Tracking the autochthonous carbon transfer in stream biofilm food webs

Food webs in the rhithral zone rely mainly on allochthonous carbon from the riparian vegetation. However, autochthonous carbon might be more important in open canopy streams. In streams, most of the microbial activity occurs in biofilms, associated with the streambed. We followed the autochthonous carbon transfer toward bacteria and grazing protozoa within a stream biofilm food web. Biofilms that developed in a second-order stream (Thuringia, Germany) were incubated in flow channels under climate-controlled conditions. Six-week-old biofilms received either 13C- or 12C-labeled CO?, and uptake into phospholipid fatty acids was followed. The dissolved inorganic carbon of the flow channel water became immediately labeled. In biofilms grown under 8-h light/16-h dark conditions, more than 50% of the labeled carbon was incorporated in biofilm algae, mainly filamentous cyanobacteria, pennate diatoms, and nonfilamentous green algae. A mean of 29% of the labeled carbon reached protozoan grazer. The testate amoeba Pseudodifflugia horrida was highly abundant in biofilms and seemed to be the most important grazer on biofilm bacteria and algae. Hence, stream biofilms dominated by cyanobacteria and algae seem to play an important role in the uptake of CO? and transfer of autochthonous carbon through the microbial food web.     

Quantification of invertebrate predation and herbivory in food chains of low complexity

Zooplankton grazing impact on algae, heterotrophic flagellates and bacteria, as well as invertebrate predation on herbivorous zooplankton, were investigated in two sub-Antarctic lakes with extremely simple food chains. The two species of herbivorous zooplankton present in the lakes (the copepods boeckella michaelseni and Pseudoboeckella poppei) exerted substantial grazing pressure on algae. However, the dominant algal species exhibited properties that enabled them to avoid (large size or extruding spines, e.g. Staurastrum sp., Tribonema sp.) or compensate (recruitment from the sediment, Mallomonas sp.) grazing. There are only two potential invertebrate predators on the herbivorous copepods in the two lakes: the copepod Parabroteas sarsi and the diving beetle Lancetes claussi. Vertebrate predators are entirely abscent from sub-Antarctic lakes. Based on our experiments, we estimated that the predators would remove at most about 0.4% of the herbivorous copepods per day, whereas planktivorous fish, if present in the lakes, would have removed 5–17% of the zooplankton each day. Consequently, the invertebrate predators in these high-latitude lakes had only a marginal predation impact compared to the predation pressure on zooplankton in the presence of vertebrate predators in temperate lakes. The study of these simple systems with only two quantitatively functionally important trophic links, suggests that high grazing pressure foreces the algal community towards forms with grazer resistant adaptations such as large size, recruitment from another habitat, and grazer avoidance spines. We propose that due to such adaptations, predictions from food web theory are only partly corroborated, i.e. algal biomass actually increases with increasing productivity, although the grazer community is released from predation. In more species-rich and complex systems, e.g temperate lakes with three functionally important links, such adaptations are likely to be even more important, and, consequently, the observable effects of trophic interactions from top predators on lower trophic levels even more obscured.     

Evaluating the need for acid treatment prior to δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N analysis of freshwater fish scales: effects of varying scale mineral content,lake productivity and CO<Subscript>2</Subscript> concentration

In order to evaluate the need for using scale acidification to remove carbonates prior to stable isotope analysis, we compared acidified and non-acidified scales of six freshwater fish species (perch, roach, rudd, pike, tench and bream) with contrasting mineral content in their scales. Fish samples were taken from six lakes with variable trophic conditions, ranging from oligotrophic to hypertrophic, and differing in CO₂ concentrations. The scale mineral content of the six species studied ranged between 31.8 and 61.3% dry weight (DW) in tench and perch, respectively. The elemental composition was characterised by high amounts of phosphorus, varying from 4.5 to 9.1% DW. The mineral fraction was dominated by apatite (range 24.4–49.2% DW), carbonates constituted a very small proportion of the total carbon content (average ± SD: 5.5 ± 1.7%). The average effect of acidification was very small for all species (average ± SD: 0.181 ± 0.122 and −0.208 ± 0.243 for carbon and nitrogen, respectively), albeit significant for five out of the six species (excepting tench that had the lowest mineral content). Linear regression slopes between acidified and untreated scales did not differ significantly from one for almost all the species and isotopes. The effects of acidification on the two isotopes were correlated with the relative carbonate content as well as with the CO₂ concentration for carbon and total phosphorus for nitrogen. We conclude that the need for scale acidification depends on the different species and on the system studied, although in most cases the acidification effect will be biologically irrelevant. However, dual analysis of acidified and untreated scales may provide useful information on differences in stable isotope composition of dissolved inorganic carbon and on phytoplankton carbon fractionation generated by varying levels of CO₂ availability.     

