Lake of flies, or lake of fish? A trophic model of Lake Malawi

Ecosystem-focused models have, for the first time, become available for the combined demersal and pelagic components of a large tropical lake ecosystem, Lake Malawi. These provide the opportunity to explore continuing controversies over the production efficiencies and ecological functioning of large tropical lakes. In Lake Malawi these models can provide important insight to the effect of fishing on fish composition, and the potential competition that the lakefly Chaoborus edulis may have with fisheries production. A mass-balanced trophic model developed for the demersal fish community of the southern and western areas of Lake Malawi was integrated with an existing trophic model developed for the open-water pelagic. Input parameters for the demersal model were obtained from a survey of fish distributions, fish food consumption studies, and from additional published quantitative and qualitative information on the various biotic components of the community. The model was constructed using the Ecopath approach and software. The graphically presented demersal food web spanned four trophic levels and was based primarily on consumption of detritus, zooplankton and sedimented diatoms. Zooplankton was imported into the system at trophic levels three and four through fish predation on carnivorous and herbivorous copepods and Chaoborus larvae. It is proposed that the primary consumption of copepods was by fish migrating into the pelagic zone. Chaoborus larvae in the demersal were probably consumed near the lakebed as they conducted a daily migration from the pelagic to seek refuge in the sediments. This evidence for strong benthic-pelagic coupling provided the opportunity for linking the demersal model to the existing model for the pelagic community so producing the first model for the complete ecosystem. Energy fluxes through the resulting combined model demonstrated that the primary import of biomass to the demersal system was detritus of pelagic origin (72.1%) and pelagic zooplankton (10.6%). Only 15.8% of the biomass consumed within the demersal system was of demersal origin. Lakefly production is efficiently utilised by the lake fish community, and any attempt to improve fishery production through introduction of a non-native plantivorous fish species would have a negative impact on the stability and productivity of the lake ecosystem.     

Energy flow model of an oxygen-deficient estuary on the Swedish west coast

The estuary Byfjord (Sweden) is characterized by high primary production, a well developed meiofauna compared to the macrofauna, high epifaunal biomass, a low number of herbivorous copepods and a small fish stock. A simplified energy flow model of the ecosystem of the fjord is given. The energy transfer is approximated to 15%. About one-fourth-300 (metric) tons of carbon — of the annual primary production is suggested to be directly consumed and to produce 5 tons of zooplankton carbon and 40 tons of epifaunal (mainly Mytilus edulis) carbon. About 500 tons of carbon from the detritus pool are probably utilized in animal production. This amount will produce 5 tons of zooplankton carbon, 6 tons of meiofaunal carbon, and 3 tons of carbon from the benthic macrofauna. Production of fish is estimated at 0.3 ton carbon per year. M. edulis seems to be the only food resource in the fjord worth harvesting by man.     

Influence des conditions hivernales sur les productions phyto-et zooplanctoniques en Méditerranée Nord-Occidentale. V. Biomasse et production zooplanctonique — relations phyto-zooplancton

