Herbivorous Food Web Research Articles

Ecotoxicological Effects of Combined UVB and Organic Contaminants in Coastal Waters: A Review

ABSTRACTOrganisms living in coastal waters are exposed to anthropogenic contaminants from terrestrial drainage, ice melting and maritime traffic and to enhanced UVB radiation (UVBR; 280–320 nm) caused by decreased concentrations of ozone in the stratosphere. This article reviews available information about the combined effects of UVBR and selected hydrosoluble contaminants potentially present in surface waters on marine species and especially on plankton community structure in high‐latitude coastal zones. Effects of UVBR on three selected pesticides (Atrazine, carbaryl and Acifluorfen) and possible induction of phototoxicity are reviewed. Most toxicological studies have been conducted under laboratory conditions with questionable relevance for coastal marine ecosystems. Similarly, photoactivation of polycyclic aromatic hydrocarbons (PAHs) has been closely examined and reported effects on aquatic species summarized. Experiments with field‐sampled communities demonstrated the complexity and the difficulty in determining the impact of multiple stressors on an aquatic ecosystem, even for ecosystems simplified by eliminating large grazers and fish. Nutrient status, specific composition and light history have influenced the different responses of planktonic assemblages exposed to enhanced UVBR and water‐soluble fraction (WSF) from crude oil or to tributyltin. Plankton assemblages subjected to changes in the ozone hole were physiologically stressed and more susceptible to WSF tox‐icity than communities from less enhanced UVBR‐impacted sites. A close relationship between phytoplankton assemblages and bacteria was observed in all experiments in mesocosms. A contaminant‐induced phytoplankton crash after a bloom event may release important carbon and nutrient sources for bacteria. The magnitude of phytoplanktonic mortality induced by a contaminant probably influenced how rapidly bacteria grew over time. The transition from a herbivorous food web to a microbial food web has significant ecological implications for carbon cycling and energy flow in pelagic systems. A high phytoplankton mortality implies a situation in which the potential for downward carbon export from surface waters is high. In contrast, high bacterial enrichment implies that the phytoplankton carbon is largely recycled in surface waters through a microbial loop and does not contribute significantly to sinking particle flux. The most ecologically relevant results were obtained with mesocosm studies using field‐collected communities. The enhancement of hydrocarbon toxicity in the presence of a high level of UVBR cannot be described as being a synergistic or an additive effect, because the WSF alone is not toxic and may even be beneficial by increasing bacterial activity. This is a case in which one stressor has the ability to modify another stressor to cause it to be toxic to target organisms. These abiotically induced interactions may be important for biological communities exposed to extreme conditions when physical, chemical or photochemical reactions modify the nature of environmental stressors before they interact with biological functions. The need for models on the impacts of multiple stressors on biodiversity and ecosystem functioning is emphasized.

Ecotoxicological Effects of Combined UVB and Organic Contaminants in Coastal Waters: A Review

Organisms living in coastal waters are exposed to anthropogenic contaminants from terrestrial drainage, ice melting and maritime traffic and to enhanced UVB radiation (UVBR; 280-320 nm) caused by decreased concentrations of ozone in the stratosphere. This article reviews available information about the combined effects of UVBR and selected hydrosoluble contaminants potentially present in surface waters on marine species and especially on plankton community structure in high-latitude coastal zones. Effects of UVBR on three selected pesticides (Atrazine, carbaryl and Acifluorfen) and possible induction of phototoxicity are reviewed. Most toxicological studies have been conducted under laboratory conditions with questionable relevance for coastal marine ecosystems. Similarly, photoactivation of polycyclic aromatic hydrocarbons (PAHs) has been closely examined and reported effects on aquatic species summarized. Experiments with field-sampled communities demonstrated the complexity and the difficulty in determining the impact of multiple stressors on an aquatic ecosystem, even for ecosystems simplified by eliminating large grazers and fish. Nutrient status, specific composition and light history have influenced the different responses of planktonic assemblages exposed to enhanced UVBR and water-soluble fraction (WSF) from crude oil or to tributyltin. Plankton assemblages subjected to changes in the ozone hole were physiologically stressed and more susceptible to WSF toxicity than communities from less enhanced UVBR-impacted sites. A close relationship between phytoplankton assemblages and bacteria was observed in all experiments in mesocosms. A contaminant-induced phytoplankton crash after a bloom event may release important carbon and nutrient sources for bacteria. The magnitude of phytoplanktonic mortality induced by a contaminant probably influenced how rapidly bacteria grew over time. The transition from a herbivorous food web to a microbial food web has significant ecological implications for carbon cycling and energy flow in pelagic systems. A high phytoplankton mortality implies a situation in which the potential for downward carbon export from surface waters is high. In contrast, high bacterial enrichment implies that the phytoplankton carbon is largely recycled in surface waters through a microbial loop and does not contribute significantly to sinking particle flux. The most ecologically relevant results were obtained with mesocosm studies using field-collected communities. The enhancement of hydrocarbon toxicity in the presence of a high level of UVBR cannot be described as being a synergistic or an additive effect, because the WSF alone is not toxic and may even be beneficial by increasing bacterial activity. This is a case in which one stressor has the ability to modify another stressor to cause it to be toxic to target organisms. These abiotically induced interactions may be important for biological communities exposed to extreme conditions when physical, chemical or photochemical reactions modify the nature of environmental stressors before they interact with biological functions. The need for models on the impacts of multiple stressors on biodiversity and ecosystem functioning is emphasized.

