The effects of eutrophication-related alterations to coral reef communities on agents and rates of bioerosion (Reunion Island,Indian Ocean)

This study investigated the variation of bioerosional processes in relation to disturbances of reefal communities due to eutrophication. La Saline fringing reef (Reunion Island) is subjected to nutrient inputs from the adjacent land. Bioerosion by grazers, microborers, and macroborers was measured using experimental substrata exposed for 1 year in three sites characterized by different levels of nutrient input and benthic community response. The relationship between bioerosion and epilithic algal cover of hard substrata and the interactions between the various agents of bioerosion were analyzed with parametric statistics. Significant variations in bioerosion were found among sites, ranging from 1.63 to 3.52 kg CaCO₃ m^-2 year^-1 for grazing rates, from 6.73 to 32.25 g m^-2 year^-1 for macroboring rates, and from 43.78 to 67.56 g m^-2 year^-1 for microboring rates. One of the major factors controlling these variations appeared to be changes in the epilithic algal cover on substrata in response to changes in reefal water chemistry. In low nutrient areas, where dead corals were colonized mainly by algal turfs, erosion by microorganisms was low (43.78 g m^-2 year^-1) due to intense grazing (3.52 kg m^-2 year^-1). In reef zones receiving high nutrient inputs, the development of encrusting calcareous algae and macroalgae was associated with the lowest grazing (1.63 kg m^-2 year^-1) and macroboring (6.73 g m^-2 year^-1) rates recorded among sites. In contrast, high microboring rates (57.54 and 67.56 g m^-2 year^-1) were found in enriched areas in association with high macroalgal cover.     

The contribution of picophytoplankton to community structure in a Mediterranean brackish environment

The seasonal composition of phytoplankton communities was investigatedin a Mediterranean brackish area (Varano lagoon). Twelve stationswere sampled monthly from March 1997 to February 1998. Numbersof prokaryotic and eukaryotic picophytoplankton cells were estimatedby epifluorescence microscopy, while larger phytoplankton (nanoand micro fractions) were enumerated by the Utermöhl settlingtechnique. Picophytoplankton densities ranged from 0.7 to 448.6cells x 10⁶ l^–1. Nano- and microphytoplankton abundancesvaried between 0.2 and 7.9 cells x 10⁶ l^–1. The picoplanktonfraction was represented mainly by cyanobacteria and the Utermöhlfraction by nano-sized phytoflagellates (56.2%) and diatoms(20.1%). The phytoflagellates had a greater abundance over timewhile diatoms reached the highest densities in summer and fall.In Varano lagoon, phytoplankton development is related to ‘nitrogen-poor'waters and to phosphorus availability. Suspension-feeding bivalves(Mytilus galloprovincialis) are sufficiently abundant to filtera volume equivalent to the volume of Varano lagoon at leastonce daily. These observations suggest that grazing exerts animportant influence on phytoplankton dynamics, mainly on themicro fraction, and that diatoms seem to play an important rolein the food web dynamics of this coastal fishery.     

The structure,diversity and somatic production of the fish community in an estuarine coastal lagoon,Ria de Aveiro (Portugal)

The present study aims to determine biological fish production of a lagoon and relate this to the commercial fisheries yield. The fish community of an estuarine lagoon in the west coast of Portugal was sampled between November 1998 and November 2000 to estimate the production ecology of the community, including somatic production, population size, species richness, species diversity, and biomass. Using the Allen curve method of determination, the total annual fish production of all fish species in the lagoon was calculated at 90.3 tonnes or 2.1 g m⁻² year⁻¹ in the first year and 106.7 tonnes or 2.5 g m⁻² year⁻¹ in the second year. The marine seasonal migrant species, sardine, Sardina pilchardus, which colonises the lagoon during the juvenile period of its life stages, produced more than 35 tonnes in each year and accounted for >39 and >33%, in the first and second year respectively, of the total fish production in this lagoon. Sardine was numerically more abundant (18,217 specimens) but due to their small size contributed only 13% to the total biomass. Sardine was thus the most important fish species in terms of the consumption and production processes of the whole fish community in this system. Commercial fisheries’ records indicate that approximately 300 tonnes per annum of fish are taken from the lagoon, which corresponds to three times more than the estimated production in the lagoon. Thus, if it exists, the sustainability of the fishery appears to depend on the immigration of fish from the adjacent coastal area and it is questioned whether the fishery is sustainable in the long-term. The findings indicate that careful and effective management of the lagoon is required to ensure a long-term healthy aquatic environment and sustainable catches in the future.     