The cruise “Mediprod I” of the R.V. “Jean Charcot” covered an area of the Mediterranean Sea situated North of the 40th parallel and West of Corsica; during two 15-day legs, the first when Winter conditions ended, in March, the second in Spring conditions, in April, a 48-station network was surveyed as to primary and secondary production, as well as hydrological conditions. The first survey revealed a higher concentration of zooplankton in hydrologically stable areas, especially in the South-East, although zooplankton values were rather low throughout the whole area surveyed. The total zooplankton per unit surface was lower at the edges of the surveyed area, where phytoplankton was scarce. An increase in zoo-plankton biomass was observed between the two surveys, mostly in the central areas and near the surface. An important difference between both legs is in the proportion of organisms of different sizes collected by the Clarke-Bumpus sampler: the-200 μ:+200 μ organisms ratio, which is around 1 during the first leg, is much lower during the second leg. Two methods were used in estimating the biomass: the +200 μ fraction of a Clarke-Bumpus sample (Cl-B towed with a 50-μ net fitted on the sampler) was collected by sieving the sample through a 200-μ mesh nylon; standard vertical WP 2 hauls were performed (200 μ mesh). Both roughly show the same zooplankton (weight per unit surface) distribution pattern. However, higher estimations of the total biomass of the volume investigated were sometimes provided by the Clarke-Bumpus method during the first leg, probably due to the distribution of the animals according to their size-classes. A graph of chlorophyll versus zooplankton for surface waters suggests that zooplankton has a limiting effect on the development of phytoplankton in April only. Apparent growth rates of cooplankton are less than those for populations in the laboratory or enclosed environments. Values of the mean secondary production vary from 18 mg C.m^-2.day^-1 for the first leg, to 230 mg C.m^-2.day^-1 for the second leg. Estimations of net efficiency for energetic transfer between phyto- and zooplankton lie between 7 and 26%. As far as our hypotheses as regards physiological coefficients are valid, we can assume that the effect of grazing relative to primary production is greater in the border areas than in the central area, thus increasing the contrast between both areas with time. Phosphorus excretion rate by zooplankton seems to be less than that measured in the Atlantic Ocean. We suggest that in Spring zooplankton excretion is not the main phosphorus recycling process. Primary productivity measurements, apparent growth coefficients, and estimated grazing rate have been used to calculate the expected mean biomass per unit area during the second leg. A 7% loss of phyto- and zooplankton from the upper 100-m layer must be assumed to explain the observed biomass variation. Vertical mixing and sinking of surface water on an isopycnal slope are responsible for such loss, which can also affect, to an undetermined degree, the phosphorus stock introduced into the surface layer by the Mediterranean deep-water formation mechanism. We suggest that nearly half the total loss of phytoplankton from March to April is attributable to animal grazing.     

Role of herbivory in controlling phytoplankton abundance: annual pigment budget for a temperate marine fjord

An annual pigment budget was constructed for Dabob Bay, Washington (USA) by comparing the downward vertical loss of phytoplankton pigments (chlorophyll and pheopigments) to the production of chlorophyll within the euphotic zone. The vertical flux of pigments was measured with sediment traps deployed at intervals of 1 to 6 wk over a 2.5 yr period yielding 763 d of trap exposure (28 November 1978–16 June 1981). The production rate of chlorophyll was calculated from measurements of algal specific growth rates, chlorophyll (chl) crops, primary production (as carbon) and appropriate C: chl ratios. Sixty one to 77% of the annual chlorophyll production was accounted for by the vertical flux of pheopigments resulting from herbivorous zooplankton grazing (macrozooplankton). Algal sinking, represented by downward chlorophyll flux, accounted for only 5 to 6% of the annual chlorophyll production. The remaining fraction of chlorophyll production was estimated to be consumed by small herbivores (microzooplankton), whose fecal material contributes to the suspended pool of pheopigments found in the euphotic zone. The suspended pheopigments are continuously removed by photodegradation. In Dabob Bay, the major process controlling phytoplankton abundance is zooplankton grazing and it appears that the ultimate fate of most phytoplankton is to be consumed by herbivores.     

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.     

Prediction of response of zooplankton biomass to climatic and oceanic changes

This paper examines the long-term variation in zooplankton biomass in response to climatic and oceanic changes, using a neural network as a nonlinear multivariate analysis method. Zooplankton data collected from 1951 to 1990 off the shore of northeastern Japan were analyzed. We considered patterns of the Kuroshio and the Oyashio, sea surface temperature, and meteorological parameters as environmental factors that affect zooplankton biomass. Back propagation neural networks were trained to generate mapping functions between environmental variables and zooplankton biomass. The performance of the network models was tested by varying the numbers of input and hidden units. Changes in zooplankton biomass could be predicted from environmental conditions. The neural network yielded predictions with smaller errors than those of predictions determined by linear multiple regression. The sensitivity analysis of networks was used to extract predictive knowledge. The air pressure, sea surface temperature, and some indices of atmospheric circulation were the primary factors for predictions. The patterns of the Kuroshio and the Oyashio demonstrated different effects among sea areas.     