Direct and indirect effects of predatory young‐of‐year fishes in a dryland river food web

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Asymmetrical food web responses in trophic-level richness, biomass, and function following lake acidification

We tested for disproportional changes in annual and seasonal species richness and biomass among five trophic levels (phytoplankton, herbivorous, omnivorous, and carnivorous zooplankton, and fish) as well as altered trophic structure and ecosystem function following the 5-year experimental acidification of Little Rock Lake (Wisconsin, USA) from pH 6.1 to 4.7. Abiotic and biotic controls of trophic level response during acidification were also identified. Asymmetric reductions of species richness among trophic levels, separated by life stage and feeding type, were evident and changes in trophic structure were most pronounced by the end of the acidification period. Relative declines in richness of fish and zooplankton were greater than phytoplankton, which were generally unaffected, leading to a reduction of upper trophic level diversity. Each of the lower four trophic levels responded to a distinct combination of abiotic and biotic variables during acidification. pH was identified as a direct driver of change for only carnivorous zooplankton, while all other trophic levels were affected more by indirect interactions caused by acidification. Fluctuations in ecosystem function (zooplankton biomass and primary production) were also evident, with losses at all trophic levels only detected during the last year of acidification. The acidified basin displayed a tendency for greater variation in biomass for upper trophic levels relative to reference conditions implying greater unpredictability in ecosystem function. Together, these results suggest that trophic asymmetry may be an important and recurring feature of ecosystem response to anthropogenic stress.     

The effect of two scales of habitat architecture on benthic grazing in a river

1. This experiment studied the effects of differing levels of the complexity of substratum architecture at two spatial scales on the distribution and abundance of benthic algae and invertebrates, and the strength of the trophic interaction between invertebrate grazers and algae. Some estimates of the effects on invertebrate colonization rates were also made. 2. Four levels of microhabitat architectural complexity were created using artificial substrata (clay tiles) and placed in Mountain River, Tasmania, in two riffle types (bedrock and boulder-cobble) of differing large-scale substratum complexity. After a colonization period, invertebrate grazers were removed from half the tiles to measure the effects of grazing. Invertebrates on the tiles were also counted and identified. At the end of the experiment, algae were removed from the tiles and analysed for chlorophyll a. 3. Invertebrate grazers did not reduce algal biomass during the experiment, and microhabitat-scale architecture influenced algal biomass more strongly than riffle-scale architecture. Highly complex microhabitat architecture increased algal biomass by providing more surface area, but once standardized for surface area, algal biomass decreased as the complexity of microhabitat architecture increased. 4. Microhabitat-scale architecture was also predominant in determining invertebrate density and the identity of the dominant grazer species. In contrast to algal biomass, invertebrate densities and species density increased with the complexity of microhabitat architecture, suggesting that refuges from flow (and possibly predation) were as important to river invertebrates as the distribution of their food source. 5. Riffle-scale architecture had some effect on the colonization of two slow-moving grazer taxa, but, overall, the colonization processes of slow-moving grazers were determined mostly by the complexity of microhabitat-scale architecture.     

Dominance of autochthonous autotrophic carbon in food webs of heterotrophic rivers

This paper addresses the river heterotrophy paradox, “How can animal biomass within riverine food webs be fueled primarily by autochthonous autotrophic production if the ecosystem as a whole is heterotrophic?”. Reviewed, stable isotope data from tropical, temperate, and arctic rivers provide evidence consistent with the revised riverine productivity model (RPM): “The primary, annual energy source supporting overall metazoan production and species diversity in mid‐ to higher‐trophic levels of most rivers (≥4th order) is autochthonous primary production entering food webs via algal‐grazer and decomposer pathways”. The revised RPM does not conflict with the heterotrophy paradox because: (a) the decomposer (microbial loop) food pathway processes most of the transported, allochthonous and autochthonous carbon and, with algal respiration in some cases, is primarily responsible for a river's heterotrophic state (P/R<1); but (b) biomass production of mid‐ to higher‐trophic levels is principally supported by an algal‐grazer (phytoplankton and benthic microalgae) pathway that is only weakly linked to the decomposer pathway. The reason the algal‐grazer pathway supports the majority of metazoan biomass is that allochthonous carbon is mostly recalcitrant, whereas carbon from autochthonous primary production, though much less plentiful, is commonly more labile (easier to assimilate), contains more energy per unit mass, and is typically preferred by metazoa.     