A quantitative analysis of the annual phytoplankton cycle of the Magdalena lagoon complex (Mexico)

An annual cycle of the micro- and nanophytoplankton cell densitiesof whole water samples in combination with the species composition,distribution and diversity of the microphytoplankton fractionin the Magdalena lagoon system, Mexico, has been studied ona bi-monthly basis. Two major patterns were detected: from Novemberto May, high microphytoplankton densities (0.5–1.5 10⁶cells l^–1 prevail throughout a large part of the investigatedarea; from late spring to late autumn cell densities are muchlower (5–250 10³ cells l^–1). During each perioda number of microplankton assemblages occur, each with its characteristicdiversity and stability. The distribution and density of themicrophytoplankton is clearly related to the presence of nutrient-richwater pockets and the prevailing winds for part of the year,as documented in earlier hydrological studies. The nanophytoplanktonplayed a minor role during most of the year; this fraction dominatedquantitatively in only a few cases. Based on these data, theMagdalena lagoon complex can be considered a very productivearea. This is further substantiated by the presence of largequantities of several sardine and mackerel species, feedingalmost exclusively on microphytoplankton. This reveals the existenceof an important short chain pathway in the food web of thiscoastal lagoon system     

Phytoplankton and its contribution to primary productivity in two coral reef areas of French Polynesia

A study of phytoplankton productivity and related parameters was carried out in two fringing and barrier reef systems around Moorea island (Tiahura lagoon) and Tahiti (Vairao lagoon), Society Islands, during July–August 1974.In Tiahura lagoon, which is the narrower and the shallower, phytoplankton standing crop and production are extremely low; photosynthetic assimilation, as measured in situ by the ¹⁴C method, ranges from 4 to 27 mg C m^?2 day^?1, presumably less than in the impoverished open ocean, but it is surprisingly high at a station just outside the barrier reef (645 mg C m^?2 day^?1) as the result of some island mass effect. As compared to the latter station, the lagoon shows a ten-times increase in particle content and glucose uptake, and a higher percentage of decomposition products of plant pigments. These data support the concept that, in such environments where benthic primary producers prevail, phytoplankton may have lost its ‘usual’ rôle in the aquatic food web.Vairao lagoon, a deeper and wider one, subjected to a greater extent to land fertilization, is more productive (103–420 mg C m^?2 day^?1). Considering this difference, as well as the considerable range of phytoplankton production in coral reef areas of the world, the need for a trophic classification of such ecosystems is emphasized.     

Dinoflagellate bloom development and collapse in Lake Kinneret: a sediment trap study