The dynamics of microbial and herbivorous food webs in a coastal sea with special reference to intermittent nutrient supply from bottom intrusion

The dynamics of microbial and herbivorous food webs in a coastal sea with special reference to intermittent nutrient supply from bottom intrusion

Mesozooplankton metabolism and feeding in the NW Iberian upwelling

Mesozooplankton size-fractionated biomass, feeding, and metabolic rates were investigated during a cruise conducted off NW Spain in August 1998. Based on different hydrographic conditions observed throughout the study area (upwelling events nearshore and stratified waters offshore), the sampling area was divided into coastal and oceanic zones. Both phytoplankton and mesozooplankton biomass were higher in the coastal upwelling than in the offshore zone. Size structure analysis shows that the small fraction (200–500 μm) of mesozooplankton was more important than the larger fractions (500–1000 μm and >1000 μm) in the open ocean in terms of both biomass and grazing activity. The relationship between copepod ingestion rate and the concentration of chlorophyll a >5 μm resembled a type II functional response. Saturation occurred at the coastal stations, where copepod gut contents were highest. The grazing impact on both phytoplankton biomass and primary production seemed to be higher offshore (5.7 and 12.9%, respectively) than in upwelled waters (5.2 and 5.2%), although the differences were not statistically significant. Phytoplankton ingestion was enough to fulfil the basal metabolism of mesozooplankton in the upwelling area (102.1% of the minimum carbon requirements met) but not in the offshore (45.9%). Feeding activity and metabolic measurements on mesozooplankton point to a herbivorous food web in the coastal upwelling zone and a multivorous food web in the stratified open oceanic waters. The amount of metabolic end products released by mesozooplankton was higher in the coastal zone, but mesozooplankton played a more important role as nutrient regenerators in the multivorous food web (30.3 and 21.7% of the nitrogen and phosphorus phytoplankton demand accounted for ammonium and phosphate excretion, respectively) than in the herbivorous one (3.8% for ammonium and 2.6% for phosphate).

The dynamics of microbial and herbivorous food webs in a coastal sea with special reference to intermittent nutrient supply from bottom intrusion

Seasonal changes in abundance of planktonic microorganisms, together with some physico-chemical variables, were monitored monthly from May 1999 to March 2002 in the surface water of a coastal bay where nutrients are mainly supplied by intermittent intrusions of deeper water (bottom intrusion). No significant bottom intrusion was detected in 1999 but large or frequent bottom intrusions were found from June to October in 2000, and again from mid-June only to late July in 2001. These results indicate that there is a different nutrient supply every year, and peaks in the abundance of dominant eukaryotic phytoplankton (diatoms and dinoflagellates) roughly corresponded to the occurrences of bottom intrusions. By contrast, there was a cyclic seasonal pattern of autotrophic picoplankton (APP) cell density, which reached maxima in August of every year at very similar levels (4.0-5.0 × 105 cells ml–1). Thus, the seasonal abundance of APP was apparently independent of the occurrence of bottom intrusions. Seasonal changes in cell densities of heterotrophic bacteria showed similar trends to the APP, and temperature-dependent growth of both was indicated. The present study suggests that the matter cycling in the bay varies as a result of shifts in the dominant food linkages, from a microbial food web to a herbivorous food web, due to intermittent nutrient supplies from bottom intrusions.

