all sampling stations, it is concluded that the human freshwater management is probably affecting the studied estuarine communities. While the higher seasonal (long-term) stability of the salinity gradient, due to the human control of the freshwater input, may facilitate the recruitment of marine species juveniles during the meteorologically unstable early-spring, the additional (short-term) salinity fluctuations during the warm period may negatively affect species that complete their lifecycle within the estuary.

The effect of aquaculture practices on the benthic macroinvertebrate community of a lagoon system in the Bay of Cádiz (southwestern Spain)

Monthly quantitative Ekman-Birge grab sampling was used to characterize and compare the composition and structure of the benthic macroinvertebrate community inhabiting semi-enclosed polyculture lagoons (SPL) (three sampling sites) and enclosed monoculture ponds (EMP) (two sampling sites) of a lagoonal system of the Bay of Cádiz. The two areas differed considerably in habitat characteristics and aquaculture management. The SPL area was characterized by low rates of water exchange, low fish densities, and the presence of a macroalgal cover. In the EMP area, there was a complete exchange of water daily (by pumping) and a supply of food pellets, density of fish was high, and no vegetative cover was present. There were considerable differences in species composition between habitats with different culture methods: 11 of the 21 most abundant species were exclusive to one or the other. Several epibenthic species were abundant in the polyculture lagoon but were low in density or were absent in monoculture ponds. Some infaunal species, on the other hand, were more abundant in the monoculture ponds. Univeriate measures of community structure (abundance and biomass, Margalef’s species richness, Shannon-Wiener diversity, and Pielou’s evenness indices) did not indicate significant differences between the SPL and EMP areas. Conversely, the abundance-biomass comparison (ABC) method indicated that, on average, the macrobenthic community was moderately disturbed in the SPL and undisturbed in the EMP areas. Multidimensional scaling (MDS) ordination and hierarchical cluster analysis (Bray-Curtis similarity measure) revealed the occurrence of two main benthic assemblages that corresponded to the aquaculture methods. The different rates of water exchange for the two aquaculture practices seem to have contributed to differences in the composition and structure of the benthic communities.

Ichthyoplankton of a shallow coastal inlet in south-west Spain: Factors contributing to colonization and retention

Abstract The ichthyoplankton of a shallow inlet was sampled for 13 months using conical tide-strained nets. The utilization of this habitat by the young stages of different fish species is analysed. Temporal and spatial distributions are discussed in relation to environmental cycles and gradients, and to feeding rhythms. A total of 110 971 individuals, belonging to 39 species and 19 families of teleosts, was collected. The postlarval stage was the most represented in the samples. Ichthyoplankton density peaked during late winter and early spring. Density was higher during flood tide, resulting in a net input of postlarvae from the bay to the inner inlet. This ecosystem functions primarily as a nursery ground for coastal pelagic spawners and, secondarily, as a spawning area for some benthic egg spawners and pouch-brooder species. Most postlarvae migrate from their spawning area (open sea) to the inlet probably cued by a decreasing gradient of water temperature and salinity, and an increasing gradient of water turbidity and suitable food concentration. The attraction of postlarvae towards the inlet mouth probably results from a combination of biotic (availability of suitable food) and abiotic (temperature, salinity and turbidity) factors. Passive transport contributes to the subsequent penetration and retention of the postlarvae within the shallow, flood-dominated inlet. Vertical migrations observed among some planktonic fish stages were related to light periods and feeding activity rather than tidal phases, and cannot act as a retention mechanism.