Warm monomictic Lake Kinneret, Israel, is characterized by awinter–spring water bloom of the large (~50 µm diameter)dinoflagellate Peridinium gatunense Nygaard. Usually the P.gatunense bloom declines in May–June and a less prominentbloom of smaller dinoflagellates (mostly Peridiniopsis spp.of ~20–30 µm diameter) develops. Water column abundancesand sedimentation losses to those dinoflagellates were followedthroughout 1994 and 1995. The objective was to quantify thevariables that describe population dynamics, that in turn willshed more light on the seasonal patterns of bloom dynamics.Sedimentation losses were measured by means of sediment trapswith and without a preservative (formaldehyde) that were exposedfor 24 h once every 2–3 weeks. Annual sedimentation lossesof Peridinium (hypolimnetic trap catches) were 209 g wet wtm^-2 year^-1 in 1994 and 187 g wet wt m^-2 year^-1 in 1995, whichconstituted 16 and 23% of Peridinium production in those years,respectively. This study revealed that increased death ratespreceded a mass sedimentation flux of Peridinium and causedthe decline of the bloom in Lake Kinneret. Annual sedimentationlosses of Peridiniopsis were 55 g wet wt m^-2 year^-1 in 1994and 34 g wet wt m^-2 year^-1 in 1995. In contrast to live Peridiniumcells, Peridiniopsis cells continued to swim to the lower trapafter the onset of thermal stratification, possibly taking advantageof the higher nutrient concentrations below the thermocline,at a time when the lake is already stratified and the epilimnionis nutrient depleted. This could be an important factor allowingPeridiniopsis spp. to peak after the decline of Peridinium.     

Salt marsh productivity with natural and altered tidal circulation

Summary The effects of altered tidal circulation on southern California salt marshes are investigated by comparing a well-flushed wetland and two modified wetlands which have reduced tidal flow. The Tijuana Estuary had continuous exchange of seawater but relatively low net aerial primary productivity (0.4–1.0 kg m^-2yr^-1) of vascular plants. Low productivity (0.6 kg m^-2yr^-1) was also found in the Flood Control Channel of the San Diego River, where tidal exchange was restricted to flow through a riprap dike. High productivity (1.2–2.9 kg m^-2yr^-1) in Los Penasquitos Lagoon was attributed to the influences of freshwater impounded behind a sand bar which blocked the mouth of the lagoon during much of the study period.It is hypothesized that elimination of tidal flow during the growing season increased primary productivity of vascular plants because freshwater runoff decreased soil salinity and because nutrients were retained within the marsh. However, we predict that sand bar obstruction can decrease productivity if below-average rainfall leads to hypersalinity of closed lagoons. Comprehensive evaluation of the effects of altered tidal circulation requires longterm study and examination of the total ecosystem.     

Pilot trophic model for subantarctic water over the Southern Plateau, New Zealand: a low biomass, high transfer efficiency system

The Southern Plateau subantarctic region, southeast of New Zealand, is an important feeding area for birds, seals and fish, and a fishing ground for commercially significant species. The Southern Plateau is a major morphometric feature, covering approximately 433,620 km² with average depth of 615 m. The region is noted for its relatively low levels of phytoplankton biomass and primary production that is iron-limited. In order to evaluate the implications of these attributes for the functioning of this ecosystem a steady-state, 19-compartment model was constructed using Ecopath with Ecosim software of Christensen et al. [www.ecopath.org]. The system is driven by primary production that is primarily governed by the supply of iron and light. The total system biomass of 6.28 g C m⁻² is very low compared with systems so far modelled with a total system throughput of 1136 g C m⁻² year⁻¹. In the model, the Southern Plateau retains 69% of the biomass in the pelagic system and 99% of total production. Although fish are caught demersally, most of their food is part of production in the pelagic system. Top predators represent about 0.3% of total biomass and account for about 0.24 g C m⁻² year⁻¹ of food consumed made up of birds 0.058 g C m⁻² year⁻¹, seals 0.041 g C m⁻² year⁻¹, and toothed 0.094 g C m⁻² year⁻¹ and baleen whales 0.051 g C m⁻² year⁻¹. This amounts to 105,803 tonnes carbon over the whole of the Southern Plateau and is about 17% of the total amount of food eaten by non-mesopelagic fish. Mean transfer efficiencies between trophic levels II and IV of 23% are at the high end of the range reported in the literature. In the model, adult fish production is almost completely accounted for by the fisheries take (32%), consumption by seals (7%), toothed whales (21%), other adult fish (13%), and squid (20%). Fish and squid catches are at the trophic levels of 4.8 and 5.0, respectively. The gross efficiency of the fishery is 0.018% (catch/primary production). Although not all data come from direct knowledge of this system, the model reflects its general characteristics, namely a low primary production system dominated by the microbial loop, low sedimentation to the seafloor, high transfer efficiencies, a long food web and supporting high-level predators.     