A spatial model of phytoplankton patchiness

The one-dimensional theory of critical-length scales of phytoplankton patchiness is developed to include phytoplankton growth and herbivore grazing as functions of time and space. The critical-length scale L _c for the pathch is then determined by the initial spatial distribution and concentration of the limiting nutrient and herbivores in addition to the daily averaged values of the growth and loss processes. The response of an initial phytoplankton patch to the stresses of turbulent diffusion, nutrient depletion, light periodicity, and nocturnal or continuous herbivore grazing is investigated numerically for several oceanic conditions. Nocturnal grazing, while less stressful on primary production than continous grazing, results in lower phytoplankton standing stocks. Increase in biomass of vertically migrating zooplankton results in a net loss of nutrient which might otherwise be egested, recycled, and utilized in the euphotic zone under continuous grazing conditions. The Ivlev constant is shown via sensitivity analysis to be a significant parameter ultimately influencing phytoplankton production. It is demonstrated numerically that diffusion of phytoplankton cells from areas of high concentration to low concentration prevents the local extinction of the standing stock, thereby rendering a positive herbivore grazing-threshold unnecessary for ecosystem stability.     

The trophic status of herbivorous fishes on coral reefs

Estimates of feeding rates, alimentary tract structure and temporal patterns of food processing obtained from twelve species of nominally herbivorous fishes on the northern Great Barrier Reef were compared. These included members of the families Acanthuridae, Scaridae and Kyphosidae. Based on an analysis of diet and short-chain fatty acid (SCFA) profiles from a previous study we initially partitioned the twelve species into four dietary categories, as follows: (a) Category 1: herbivores with a diet of macroscopic brown algae and high SCFA profiles in the hindgut region (Naso unicornis, Kyphosus vaigiensis); (b) Category 2: herbivores feeding on turfing and filamentous red and green algae with moderate SCFA profiles in the hindgut region (N. tonganus, K. cinerascens, Zebrasoma scopas, Acanthurus lineatus); (c) Category 3: zooplankton feeders with moderate SCFA profiles (N. vlamingii, N. brevirostris); (d) Category 4: species feeding on detrital and sedimentary materials with low levels of SCFA (Chlorurus microrhinos, Scarus schlegeli, Ctenochaetus striatus, A. olivaceus). The purpose of this comparison was to determine whether measures of feeding activity, alimentary tract structure, and food processing were concordant with diet. A dichotomy in feeding rates was observed. Species with a diet of algae and zooplankton (categories 1–3) had slower feeding rates than those feeding on detrital aggregates and sediment (category 4). The pattern of food processing also followed the same dichotomy with species of categories 1–3 retaining food in the alimentary tract overnight and commencing the feeding day with substantial amounts of food in the intestine and hindgut. Category-4 species commenced the feeding day with empty alimentary tracts suggesting a rapid turnover of gut contents. Within the herbivorous and zooplankton-feeding species neither alimentary tract structure nor food processing mode were predicted by diet or SCFA profiles. A hindgut fermentation chamber was present in K. vaigiensis but not in N. unicornis, a species with high levels of SCFA in the hindgut region and a diet of brown macroscopic algae. In contrast N. vlamingii, with a diet dominated by animal matter, retained large amounts of food material in a hindgut chamber over the entire feeding cycle. In tropical perciform fishes, herbivory and fermentation are not associated with the alimentary tract structures that characterise herbivorous terrestrial vertebrates. Estimates of the abundance of the different groupings of nominally herbivorous fishes indicated that the dominant elements in the reef grazing and browsing fauna were consumers of detrital and sedimentary materials. These could not be classified as herbivores. Members of this group were dominant in all habitats investigated. Explicitly herbivorous taxa were a minority component in all habitats investigated.Communicated by G.F. Humphrey, Sydney     