Burrowing rodents as ecosystem engineers: the ecology and management of plateau zokors Myospalax fontanierii in alpine meadow ecosystems on the Tibetan Plateau

ABSTRACT 1. Plateau zokors, Myospalax fontanierii, are the only subterranean herbivores on the Tibetan plateau of China. Although the population biology of plateau zokors has been studied for many years, the interactions between zokors and plants, especially for the maintenance and structure of ecological communities, have been poorly recognized. In the past, plateau zokors have been traditionally viewed as pests, competitors with cattle, and agents of soil erosion, thus eradication programmes have been carried out by local governments and farmers. Zokors are also widely and heavily exploited for their use in traditional Chinese medicine. 2. Like other fossorial animals, such as pocket gophers Geomys spp. and prairie dogs Cynomys spp. in similar ecosystems, zokors may act to increase local environmental heterogeneity at the landscape level, aid in the formation, aeration and mixing of soil, and enhance infiltration of water into the soil thus curtailing erosion. The changes that zokors cause in the physical environment, vegetation and soil clearly affect the herbivore food web. Equally, plateau zokors also provide a significant food source for many avian and mammalian predators on the plateau. Zokor control leading to depletion of prey and secondary poisoning may therefore present problems for populations of numerous other animals. 3. We highlight the important role plateau zokors play in the Tibetan plateau ecosystem. Plateau zokors should be managed in concert with other comprehensive rangeland treatments to ensure the ecological equilibrium and preservation of native biodiversity, as well as the long‐term sustainable use of pastureland by domestic livestock.

Phytoplankton production in the North Water Polynya: size-fractions and carbon fluxes, April to July 1998

In order to understand the mechanisms responsible for the high productivity and bio- geochemical cycling of carbon in the North Water Polynya (NOW), we determined physical proper- ties and nutrient concentrations of the upper water column, and phytoplankton production, during spring/summer (April to July) 1998. Phytoplankton production of total organic carbon (PTOC) was par- titioned into production of dissolved and particulate organic carbon (PDOC and PPOC, respectively), the latter being further partitioned into production of large and small phytoplankton (PL and PS, respectively) using 5 µm as threshold. The highest PTOC was 6 g C m -2 d -1 at peak bloom. The fraction of PDOC in PTOC was lower in periods of high PTOC than those of low PTOC. Averaged over the whole polynya for the sampling period, PDOC and PPOC accounted for 34 and 66% of the fixed carbon, respec- tively, and 81 and 19% of PPOC were in the PL and PS fractions, respectively. Variations in the inte- grated assimilation numbers of large and small phytoplankton were mostly explained by nutrients and irradiance. Even though PPOC was dominated by large phytoplankton, the sinking rates of the phytoplankton cells were relatively low (0 to 0.7 m d -1 ), hence low export of PPOC to depth (17%), and relatively high potential transfer to large pelagic organisms through the herbivorous food web. This explains why the NOW is a major feeding and spawning area for fish, mammals and birds.

Mesozooplankton features in a frontal area off northern Patagonia (Argentina) during spring 1995 and 1998

Hydrography and spatial patterns of mesozooplankton communities in a frontal area off northern Patagonian were investigated in spring 1995 and 1998. Differential wind/tidal relative forcing appeared to drive along-shelf variations in the physical structure of the front and hence, planktonic patterns. Approaching the northern border of the system, the conspicuous presence of Oithona helgolandica , cladocerans and appendicularians, and the low fecundities of calanoid copepods suggest that a high proportion of the primary production may be channelled through microheterotrophic processes. Towards the southern area, calanoids dominated over cyclopoids, `large´ species ( Drepanopus forcipatus, Calanoides cf. carinatus ) were abundant, and copepod fecundities were high and steady, suggesting that there may be a more classical-like herbivorous food web. Paracalanus parvus and Ctenocalanus vanus occurred over the whole system but seemed to be key species in mixed and weakly stratified waters. Copepods peaked in stratified waters away from the front itself, while the abundance of eggs and nauplii was highest in weakly stratified waters next to the front. Apparently, the general heating occurring over the region in spring 1998 strongly affected both the physical and biological structure of the system. Unlike in 1995, a massive bloom of gelatinous zooplankton took place in 1998 and a bloom of the phagotrophic dinoflagellate Noctiluca sp. occurred in the northern area. Co-occurring copepod populations were depressed and their reproductive patterns seemed to be highly disrupted. Present observations stress the significance of variable external forcing in driving satisfactory habitat conditions for juvenile fish in the area.