Structure and Function of an Age Series of Poplar Plantations in Central Himalaya: I Dry Matter Dynamics

The biomass and net primary productivity (NPP) of 5- to 8-year-oldpoplar (Populus deltoides Marsh, Clone D₁₂₁) plantations growingin the Tarai belt (low-lying plains with high water table adjacentto foothills of central Himalaya) were estimated. Allometricequations for all the above-ground and below-ground componentsof trees and shrubs were developed for each stand. Understorey,forest floor biomass, and litter fall were also estimated fromstands. The biomass of plantation, forest floor litter mass,tree litter fall and net primary productivity (NPP) of treesand shrubs increased with increase in plantation age, whereasherb biomass and NPP significantly (P < 0·01) decreasedwith increasing plantation age. The total plantation biomassincreased from 84·0 in the 5-year-old to 170·0t ha^-1 in the 8-year-old plantation and NPP from 16·8t ha^-1 year^-1 in the 5- and 6-year-old to 21·8 t ha^-1year^-1 in the 8-year-old plantation. The biomass accumulationratio (biomass: net production, BAR) for different tree componentsincreased with the age of plantation increase. The BAR ratioranged from 4·9 in the 5-year-old to 7·7 in the8-year-old plantation.Copyright 1995, 1999 Academic Press Populus deltoides plantations (Clone D₁₂₁), biomass, dry matter turnover, net primary productivity, Tarai belt of Central Himalaya     

Trophic ecology and food consumption of fishes in a hypersaline tropical lagoon

This study evaluated the trophic ecology (diet composition, trophic strategy, similarities and overlap between species, feeding period and food consumption) of six benthivorous fish species in Araruama Lagoon, the largest hypersaline tropical lagoon on the east coast of South America, with an area of 210 km² and an average salinity of 52. The burrfish Chilomycterus spinosus fed on Anomalocardia flexuosa shell deposits, ingesting associated fauna. The caitipa mojarra Diapterus rhombeus differed from all other species, having not only the highest proportions of algae and Nematoda, but also feeding on polychaete tentacles. The two mojarras Eucinostomus spp. showed similar trophic strategies, feeding mostly on Polychaeta. The corocoro grunt Orthopristis ruber also fed mainly on Polychaeta, but differed from Eucinostomus spp. in secondary items. The whitemouth croacker Micropogonias furnieri fed mainly on small Crustacea at night, showing a high number of secondary prey items with low frequencies and high prey‐specific abundance. The daily food consumption (g food g^?1 fish mass) for Eucinostomus argenteus was 0·012 and was 0·031 and 0·027 for M. furnieri in two different sampling events. The diet similarities between Araruama Lagoon and other brackish and marine environments indicate that hypersalinity is not a predominant factor shaping the trophic ecology of fishes in this lagoon. The stability of hypersaline conditions, without a pronounced gradient, may explain the presence of several euryhaline fishes and invertebrates well adapted to this condition, resulting in a complex food web.     

Food web analysis of southern California coastal wetlands using multiple stable isotopes