Phytoplankton carbon dynamics during a winter-spring diatom bloom in an enclosed marine ecosystem: primary production,biomass and loss rates

Phytoplankton production, standing crop, and loss processes (respiration, sedimentation, grazing by zooplankton, and excretion) were measured on a daily basis during the growth, dormancy and decline of a winter-spring diatom bloom in a large-scale (13 m³) marine mesocosm in 1987. Carbonspecific rates of production and biomass change were highly correlated whereas production and loss rates were unrelated over the experimental period when the significant changes in algal biomass characteristic of phytoplankton blooms were occurring. The observed decline in diatom growth rates was caused by nutrient limitation. Daily phytoplankton production rates calculated from the phytoplankton continuity equation were in excellent agreement with rates independently determined using standard ¹⁴C techniques. A carbon budget for the winter bloom indicated that 82.4% of the net daytime primary production was accounted for by measured loss processes, 1.3% was present as standing crop at the end of the experiment, and 16.3% was unexplained. Losses via sedimentation (44.8%) and nighttime phytoplankton respiration (24.1%) predominated, while losses due to zooplankton grazing (10.7%) and nighttime phytoplankton excretion (2.8%) were of lesser importance. A model simulating daily phytoplankton biomass was developed to demonstrate the relative importance of the individual loss processes.     

Production of tropical copepods in Kingston Harbour, Jamaica: the importance of small species

The copepod community observed during an 18-month period at the mouth of eutrophic Kingston Harbour, Jamaica, was dominated by small species of Parvocalanus, Temora, Oithona, and Corycaeus. Mean copepod biomass was 22.1 mg AFDW m⁻³ (331 mg m⁻²). Annual production was 1679 kJ m⁻², partitioned as 174 kJ m⁻² naupliar, 936 kJ m⁻² copepodite, 475 kJ m⁻² egg and 93 kJ m⁻² exuvial production. All nauplii, most copepodites and many adults, equivalent to half of the biomass and production, were missed by a standard 200-μm plankton net, emphasizing the importance of nauplii and small species in secondary production estimates. The evidence suggests that growth rates and production are generally not food limited, and we speculate that size-selective predation shapes the structure of the harbour community. Biomass and production are higher than previous estimates for tropical coastal waters, but comparable to other eutrophic tropical embayments and many productive temperate ecosystems. Far from being regions of low productivity, tropical zooplankton communities may have significant production and deserve greater research attention than they currently receive. Received: 19 September 1997 / Accepted: 21 October 1997     

Are closure terms appropriate or necessary descriptors of zooplankton loss in nutrient–phytoplankton–zooplankton type models?

Our current knowledge of plankton ecology ascribes a large proportion of zooplankton losses to zooplankton cannibalism and carnivory, rather than via the activity of higher trophic levels beyond the plankton. However, planktonic ecosystem models, such as the widely used nutrient–phytoplankton–zooplankton (NPZ) type models, typically represent all zooplankton losses by mathematically (rather than biologically) justified closure functions. Even where it is assumed that these closure functions include zooplanktonic cannibalism and carnivory, these processes are not explicitly implemented within the grazing function of the zooplankton. Here it is argued that this representation of zooplankton losses through “closure” terms within planktonic food web models is neither appropriate nor necessary. The general consequences of implementing a simple function incorporating zooplankton cannibalism and carnivory (intra-guild predation) within a planktonic food web model are compared against models implementing different types of traditional closure functions. While the modelled biomass outputs may appear similar, the fate of annual primary production and f-ratios vary widely. There appears no justification for the continued use of traditional closure term to depict zooplankton loss processes on biological or modelling arguments. To do so can seriously misrepresent the fate of primary production and thence trophic dynamics.     