Particulate matter and plankton dynamics in the Ross Sea Polynya of Terra Nova Bay during the Austral Summer 1997/98

The structure and variability of the plankton community and the distribution and composition of suspended particulate matter, were investigated in the polynya of Terra Nova Bay (western Ross Sea) during the austral summer 1997/1998, with the ultimate objective of understanding the trophic control of carbon export from the upper water column. Sampling was conducted along a transect parallel to the shore, near the retreating ice edge at the beginning of December, closer to the coast at the beginning of February, and more offshore in late February. Hydrological casts and water sampling were performed at several depths to measure total particulate matter (TPM), particulate organic carbon (POC), biogenic silica (BSi), chlorophyll a (Chl a) and phaeopigment (Phaeo) concentrations. Subsamples were taken for counting autotrophic and heterotrophic pico- and nanoplankton and to assess the abundance and composition of microphyto- and microzooplankton. Statistical analysis identified two major groups of samples: the first included the most coastal surface samples of early December, characterized by the prevalence of autotrophic nanoplankton biomass; the second included all the remaining samples and was dominated by microphytoplankton. With regard to the relation of the plankton community composition to the biogenic suspended and sinking material, we identified the succession of three distinct periods. In early December Phaeocystis dominated the plankton assemblage in the well-mixed water column, while at the retreating ice-edge a bloom of small diatoms (ND) was developing in the lens of superficial diluted water. Concentrations of biogenic particulates were generally low and confined to the uppermost layer. The very low downward fluxes, the near absence of faecal pellets and the high Chl a/Phaeo ratios suggested that the herbivorous food web was not established yet or, at least, was not working efficiently. In early February the superficial pycnocline and the increased water column stability favoured the development of the most intense bloom of the season, essentially sustained by micro-sized diatoms (MD). The shift of the autotrophic community toward this size component produced major changes in the composition of particulate matter and determined its export to depth. The particulate organic carbon (POC)/chlorophyll a (Chl a) and Chl a/Phaeo ratios more than halved, biogenic silica (BSi)/POC and BSi/Chl a strongly increased. Downward fluxes were greatly enhanced (reaching the yearly maximum) and essentially occurred via faecal pellets, underscoring the high efficiency of the herbivorous food web. In late February the deepening of the pycnocline, together with the decrease in light intensity, contributed to halting the diatom bloom. The biomass of small heterotrophs (HNF and MCZ) significantly increased relative to the previous period, favouring the shift toward a mistivorous food web (sensu [Ophelia 41 (1995) 153]) and resulting in the retention of biogenic matter in the superficial layer. Only in early February, with the increase in the size of primary producers (essentially represented by micro-sized diatoms), did the grazing food web become efficient [S. Afr. J. Mar. Sci. 12 (1992) 477], fuelling the long-lived carbon pool and enhancing export to depth (and hence carbon sequestration) via the sinking of large diatoms and high amounts of faecal pellets. The conditions predominating in the Terra Nova Bay polynya in mid-summer probably increased the efficiency of the CO 2 pump, possibly causing the Bay to act as a carbon sink.

Climatic and oceanic forcing of new, net, and diatom production in the North Water

Abstract New, net, and diatom production in the North Water were estimated during May to July 1998 from in vitro measurements of nitrate uptake and mesoscale temporal changes in the inventories of nitrate, silicate, oxygen, and inorganic carbon (DIC). Sampling stations were divided into two domains according to the position of the dominant water types: the silicate-rich Arctic water (SRAW) and Baffin Bay Water (BBW). BBW dominated in the southeast and was associated with relatively shallow upper mixed layers (UMLs) and weak horizontal advection. A phytoplankton bloom started in late April in BBW and grew slowly over 7 weeks, during which time the build-up of particulate organic nitrogen and carbon accounted for ca. 80% of the nitrate and DIC deficit, respectively. Over half of the new production (1.37 g C m −2 d −1 ) during this period was attributed to wind-driven replenishment of nitrate in the euphotic zone. The bloom culminated when seasonally declining winds and rising temperatures severed the UML from the deep nutrient reservoir. The same change in weather induced ice melt, stratification, and bloom development in northern SRAW, which had previously been characterized by deep UMLs. Collectively, the results imply that the timing and magnitude of blooms in the North Water are controlled by a succession of oceanic and climatic forcings. New C production in the North Water during April to July (1.11 g C m −2 d −1 ) was an order of magnitude higher than in adjacent waters and up to 8 times higher than in the Northeast Water polynya. As much as 80% of this production was mediated by diatoms >5 μm, suggesting potentially high and efficient C transfer to the herbivorous food web and deep waters.

Assessing the trophic pathways that dominate planktonic food webs: an approach based on simple ecological ratios

It was recently proposed that there is, in the pelagic environment, a continuum of trophic pathways ranging from the herbivorous food web to the multivorous food web, the microbial food web and the microbial loop. It was also suggested that combining specific ecological ratios could provide a way of assessing the dominance of planktonic food webs by specific trophic pathways. Three large data sets, collected in different but adjacent marine systems (the Scotian Shelf, off eastern Canada; the Gulf of St Lawrence; and nearshore waters of the Baie des Chaleurs, in the northwestern Gulf), are used to compute two ratios: small-sized ( 5 μm) particulate phytoplankton production, and phaeopigments in small- to phaeopigments in large-sized particles. By combining the two ecological ratios it is possible to delineate successfully, in each studied system, coherent periods during the year; to interpret the combined ratios in terms of dominance by specific trophic pathways; and to show that the interpretation is consistent with additional information collected in the three systems.