Food web pathways for fish communities in small tropical streams

1. It is often assumed that invertebrate consumers in small tropical streams are dependent on allochthonous sources, although recent studies indicate that algae can form the base of food webs in tropical streams. Fish in tropical streams can feed across several trophic levels and the origin and path of energy and nutrient flow is uncertain for many species.
2. We collected fish, insects, periphyton, and leaf litter from 20 streams across four Atlantic Forest catchments. We analysed stomach contents of fish to define trophic guild and fish dietary trophic position. We also analysed stable isotopes of carbon and nitrogen of fish and their resources to identify the main basal resources of the food web and to estimate trophic positions and identify the path of energy flow.
3. We found that autochthonous sources were the primary resource base for fish communities. Trophic positions estimated from diet and isotopes were similar and correlated for insectivore and algivore–insectivore fish, but not for algivore–detritivore or omnivore fish. Using path analysis, fish classified as algivore–detritivores appear to have derived their biomass through a diet of primary consumer insects and periphytic algae and thus, are more likely to play a trophic role as algivore–insectivores in these streams. However, omnivores probably derived much of their biomass from aquatic insects.
4. Our findings support other studies of tropical systems in which the main basal resource is autochthonous, even in small streams. We also show that the assignment to a specific trophic guild for some fish species, based on gut contents, does not reflect what they assimilate into their bodies. In some species, food sources that are uncommon can make a disproportionately important contribution to their biomass.
5. This study affirms the important role of inconspicuous algal resources in aquatic food webs, even in small forested streams, and demonstrates the effectiveness of taking a combined approach of diet analysis, isotopic tracing, and modelling to resolve food web pathways where the level of omnivory is high.

     

Carbon Sources in Riverine Food Webs: New Evidence from Amino Acid Isotope Techniques

A nearly 40-year debate on the origins of carbon supporting animal production in lotic systems has spawned numerous conceptual theories emphasizing the importance of autochthonous carbon, terrestrial carbon, or both (depending on river stage height). Testing theories has been hampered by lack of adequate analytical methods to distinguish in consumer tissue between ultimate autochthonous and allochthonous carbon. Investigators initially relied on assimilation efficiencies of gut contents and later on bulk tissue stable isotope analysis or fatty acid methods. The newest technique in amino acid, compound specific, stable isotope analysis (AA-CSIA), however, enables investigators to link consumers to food sources by tracing essential amino acids from producers to consumers. We used AA-CSIA to evaluate nutrient sources for 5 invertivorous and 6 piscivorous species in 2 hydrogeomorphically contrasting large rivers: the anastomosing Upper Mississippi River (UMR) and the mostly constricted lower Ohio River (LOR). Museum specimens we analyzed isotopically had been collected by other investigators over many decades (UMR: 1900–1969; LOR: 1931–1970). Our results demonstrate that on average algae contributed 58.5% (LOR) to 75.6% (UMR) of fish diets. The next highest estimated contributions of food sources were from C₃ terrestrial plants (21.1 and 11.5% for the LOR and UMR, respectively). Moreover, results from 11 individually examined species consistently demonstrated the importance of algae for most fish species in these trophic guilds. Differences among rivers in relative food source availability resulting from contrasting hydrogeomorphic complexity may account for relative proportions of amino acids derived from algae.     

Allochthonous and autochthonous carbon sources for fish in floodplain lagoons of an Australian dryland river

Dryland rivers associated with arid and semi-arid land areas offer an opportunity to explore food web concepts and models of energy sources in systems that experience unpredictable flooding and long dry spells. This study investigated the sources of energy supporting three species of fish feeding at different trophic levels within floodplain lagoons of the Macintyre River in the headwaters of the Murray-Darling river system, Australia. Stable isotope analyses revealed that fish consumers derived, on average, 46.9% of their biomass from zooplankton, 38.1% from Coarse Particulate Organic Matter (CPOM) and 24.0% from algae. Ambassis agassizii derived on average 57.6% of its biomass carbon from zooplankton and 20.4–27.8% from algae or CPOM. Leiopotherapon unicolor derived most of its carbon from zooplankton and CPOM (38.3–39.5%), with relatively high contributions from algae compared to the other species (33.3%). An average of 48.4% of the biomass of Nematalosa erebi was derived from zooplankton, with CPOM contributing another 38.1%. Zooplankton was the most important source of organic carbon supporting all three fish species in floodplain lagoons. Phytoplankton, and possibly, particulate organic matter in the seston, are the most likely energy sources for the planktonic suspension feeders (zooplankton) and, consequently, the fish that feed on them. These results indicate a stronger dependence of consumers on autochthonous sources and on locally produced organic matter from the riparian zone (i.e., the Riverine Productivity Model), than on other resources.     