A simulation analysis of continental shelf food webs

Energy flow through continental shelf food webs was examined using a simulation model. The model structure expands the two traditional marine food chains of phytoplankton-zooplankton-pelagic fish and benthos-demersal fish into a complex web which includes detritus, dissolved organic matter (DOM), bacteria, protozoa, and mucus net feeders. Simulation of energy flux for different shelf systems using the expanded web revealed that heterotrophic microorganisms and their predators account for a significant component of the energy flux in the continental shelf ecosystem. Contrary to previous models, where all phytoplankton were considered to be grazed by zooplankton, our simulation results indicate that only slightly more than 50% of the annual net primary production is grazed. A substantial quantity of the phytoplankton production directly becomes detritus. Bacteria mineralize detritus and DOM produced by phytoplankton and other components of the food web, converting these to biomass with high efficiency. Consequently, the model predicts that planktonic bacterial production is equivalent to zooplankton production. Exclusion of the bacteria requires the assumption that all DOM is either exported from the system or consumed by another component of the food web. Neither of these assumptions can be supported by present knowledge of the dynamics of DOM in the sea. Model simulations were also employed to test the hypothesis that production exceeds consumption on continental shelves, resulting in exports of 50% of the annual primary production. Simulations of shelves with high rates of primary production resulted in a particulate export of 27% and realistic estimates of secondary production. Results of other simulations suggest that shelves with lower primary production cannot export production and still maintain the macrobenthos and their predators. General properties about continental shelves can also be inferred from the model. From simulations of shelves of differing primary production, nanoplankton are predicted to account for a greater proportion of the primary production in nutrient limited systems. Benthic production appears to be related to both the quantity of primary production and the sinking rates of the phytoplankton. The model indicates that zooplankton fecal inputs to the shelf benthos are only a small portion of the total detrital flux, leading to the prediction that fecal pellets are of little significance in determining benthic production. Finally, the model generates production efficiencies that are highly variable depending on the type of system and kind of populations involved. We argue that the assumed ecological efficiency of 10% should be abandoned for continental shelves and other ecosystems.     

Lack of avoidance of phenolic-rich brown algae by tropical herbivorous fishes

High levels of polyphloroglucinol phenolics in marine brown algae are usually interpreted as a defensive response to herbivory. However, tropical brown algae generally contain very low levels of phenolics, even though herbivory in many tropical systems (e.g. coral reefs) is intense. This apparent paradox would be explained if polyphenolics did not deter tropical herbivores, in which case selection by herbivores for high levels of phenolics in tropical algae would be weak. To examine this hypothesis, in February 1989 we presented mixed assemblages of herbivorous fishes on the Great Barrier Reef with tropical, phenolic-poor brown algae (primarilySargassum spp.) and closely related (conspecifics in one instance) phenolic-rich temperate species. Different species of brown algae were eaten at very different rates, but these differences were not correlated with variation in the phenolic levels among the plants. TLC and NMR analyses showed no evidence of other, non-polar, metabolites in these algae, with the exception of the temperate speciesHomoeostrichus sinclairii. Thus, variation in non-polar metabolites also did not explain the differences in susceptibility to herbivores among these algae. We conclude that the herbivorous fishes studied here were not deterred by phenolic-rich algae, which suggests that levels of phenolics in many tropical algae may generally be low due to their ineffectiveness as defences. However, alternative explanations for the pattern are possible, and these are discussed.     

Impact of conservation areas on trophic interactions between apex predators and herbivores on coral reefs