Eco-evolutionary Dynamics of Individual-Based Food Webs

The past decade has seen the rise of high resolution datasets. One of the main surprises of analysing such data has been the discovery of a large genetic, phenotypic and behavioural variation and heterogeneous metabolic rates among individuals within natural populations. A parallel discovery from theory and experiments has shown a strong temporal convergence between evolutionary and ecological dynamics, but a general framework to analyse from individual-level processes the convergence between ecological and evolutionary dynamics and its implications for patterns of biodiversity in food webs has been particularly lacking. Here, as a first approximation to take into account intraspecific variability and the convergence between the ecological and evolutionary dynamics in large food webs, we develop a model from population genomics and microevolutionary processes that uses sexual reproduction, genetic-distance-based speciation and trophic interactions. We confront the model with the prey consumption per individual predator, species-level connectance and prey–predator diversity in several environmental situations using a large food web with approximately 25,000 sampled prey and predator individuals. We show higher than expected diversity of abundant species in heterogeneous environmental conditions and strong deviations from the observed distribution of individual prey consumption (i.e. individual connectivity per predator) in all the environmental conditions. The observed large variance in individual prey consumption regardless of the environmental variability collapsed species-level connectance after small increases in sampling effort. These results suggest (1) intraspecific variance in prey–predator interactions has a strong effect on the macroscopic properties of food webs and (2) intraspecific variance is a potential driver regulating the speed of the convergence between ecological and evolutionary dynamics in species-rich food webs. These results also suggest that genetic–ecological drift driven by sexual reproduction, equal feeding rate among predator individuals, mutations and genetic-distance-based speciation can be used as a neutral food web dynamics test to detect the ecological and microevolutionary processes underlying the observed patterns of individual and species-based food webs at local and macroecological scales.

Assessment of the interaction between the white shrimp, Palaemon longirostris, and the exotic oriental shrimp, Palaemon macrodactylus, in a European estuary (SW Spain)

Results of field surveys, stomach content analysis, and laboratory measurements of oxygen consumption at different salinities in the exotic shrimp Palaemon macrodactylus and its native counterpart P. longirostris from the Guadalquivir estuary (SW Spain) were used to assess interaction between both species. After its first record in this estuary in 1998, P. macrodactylus has shown a gradual increase in density and its estuarine population has been clearly dominated by juveniles. Feeding habits of both shrimps indicated a strong trophic overlap between them, with mysids as main prey. Their salinity-related and spatial distribution patterns show that maximal inter-specific overlap between the two populations occurs in the inner, less saline part of the estuary. Specific oxygen consumption rates under different salinities and water oxygen concentrations suggest a more efficient metabolism and a higher tolerance to hypoxic conditions in brackish waters by P. macrodactylus than by P. longirostris. Such inter-specific physiological differences could have made it possible for the exotic P. macrodactylus to invade an estuarine stretch that, prior to its arrival, was infra-utilized by native species.

Effects of the river discharge management on the nursery function of the Guadalquivir river estuary (SW Spain)

Within the Guadalquivir estuary, young recruits of marine species seem to respond to changes in freshwater flow by moving with the mass of estuarine water that is most “suitable” for them. The control of the river flow, from a dam 110 km upstream from the river mouth, has an immediate effect on the estuarine salinity gradient, displacing it either seaward or upstream. Consequently, there is a reduction or enlargement of the estuarine area that is used as nursery grounds. The analysis of the temporal estuarine recruitment and spatial distribution of young stages of marine species, during six annual cycles, provides evidence that the estuarine zone used as nursery grounds is mainly that part situated seaward from an isohaline value of 5. The relationship between the position of that isohaline (D5) and the freshwater discharges from the dam was also examined during high and low tides. It was found that a high percentage of the isohaline position variation (75% and 73% at high and low tides, respectively) can be explained by the freshwater volume discharged from the dam during the previous week. These preliminary results suggest that an accurate model of the relationship between these two variables may be a useful tool for future management strategies of freshwater discharges to the estuary.