Soil acidification reduces the effects of short‐term nutrient enrichment on plant and soil biota and their interactions in grasslands

Soil nitrogen (N) and phosphorus (P) contents, and soil acidification have greatly increased in grassland ecosystems due to increased industrial and agricultural activities. As major environmental and economic concerns worldwide, nutrient enrichment and soil acidification can lead to substantial changes in the diversity and structure of plant and soil communities. Although the separate effects of N and P enrichment on soil food webs have been assessed across different ecosystems, the combined effects of N and P enrichment on multiple trophic levels in soil food webs have not been studied in semiarid grasslands experiencing soil acidification. Here we conducted a short‐term N and P enrichment experiment in non‐acidified and acidified soil in a semiarid grassland on the Mongolian Plateau. We found that net primary productivity was not affected by N or P enrichment alone in either non‐acidified or acidified soil, but was increased by combined N and P enrichment in both non‐acidified and acidified soil. Nutrient enrichment decreased the biomass of most microbial groups in non‐acidified soil (the decrease tended to be greatest with combined N and P enrichment) but not in acidified soil, and did not affect most soil nematode variables in non‐acidified or acidified soil. Nutrient enrichment also changed plant and microbial community structure in non‐acidified but not in acidified soil, and had no effect on nematode community structure in non‐acidified or acidified soil. These results indicate that the responses to short‐term nutrient enrichment were weaker for higher trophic groups (nematodes) than for lower trophic groups (microorganisms) and primary producers (plants). The findings increase our understanding of the effects of nutrient enrichment on multiple trophic levels of soil food webs, and highlight that soil acidification, as an anthropogenic stressor, reduced the responses of plants and soil food webs to nutrient enrichment and weakened plant–soil interactions.     

Hydrologic Variability Affects Invertebrate Grazing on Phototrophic Biofilms in Stream Microcosms

The temporal variability of streamflow is known to be a key feature structuring and controlling fluvial ecological communities and ecosystem processes. Although alterations of streamflow regime due to habitat fragmentation or other anthropogenic factors are ubiquitous, a quantitative understanding of their implications on ecosystem structure and function is far from complete. Here, by experimenting with two contrasting flow regimes in stream microcosms, we provide a novel mechanistic explanation for how fluctuating flow regimes may affect grazing of phototrophic biofilms (i.e., periphyton) by an invertebrate species (Ecdyonurus sp.). In both flow regimes light availability was manipulated as a control on autotroph biofilm productivity and grazer activity, thereby allowing the test of flow regime effects across various ratios of biofilm biomass to grazing activity. Average grazing rates were significantly enhanced under variable flow conditions and this effect was highest at intermediate light availability. Our results suggest that stochastic flow regimes, characterised by suitable fluctuations and temporal persistence, may offer increased windows of opportunity for grazing under favourable shear stress conditions. This bears important implications for the development of comprehensive schemes for water resources management and for the understanding of trophic carbon transfer in stream food webs.     

Stable isotopes, mesocosms and gut content analysis demonstrate trophic differences in two invasive decapod crustacea

1. Improving our understanding of dietary differences among omnivorous, benthic crustacea can help to define the scope of their trophic influence in benthic food webs. In this study, we examined the trophic ecology of two non‐native decapod crustaceans, the Chinese mitten crab (Eriocheir sinensis) (CMC) and the red swamp crayfish (Procambarus clarkii) (RSC), in the San Francisco Bay ecosystem to describe their food web impacts and explore whether these species are functionally equivalent in their impacts on aquatic benthic communities. 2. We used multiple methods to maximise resolution of the diet of these species, including N and C stable isotope analysis of field data, controlled feeding experiments to estimate isotopic fractionation, mesocosm experiments, and gut content analysis (GCA). 3. In experimental enclosures, both CMC and RSC caused significant declines in terrestrially derived plant detritus (P < 0.01) and algae (P < 0.02) relative to controls, and declines in densities of the caddisfly Gumaga nigricula by >50% relative to controls. 4. Plant material dominated gut contents of both species, but several sediment‐dwelling invertebrate taxa were also found. GCA and mesocosm results indicate that CMC feed predominantly on surface‐dwelling invertebrates, suggesting that trophic impacts of this species could include a shift in invertebrate community composition towards sediment‐dwelling taxa. 5. Stable isotope analysis supported a stronger relationship between CMC and both algae and algal‐associated invertebrates than with allochthonous plant materials, while RSC was more closely aligned with terrestrially derived detritus. 6. The trophic ecology and life histories of these two invasive species translate into important differences in potential impacts on aquatic food webs. Our results suggest that the CMC differs from the RSC in exerting new pressures on autochthonous food sources and shallow‐dwelling invertebrates. The crab's wide‐ranging foraging techniques, use of intertidal habitat, and migration out of freshwater at sexual maturity increases the distribution of the impacts of this important invasive species.     