Apex predators are declining at alarming rates due to exploitation by humans, but we have yet to fully discern the impacts of apex predator loss on ecosystem function. In a management context, it is critically important to clarify the role apex predators play in structuring populations of lower trophic levels. Thus, we examined the top‐down influence of reef sharks (an apex predator on coral reefs) and mesopredators on large‐bodied herbivores. We measured the abundance, size structure, and biomass of apex predators, mesopredators, and herbivores across fished, no‐take, and no‐entry management zones in the Great Barrier Reef Marine Park, Australia. Shark abundance and mesopredator size and biomass were higher in no‐entry zones than in fished and no‐take zones, which indicates the viability of strictly enforced human exclusion areas as tools for the conservation of predator communities. Changes in predator populations due to protection in no‐entry zones did not have a discernible influence on the density, size, or biomass of different functional groups of herbivorous fishes. The lack of a relationship between predators and herbivores suggests that top‐down forces may not play a strong role in regulating large‐bodied herbivorous coral reef fish populations. Given this inconsistency with traditional ecological theories of trophic cascades, trophic structures on coral reefs may need to be reassessed to enable the establishment of appropriate and effective management regimes. El Impacto de las Áreas de Conservación sobre las Interacciones Tróficas entre los Depredadores Dominantes y los Herbívoros en los Arrecifes de Coral     

Structure and function of a warm monomictic lake

Construction and simulation of a model of Lake Conway, Florida, U.S.A., provided a framework for defining major characteristics of this ecosystem. The relationships that are formalized in this model comprise a set of hypotheses about the nature of a warm monomictic lake. The data that were used to parameterize the model came primarily from literature estimates, although approximations of biomass levels were available from associated research conducted on the lake.Submersed macrophytes are a major biomass component in Lake Conway; simulation suggested that their role in nutrient recycling overshadows their importance in the grazing food chain. Phytoplankton biomass and degree of fluctuation are considerably lower than are observed in most cool temperate lakes, although simulated respiration and herbivory rates are closer to temperate values than tropical values. Simulated epipelic algae biomass varies an order of magnitude during the year, and this group appears to be a significant part of the food chain. Simulated zooplankton consumption and turnover rates are very high, in part because of the relatively small biomass per individual. Simulation of the model suggests that slightly more carbon is processed through the grazing food chain in Lake Conway than through the detritus food chain.     

On plankton production in Kungsbacka Fjord,an estuary on the Swedish west coast

Environmental conditions, primary production, and zooplankton populations were studied from May, 1969 to November, 1970 at one station in Kungsbacka Fjord, Sweden. The fjord, with an arca of 53 km², is a moderately polluted estuary, with a small tidal range. Data for primary production and environmental parameters were correlated using Spearman's rank correlation coefficient. The annual rate of primary production in 1970 was about 100 gC·m^-2. Carbon fixation was about 80 g·m^-2 in May–November in1969 and 1970. The average monthly rate was highest in June, 1970, with 25 gC·m^-2; about 15 gC·m^-2 was recorded in August–October of both years. Carbon fixation by the phytoplankton was estimated to be about 2,800 tons in the whole fjord in 1970. The average fresh-water inflow to the fjord, amounting to about 13 m³·sec^-1, added about 380 tons of organic carbon, 45 tons of nitrogen, and 4.5 tons of phosphorus per month. Primary production displayed strong correlation with temperature at different depths (P<0.05 to 0.001), indicating the sediments to be the most important nutrient source. A total of 19 holoplanktonic zooplankton species was identified, copepods being the dominant group. The highest zooplankton biomass, 800 to 900 mg·m^-3, was recorded in June of both years. The production of copepods in May–October was about 1 gC·m^-2 in both years. The total secondary production of the zooplankton was calculated as only 1.8 gC·m^-2 in 1970.     