Carbon, nitrogen, and sulfur stable isotopes were used to characterize the food webs (i.e., sources of carbon and trophic status of consumers) in Tijuana Estuary and San Dieguito Lagoon. Producer groups were most clearly differentiated by carbon, then by sulfur, and least clearly by nitrogen isotope measurements. Consumer ¹⁵N isotopic enrichment suggested that there are four trophic levels in the Tijuana Estuary food web and three in San Dieguito Lagoon. A significant difference in multiple isotope ratio distributions of fishes between wetlands suggested that the food web of San Dieguito Lagoon is less complex than that of Tijuana Estuary. Associations among sources and consumers indicated that inputs from intertidal macroalgae, marsh microalgae, and Spartina foliosa provide the organic matter that supports invertebrates, fishes, and the light-footed clapper rail (Rallus longirostris levipes). These three producers occupy tidal channels, low salt marsh, and mid salt marsh habitats. The only consumer sampled that appears dependent upon primary productivity from high salt marsh habitat is the sora (Porzana carolina). Two- and three-source mixing models identified Spartina as the major organic matter source for fishes, and macroalgae for invertebrates and the light-footed clapper rail in Tijuana Estuary. In San Dieguito Lagoon, a system lacking Spartina, inputs of macroalgae and microalgae support fishes. Salicornia virginica, S. subterminalis, Monanthochloe littoralis, sewage- derived organic matter, and suspended particulate organic matter were deductively excluded as dominant, direct influences on the food web. The demonstration of a salt marsh–channel linkage in these systems affirms that these habitats should be managed as a single ecosystem and that the restoration of intertidal marshes for endangered birds and other biota is compatible with enhancement of coastal fish populations; heretofore, these have been considered to be competing objectives. Received: 24 April 1996 / Accepted: 24 October 1996     

An estimate of global primary production in the ocean from satellite radiometer data

An estimate of global net primary production in the ocean hasbeen computed from the monthly mean near-surface chlorophyllfields for 1979–1986 obtained by the Nimbus 7 CZCS radiometer.Our model required information about the subsurface distributionof chlorophyll, the parameters of the photosynthesis-light relationship,the sun angle and cloudiness. The computations were partitionedamong 57 biogeochemical provinces that were specified from regionaloceanography and by examination of the chlorophyll fields. Makingdifferent assumptions about the overestimation of chlorophyllby the CZCS in turbid coastal areas, the global net primaryproduction from phytoplankton is given as 45–50 Gt C year^–1.This may be compared with current published estimates for landplants of 45–68 Gt C year^–1 and for coastal vegetationof 1.9 Gt C year^–1.     

Carbon flux by seasonal vertical migrant copepods is a small number

The abundant species of Calanus that dominate the mesozooplanktonof high North Atlantic latitudes overwinter at depths >500m, when the population loses 70–80% of its biomass bypredation and physiological stress. This represents an annualflux of carbon, obtained in the photic zone, into the interiorof the ocean of 274.5 mg C m^–2 year^–1, or 0.0018Gt C year^–1 for the North Atlantic. This is a small valuecompared with the flux of respiratory carbon by diel migrantsin warmer oceans and, when extrapolated to a global flux (0.012–0.018Gt C year^–1 over areas where winter migrations are importantis also small compared with computations of the global sinkingflux of particles through 200 m (1.6–3.8 Gt C year^–1or other relevant global carbon fluxes in the oceans.     

Scaling-up carbon and carbonate metabolism of coral reefs using in-situ data and remote sensing

Areal up-scaling at reef-scale of organic and inorganic metabolism is possible using in-situ measurements and remote sensing data providing the extent of each bottom type inside the reef. Using a SPOT image and published values of metabolism, the gross production (93,560᎒³ kg C year^-1), excess production (10,017᎒³ kg C year^-1) and calcification (165,348᎒³ kg CaCO₃ year^-1) over 35 km² of coral reef environment in Moorea Island (French Polynesia) are estimated. While the computations are straightforward, certain assumptions must be made in order to conduct the scaling exercise. The exercise is valid only if the metabolism of reef benthos is additive through increasing spatial scale. Despite the difficulty of quantitatively assessing our extrapolations, spatial additivity seems to represent the reality. The other limitation is that the reef must be considered as a closed system, in an equilibrium state supposedly accurately described by the few available in-situ measurements. To consider the reef an open system, long-term metabolic measurements coupled with knowledge of oceanic and land forcing processes are required. These theoretical considerations point to the necessity of integrated multi-scale studies based on both remote sensing and in-situ data in order to better understand the productivity and calcification of reefs in the current global change context.     

