Mass Coral Reef Bleaching: A Recent Outcome of Increased El Niño Activity?

Coral reefs are generally considered to be the most biologically productive of all marine ecosystems, but in recent times these vulnerable aquatic resources have been subject to unusual degradation. The general decline in reefs has been greatly accelerated by mass bleaching in which corals whiten en masse and often fail to recover. Empirical evidence indicates a coral reef bleaching cycle in which major bleaching episodes are synchronized with El Niño events that occur every 3–4 years on average. By heating vast areas of the Pacific Ocean, and affecting the Indian and Atlantic Oceans as well, El Niño causes widespread damage to reefs largely because corals are very sensitive to temperature changes. However, mass bleaching events were rarely observed before the 1970s and their abrupt appearance two decades ago remains an enigma. Here we propose a new explanation for the sudden occurrence of mass bleaching and show that it may be a response to the relative increase in El Niño experienced over the last two decades.     

Size-scaling of zooplankton foraging in Arctic charr

Prey capture rate (number of prey s⁻¹) and the mode of feeding of Arctic charr Salvelinus alpinus were studied by performing foraging experiments with two sizes (1·1 and 1·8 mm) of Daphnia longispina prey. Arctic charr were particulate feeders at all densities tested. Adjusted for the effect of prey density, the capture rate showed a hump-shaped relationship with Arctic charr size for both sizes of D. longispina . Estimated attack rates ( a ) also tended to show a hump-shaped relationship with fish size. The estimated size-scaling exponent of the attack rate function, however, was relatively small, implying small changes in attack rate over fish sizes. Simultaneous estimations of a and handling time were used in combination with published data on fish metabolism and dry mass rations of prey to estimate maintenance resource density of prey as a function of Arctic charr mass. Maintenance resource densities increased monotonically with Arctic charr size, and rapidly as optimum fish size relative to attack rate on prey was passed.     

Coexistence of natural enemies in a multitrophic host–parasitoid system

Abstract.  1. This study explored the temporal and spatial aspects of coexistence over many generations in a multispecies host–parasitoid assemblage.
2. The long-term interaction between the cabbage root fly, Delia radicum (Diptera: Anthomyiidae), and two of its natural enemies, Trybliographa rapae (Hymenoptera: Fitigidae) and Aleochara bilineata (Coleoptera: Staphylinidae), in a cultivated field at Silwood Park over 19 years was explored.
3. Although time series showed that the populations were regulated, the impact of the natural enemies was highly variable. Within-year determinants showed that the spatial response of the specialist parasitoid, T. rapae , was predominantly independent of host density while A. bilineata acted simply as a randomly foraging generalist parasitoid.
4. These findings are compared and contrasted with an earlier investigation of the same system when only the first 9 years of the time series were available. This study demonstrated the potential of long-term field studies for exploring hypotheses on population regulation, persistence, and coexistence.     

Estimating woody and herbaceous vegetation cover from time series satellite observations

In this paper we test a method to estimate the tree and grass vegetation cover over Australia from satellite-derived normalized difference vegetation index (NDVI) time series (monthly 1981–91, ≈5 km pixels) observations. The evergreen cover is assumed to track along the base of the NDVI time series, which is assumed to be equivalent to the woody vegetation cover. The base of the NDVI time series is estimated using modifications to a classical econometric model (i.e. time series is the sum of trend, seasonal and random components). Estimates of the average evergreen component during 1982–85 and 1986–89 were generally consistent with known vegetation distributions. Changes in evergreen cover were largely restricted to the south-west and south-east of Australia. Those changes were largely the result of differences in rainfall between the two periods. The proposed method for estimating woody vegetation cover is found to be generally robust. However, there are some regions where the grass (or pasture) is mostly evergreen. Some possible refinements are proposed to handle such cases.     

Empirical relationships to estimate asymptotic length, length at first maturity and length at maximum yield per recruit in fishes, with a simple method to evaluate length frequency data

Empirical relationships are presented to estimate in fishes, asymptotic length (L∞) from maximum observed length (L_max), length at first maturity (L_m) from L_∞, life span (t_max) from age at first maturity (t_m), and length at maximum possible yield per recruit (L_opt) from L_∞ and from L_m, respectively. The age at L_opt is found to be a good indicator of generation time in fishes. A spreadsheet containing the various equations can be downloaded from the Internet at http://www.fishbase.org/download as popdynJFB.zip. A simple method is presented for evaluation of length–frequency data in their relationship to L_∞, L_m and L_opt. This can be used to evaluate the quality of the length–frequency sample and the status of the population. Three examples demonstrate the usefulness of this method. 2000 The Fisheries Society of the British Isles     

Environmental factors influencing the occurrence of juvenile fish in the mangroves of Pagbilao,Philippines

128 fish species belonging to 54 families were collected from the mangroves of Pagbilao. Ambassis kopsi (Ambassidae) was the most abundant species.Fourteen environmental factors were correlated to the catch per unit time for some species. The occurrence of A. kopsi was positively correlated to nitrate in the water, carbon in the sediments and litter fall. Out of the 8 species investigated, 6 were positively correlated to litter fall. Three species showed positive correlation to phosphate in the water, two to organic carbon in the sediments, nitrate, silicate and pH, and one to salinity and carotenoids in water. The biomass of the total catch was correlated to carbon in the sediments and litter fall.     

Effects of sulphuric acid exposure on the behaviour of largemouth bass,Micropterus salmoides

Synopsis Young-of-the-year largemouth bass,Micropterus salmoides, were exposed to four concentrations of sulphuric acid (pH levels 7.2, 6.1, 4.8, and 3.7) for 30 days, and the frequencies of feeding acts and activity bouts, and time budgets were recorded. Juveniles at pH 6.1 and at pH 4.8 performed the two feeding acts, bites and orientations, more often, and spent more time feeding than bass at pH 7.2. Bass at pH 3.7, however, reduced feeding, and spent a significantly larger portion of their time hovering in the water column. Frequencies of comfort and agonistic acts increased with a decline in pH. Alterations of behavioural repertoires of young-of-the-year largemouth bass were useful indicators of sulphuric acid exposure.     

A stochastic model for the membrane potential of a stimulated neuron

We present a simple model describing the transition between the prefiring, firing and postfiring phases of a single neuron in a large neural net. Using typical values for the physiological parameters that enter the model, we find average interspike times that are close to those reported in experimental measurements.     

Synchrotron-excited time-resolved fluorescence spectroscopy of adenosine,protonated adenosine and 6N, 6N-dimethyladenosine in aqueous solution at room temperature

The fluorescence behavior of adenosine in neutral solution has been studied by time-resolved spectroscopy using synchrotron excitation and timecorrelated single photon counting, and by decay time measurements. Three emissions have been identified and correlated with three excitation spectra. The assignment of these transitions has been made by comparison with similar measurements on ⁶N, ⁶N-dimethyladenosine (6 DMA), and on adenosine in acid solution (ADO H⁺). It is proposed that two of the transitions of adenosine which correlate with 6DMA originate from coplanar and orthogonal rotational conformers of the amino group. The other transition, correlating with ADO H⁺ may originate either from the ³H-imino tautomer, or from a differently solvated rotational conformer.A partial presentation of this work has been made at the Second Congress of the European Society for Photobiology Padova, Italy, 6–10 September 1987