Effects of Artisanal Fishing on Caribbean Coral Reefs

Abstract:  Although the impacts of industrial fishing are widely recognized, marine ecosystems are generally considered less threatened by artisanal fisheries. To determine how coral reef fish assemblages and benthic communities are affected by artisanal fishing, we studied six Caribbean islands on which fishing pressure ranged from virtually none in Bonaire, increasing through Saba, Puerto Rico, St Lucia, and Dominica, and reaching very high intensities in Jamaica. Using stationary-point fish counts at 5 m and 15 m depth, we counted and estimated the lengths of all noncryptic, diurnal fish species within replicate 10-m-diameter areas. We estimated percent cover of coral and algae and determined reef structural complexity. From fish numbers and lengths we calculated mean fish biomass per count for the five most commercially important families. Groupers (Serranidae), snappers (Lutjanidae), parrotfish (Scaridae), and surgeonfish (Acanthuridae) showed order-of-magnitude differences in biomass among islands. Biomass fell as fishing pressure increased. Only grunts (Haemulidae) did not follow this pattern. Within families, larger-bodied species decreased as fishing intensified. Coral cover and structural complexity were highest on little-fished islands and lowest on those most fished. By contrast, algal cover was an order of magnitude higher in Jamaica than in Bonaire. These results suggest that following the Caribbean-wide mass mortality of herbivorous sea urchins in 1983–1984 and consequent declines in grazing pressure on reefs, herbivorous fishes have not controlled algae overgrowing corals in heavily fished areas but have restricted growth in lightly fished areas. In summary, differences among islands in the structure of fish and benthic assemblages suggest that intensive artisanal fishing has transformed Caribbean reefs.     

Predator diversity and identity drive interaction strength and trophic cascades in a food web

Declining predator diversity may drastically affect the biomass and productivity of herbivores and plants. Understanding how changes in predator diversity can propagate through food webs to alter ecosystem function is one of the most challenging ecological research topics today. We studied the effects of predator removal in a simple natural food web in the Sierra Nevada mountains of California (USA). By excluding the predators of the third trophic level of a food web in a full-factorial design, we monitored cascading effects of varying predator diversity and composition on the herbivorous beetle Chrysomela aeneicollis and the willow Salix orestera, which compose the first and second trophic levels of the food web. Decreasing predator diversity increased herbivore biomass and survivorship, and consequently increased the amount of plant biomass consumed via a trophic cascade. Despite this simple linear mean effect of diversity on the strength of the trophic cascade, we found additivity, compensation, and interference in the effects of multiple predators on herbivores and plants. Herbivore survivorship and predator-prey interaction strengths varied with predator diversity, predator identity, and the identity of coexisting predators. Additive effects of predators on herbivores and plants may have been driven by temporal niche separation, whereas compensatory effects and interference occurred among predators with a similar phenology. Together, these results suggest that while the general trends of diversity effects may appear linear and additive, other information about species identity was required to predict the effects of removing individual predators. In a community that is not temporally well-mixed, predator traits such as phenology may help predict impacts of species loss on other species. Information about predator natural history and food web structure may help explain variation in predator diversity effects on trophic cascades and ecosystem function.     

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.     

Weight and chemical composition of some important oceanic zooplankton in the North Pacific Ocean

Wet and dry weight, total carbon, nitrogen, hydrogen, and ash contents were determined on 33 species of zooplankton distributed predominantly in the open sea region of the North Pacific. Sampling covered the waters from 44°N to the equator. Average percentage of dry weight to wet weight was about 19% of all samples from the whole area. Percentage dry weight of carbon in copepods was on an average 51.5%. The highest value, 66.6%, was obtained in eggs of the copepod Pareuchaeta sarsi. Mixed zooplankton was assumed to contain carbon comprising about 35 to 45% of the dry weight. Carbon contained in the zooplankton biomass existing in the upper 200 m in the western parts of the northern North Pacific and Bering Sea during spring and summer was estimated to range from 20 to 85 mg C/m³. Nitrogen content varied considerably with localities. Average ratio of carbon to nitrogen was 8.5 in subarctic copepods, and 4.1 in subtropic-tropic copepods. This ratio also varied with season. In the copepod Calanus cristatus the ratio was highest (10.0) in May, immediately after the spring bloom of phytoplankton, when the animals contained much fat. The ratio fell to 5.1 in December. There seemed to be a large seasonal variation in boreal zooplankton due to great fluctuations of environmental conditions, especially the amount of food available; in tropical species the range was small because of environmental uniformity. Average hydrogen content was about 6 to 10%. The percentage of ash to dry weight amounted to 39.3% in pteropods and 3.4% in copepods.