Distribution and production ofChironomus salinarius (Diptera: Chironomidae) in a shallow coastal lagoon in the Bay of Cádiz

The abundance, generation time and production ofChironomus salinarius larvae in a lagoon fish-pond system in the Bay of Cádiz were studied by taking monthly samples at 3 sites during 1991 and 1992. Numerical abundance and biomass of larvae showed considerable spatial, seasonal and interannual variation (ANCOVAs,P<0.001). The maximum mean annual density was 7048 larvae m−2, and corresponded to a biomass of 3.08 g dry weight (DW) m−2. It was recorded at the site with the lowest rate of water renewal. Seasonal patterns were similar at all sites, with main annual peaks of abundance and biomass in autumn-early winter. Chironomid density was positively related to the biomass of benthic macroalgae (P<0.001). The population studied was multivoltine with a probable average of five generations per year, with overlapping cohorts and a predominance of third- and fourth-instar larvae. Estimates of annual production ranged between 72.2 g DW m−2 yr−1 at the site with the lowest rate of water renewal in 1991 and 0.1 g DW m−2 yr−1 at the site with the highest rate of water renewal in 1992. Mean annual production and the production/biomass ratio for the system was estimated to be 16.8 g DW m−2 yr−1 and 12.7, respectively. Possible factors leading to the observed density fluctuations are discussed, as well as possible sources of error in production estimates.

Macrobenthic community structure in estuarine pollution assessment on the Gulf of Cádiz (SW Spain): Is the Phylum-level Meta-analysis approach applicable?

Abstract Macrobenthic species abundances and biomasses were determined at twelve stations from three estuarine areas of the Gulf of Cadiz (SW Spain). They are subject to different sources of pollution: urban effluents (Bay of Cadiz), agricultural/urban sewages (Barbate River Estuary) and mining/industrial sewages (Odiel River Estuary). Different univariate and multivariate techniques were used in the assessment of the community disturbance status. At the Species-level, inferences from Gray and Pearson’s graphics together with multivariate MDS ordinations provided a reliable picture of the severity of the community disturbance at the twelve sampling sites. At the Phylum-level, the meta-analysis of “production” also gave a reliable disturbance status for all the stations when new samples were introduced in the MDS analysis one by one. These results suggest a more general applicability of the Phylum-level meta-analysis, provided that the original NE Atlantic data ordination remains unmodified.

DISTRIBUTION AND PRODUCTION OF MICRODEUTOPUS GRYLLOTALPA (AMPHIPODA: AORIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

ABSTRACT The spatial and temporal fluctuations of density, life cycle, and production of Microdeutopus gryllotalpa in a seminatural lagoon system were studied by taking monthly samples at 3 sites during 1991 and 1992. Its numerical abundance and biomass, which showed considerable spatial and temporal variation, were positively related to the biomass of benthic macroalgae and water salinity (stepwise multiple regression analysis, P 17°C) than for the rest of the year (0.026 mm d-1; T < 17°C). The population studied was multivoltine, with overlapping cohorts and a possible maximum of 5 generations per year. Using the size-frequency method, the mean annual production and production/biomass ratio for the system were estimated to be 22.42 g DW (13.49 g AFDW) m-2 yr-1 and 12.8 (11.7), respectively. These results indicated that in warm temperate lagoons secondary production for species such as M. gryllotalpa may be high, despite its moderate mean biomass.

Multivariate methods and artificial neural networks in the assessment of the response of infaunal assemblages to sediment metal contamination and organic enrichment.

A 4-year annual sediment survey was conducted in an organically enriched tidal channel to compare the performance of univariate community descriptors, traditional multivariate techniques (TM) and artificial neural networks (AANs), in the assessment of infaunal responses to moderate levels of sediment metal contamination. Both TM approaches and the SOM ANN revealed spatiotemporal patterns of environmental and biological variables, suggesting a causal relationship between them and further highlighting subsets of taxa and sediment variables as potential main drivers of those patterns. Namely, high values of non-natural metals and organic content prompted high abundances of opportunists, while high values of natural metals yielded typical tolerant assemblages of organically enriched areas. The two approaches yielded identical final results but ANNs showed the following advantages over TM: ability to generalise results, powerful visualization tools and the ability to account simultaneously for sediment and faunal variables in the same analysis. Therefore, the SOM ANN, combined with the K-means clustering algorithm, is suggested as a promising tool for the assessment of the ecological quality of estuarine infaunal communities, although further work is needed to ensure the accuracy of the method.