Experimental test of the effect of ultraviolet‐B radiation in a planktonic community

Ultraviolet‐B (UVB, 280–320 nm) radiation is a natural component of sunlight that harms organisms and disturbs natural communities in surface waters. A natural planktonic assemblage of organisms (<240 μm) was studied in a mesocosm experiment for 7 d under varying conditions of UVB radiation: UVB excluded, natural radiation, and UVB enhanced at two different levels. The dynamics of several populations at different trophic levels comprising heterotrophic bacteria (<1 μm), heterotrophic flagellates (2–10 μm), small phytoplankton (<5 μm), large phytoplankton (5–20 μm), and ciliates (15–35 μm) were monitored during the experiment. Enhanced UVB provoked a significant decrease in the number of ciliates (66%) and large phytoplankton (63%) relative to natural UVB con‐ditions. The severe effects of UVB radiation on ciliates and large phytoplankton communities shown here would strongly limit upward transfer of mass and energy. The decline of predator abundance (ciliates) under UVB stress relative to natural conditions resulted in a positive feedback between enhanced UVB radiation and prey abundances, shown by increased abundances of bacteria (49%), heterotrophic flagellates (up to 300%), and small phytoplankton (41%). Similarly, with respect to carbon partitioning, the decrease in ciliate and diatom carbon biomass (64 and 56%, respectively) under enhanced UVB exposure was balanced by an increase in the carbon biomass of hetero‐trophic bacteria (48%), heterotrophic flagellates (126%), and autotrophic flagellates (162%). As a manifestation of enhanced UVB at the community level, the ecosystem develops toward a microbial food web in preference to an herbivorous food web. Thus, enhanced UVB radiation can change the structure and dynamics of the pelagic food web.

Planktonic herbivorous food webs in the catalan Sea (NW Mediterranean): temporal variability and comparison of indices of phyto-zooplankton coupling based on state variables and rate processes

The structural properties and functional relationships between phyto- and zooplankton were studied during two cruises, FR92 (October–November 1992) and ME93 (June 1993), at two fixed stations placed in a transect crossing the Catalan density front (NW Mediterranean), one located in the vicinity of the density front and another offshore. Both stations were sampled at variable intervals during each cruise in order to determine possible tendencies in the temporal changes in the phyto-zoo-plankton coupling. This coupling (i.e. the matter and energy transfer through planktonic herbivorous food webs) was estimated by means of two categories of quantitative indicators: (i) structural indices, based on the relationships between state variables of producers and consumers; (ii) functional indices, based on their rate processes. Structural indices showed lower temporal variability, both at a short time scale (within cruises) and interannually (amongst cruises), than those based on rate processes. The values of both categories of indices at offshore stations coincided for the two cruises, and were similar to the average values for the whole area observed in previous cruises. At stations near the front, while structural indices suggested a more intense phyto-zooplankton coupling during FR92 (although differences between cruises were not statistically significant), functional indices indicated an opposite trend, being significantly higher during ME93. The different functional indices allowed coincident tendencies on the phyto-zooplankton coupling and seemed to respond faster to any change in the relative importance of planktonic food webs than structural ones. This suggests that the relative importance of planktonic herbivorous food webs can be better estimated through the relationships between rate processes of producers and consumers, than through the relationships between more conservative state variables, like biomass or community structure.

Plankton and nutrient dynamics in marine waters

Abstract Biological oceanographers generally distinguish between two contrasting trophic pathways in the pelagic environment, i.e. the herbivorous and the microbial food webs. The former goes from large phytoplankton and zooplankton to fish, whereas the latter comprises small eukaryotic algae and cyanobacteria as well as heterotrophic bacteria and protozoa. The present paper describes a continuum of trophic pathways, between systems dominated by the herbivorous food web and those dominated by the microbial loop (i.e. almost closed system of heterotrophic bacteria and zooflagel-late grazers, the latter releasing dissolved organic matter used as substrate by the bacteria). It is proposed that the continuum goes from the herbivorous web (or chain) to a “multivorous food web”, to the microbial web, and finally the microbial loop. Characteristics of the various pathways maybe summarized as a series of interconnected ratios. It is hypothesized that systems dominated by the herbivorous food web or the microbial ...