The seasonal productivity of phytoplankton in a hypertrophic, gravel-pit lake

The seasonal time course of phytoplankton primary productivitywas studied weekly in a hypertrophic, gravel-pit lake closeto Madrid, Spain. Chlorophyll a ranged 22–445 mg m^–2.Gross primary productivity attained 0.28±0.14 g C m^–2h^–1 (range: 0.06–0.60), its yearly value being 900g C m^–2, but the shallow euphotic depths and the highplankton respiration ensured that net productivity was generallylow. Respiration losses amounted to 0.31±0.24 g O₂ m^–2h^–1, with phytoplankton respiration roughly attainingone-half of overall plankton respiration. Areal phytoplanktonproductivity and plankton respiration followed a seasonal trendbut this was not the case for photosynthetic capacity. Surfacephotoinhibition was evenly distributed throughout the study.Quantum yields showed an increasing depth trend, but no seasonaltrend. Both P_max and I_k were both temperature- and irradiance-dependent.As compared with lakes of lesser trophic degree, phytoplanktonprimary production in hypertrophic lakes might be increasednot only by higher nutrient contents but also by low chlorophyll-specificattenuation coefficients and low background, non-algal attenuation,thereby allowing for higher areal chlorophyll contents and hencehigher areal productivity. Our study suggests that physical(irradiance and water column stability) as well as chemicalfeatures (dissolved inorganic carbon and soluble reactive phosphorus)may control seasonality of phytoplankton primary productionin this lake despite recent claims that only physical factorsare of significance in hypertrophic lakes. However, this doesnot explain all the variability observed and so a food web controlis also likely to be operating.     

Structure and Function of High Altitude Forests of Central Himalaya I. Dry Matter Dynamics

The present study deals with the structure and functioning ofthree different forest communities, viz., horse chestnut, silverfir and kharsu oak forests, in a high altitude region of CentralHimalaya. The tree density and total basal cover of horse chestnutforest was 280 and 76, silver fir forest 355 and 106, and kharsuoak forest 480 trees ha^-1 and 73 m² ha^-1, respectively. Allometricequations relating biomass of different tree components to cbh(circumference at breast height) were significant. Total vegetationbiomass was 505 t ha^-1 in horse chestnut, 566 t ha^-1 in silverfir and 593 t ha^-1 in kharsu oak forests, of which maximum contributionwas by tree layer followed by shrub, herb, sapling and seedlinglayers. The forest floor litter biomass was 2·1, 4·7and 4·2 t ha^-1 in horse chestnut, silver fir and kharsuoak forests, respectively. The total litter fall was 7·3,6·7 and 9·4 t ha^-1 year^-1, of which leaf littercontributed 48, 39 and 64% in horse chestnut, silver fir andkharsu oak forests, respectively. Turnover rate of tree litterwas 0·80 in horse chestnut, 0·61 in silver firand 0·71 in kharsu oak forests. Net primary productionof total vegetation was 19·6, 18·9 and 24·9t ha^-1 year-1, of which tree layer contributed maximum proportionfollowed by herb, shrub, sapling and seedling layers. To showdry matter storage and flow of dry matter within the system,compartment models were developed for all forests.Copyright1995, 1999 Academic Press Total basal cover, biomass, productivity, Quercus, Aesculus, Abies, high altitude, litter, compartmental transfer     

Simulated annual plankton production in the northeastern Pacific Coastal Upwelling Domain

A microcomputer simulation model is presented that describesthe generalized plankton production dynamics, in the surfacemixed layer, of the Juan de Fuca Eddy located on the southwesternBritish Columbia continental shelf. The Juan de Fuca Eddy simulationmodel evaluates how the annual biomass production of diatoms,copepods and euphausiids is forced by plankton feeding interactions,seasonal variability in upwelling, water temperature and solarradiation, and generalized fish predation. The model estimatesannual primary production of 345 g C m^–2 year^–1and secondary production of 19.4 g C m^–2 year^–1for copepods and 6 g C m^–2 year^–1 for euphausiids,during 1985–89; -90% of the annual plankton productionwas generated during the April-October upwelling season. Perturbationsof 22 abiotic and biotic parameters, one at a time by ±10%of nominal values, indicated that oceanic variability (e.g.upwelling rate) most strongly affected primary production. Conversely,zooplankton production was most sensitive to variability inbiological parameters describing zooplankton grazing potentialand growth (e.g. gross growth efficiency). Simulated seasonalbiomass patterns of diatoms, copepods and euphausiids were foundto closely match empirical data. However, euphausiid biomassproduction in the Juan de Fuca Eddy alone was unable to meetthe demands of estimated pelagic fish consumption. Local Eddyeuphausiid populations had to be supplemented, from regionaleuphausiids. by a mechanism that is proposed to be linked tothe seasonal pattern and intensity of positive Ekman transport(upwelling).     

Contribution of heterotrophic plankton to nitrogen regeneration in the upwelling ecosystem of A Coruna (NW Spain)

The contribution of heterotrophic plankton to nitrogen (N) regenerationin the water column, and its significance for the requirementsof phytoplankton, were studied at the seasonal scale in thecoastal upwelling ecosystem of A Coruña (Galicia, NWSpain). During 1995–1997, monthly measurements were takenof hydrographic conditions, dissolved nutrients, and abundanceand biomass of microplanktonic heterotrophs (bacteria, flagellatesand ciliates), phytoplankton and mesozooplankton (>200 µm).Additionally, series of experiments were conducted to quantifyN fluxes, including primary production (¹⁴C method), phytoplanktonuptake of nitrate, ammonium and urea (¹⁵N-labelling techniques),microheterotrophic regeneration of ammonium, mesozooplanktongrazing (chlorophyll gut-content method) and excretion of ammoniumby mesozooplankton. Two N budgets were built for the averagesituations of high (>100 mg C m^-2 h^-1) and low (<100 mgC m^-2 h^-1) primary production. The results revealed that phytoplanktonrelied strongly on regenerated ammonium all year round (33 and43% of total N uptake in high and low production situations,respectively). This demand for ammonium was closely matchedby regeneration rates of microplankton (0.14–0.25 mmolN m^-2 h^-1), whereas zooplankton contributed on average <10%to N regeneration. Likewise, zooplankton grazing had littledirect control on phytoplanktonic biomass. The results obtainedindicate that in the A Coruña upwelling system, N biomassof heterotrophic plankton is generally higher than phytoplanktonN biomass. The high rates of N regeneration measured also suggestthat a large proportion of the organic matter produced afteran upwelling pulse is recycled in the water column through themicrobial food web.     

Stable isotopic structure of aquatic ecosystems

Isotopic, biogeochemical and ecological structure can provide a new dimension for understanding material flows, and the simultaneous function and structure of an ecosystem. Distributions ofδ ¹³C andδ ¹⁵N for biogenic substances in the Nanakita river estuary involving Gamo lagoon in Japan were investigated to construct isotope biogeochemical and ecological structure for assessing fate and transfer of organic matter, and food web structure. The isotopic framework of the ecosystem was successfully described in aδ ¹⁵N–δ ¹³C map. In this estuary the variations of isotope ratios of biogenic substances were clearly explained by the mixing of land-derived organic matter, and marine-derived organic matter. A trophic-level effect of¹⁵N enrichment was clearly observed. Organisms were classified into three groups depending upon the contribution of land-derived organic matter in a food chain. Almost all biota except mollusca in the lagoon depend on organic matter of marine origin. The contributions of both land and marine organic matter were comparable for mollusca in the lagoon.