Trophic Dependence of Ecosystem Resistance and Species Compensation in Experimentally Acidified Lake 302S (Canada)

Ecosystem resistance to the impacts of diverse human insults depends on the replacement of sensitive species by ones more tolerant of the stressor. Here we present evidence from a whole-lake acidification experiment (Lake 302S, Experimental Lakes Area, Canada) that resistance and species compensation decline with increasing trophic level. Diverse and fast-growing algal and rotifer assemblages with high dispersal potentials showed significant compensatory species dynamics, resulting in the maintenance of total biomass despite 30%–80% declines in species richness. Canonical correspondence analysis showed that significant compensatory algal and rotifer dynamics were best explained by differential species tolerances of acidified chemical conditions coupled with release from resource limitation and predation. However, less diverse cladoceran, copepod, and fish assemblages showed significant declines in total biomass and weak species compensation with loss of species during acidification. In comparison, algal and zooplankton species dynamics remained relatively synchronized in a nearby unperturbed reference lake (Lake 239) during the experiment. As a result, Lake 302S showed limited ecosystem resistance to anthropogenic acidification. Therefore, we hypothesize that lost species will increase the susceptibility of acidified lakes to the adverse impacts of other environmental stressors (for example, climate warming, stratospheric ozone depletion, invasive species). Consequently, the ecosystem stability of boreal lakes is expected to decline as global change proceeds. Received 2 January 2001; accepted 12 July 2002.     

Biofilm formation at warming temperature: acceleration of microbial colonization and microbial interactive effects

River biofilms that grow on wet benthic surface are mainly composed of bacteria, algae, cyanobacteria and protozoa embedded in a polysaccharide matrix. The effects of increased river water temperature on biofilm formation were investigated. A laboratory experiment was designed employing two temperatures (11.1-13.2°C, night-day; 14.7-16.0°C, night-day) and two nutrient levels (0.054 mg P l(-1), 0.75 mg N l(-1); 0.54 mg P l(-1), 7.5 mg N l(-1)). Biofilm formation at the higher temperature was faster, while the biomass of the mature biofilm was mainly determined by nutrient availability. The specific response of the three microbial groups that colonized the substrata (algae, bacteria and ciliates) was modulated by interactions between them. The greater bacterial growth rate and earlier bacterial colonization at the higher temperature and higher nutrient status was not translated into the accrual of higher bacterial biomass. This may result from ciliates grazing on the bacteria, as shown by an earlier increase in peritrichia at higher temperatures, and especially at high nutrient conditions. Temperature and ciliate grazing might determine the growth of a distinctive bacterial community under warming conditions. Warmer conditions also produced a thicker biofilm, while functional responses were much less evident (increases in the heterotrophic utilization of polysaccharides and peptides, but no increase in primary production and respiration). Increasing the temperature of river water might lead to faster biofilm recolonization after disturbances, with a distinct biofilm community structure that might affect the trophic web. Warming effects would be expected to be more relevant under eutrophic conditions.     

Identifying qualitative effects of different grazing types on below‐ground communities and function in a long‐term field experiment

Herbivory is an important modulator of plant biodiversity and productivity in grasslands, but our understanding of herbivore‐induced changes on below‐ground processes and communities is limited. Using a long‐term (17 years) experimental site, we evaluated impacts of rabbit and invertebrate grazers on some soil functions involved in carbon cycling, microbial diversity, structure and functional composition. Both rabbit and invertebrate grazing impacted soil functions and microbial community structure. All functional community measures (functions, biogeochemical cycling genes, network association between different taxa) were more strongly affected by invertebrate grazers than rabbits. Furthermore, our results suggest that exclusion of invertebrate grazers decreases both microbial biomass and abundance of genes associated with key biogeochemical cycles, and could thus have long‐term consequences for ecosystem functions. The mechanism behind these impacts are likely to be driven by both direct effects of grazing altering the pattern of nutrient inputs and by indirect effects through changes in plant species composition. However, we could not entirely discount that the pesticide used to exclude invertebrates may have affected some microbial community measures. Nevertheless, our work illustrates that human activity that affects grazing intensity may affect ecosystem functioning and sustainability, as regulated by multi‐trophic interactions between above‐ and below‐ground communities.     

The effects of sea urchin grazing and drift algal blooms on a subtropical seagrass bed community

Subtropical seagrass beds can be subject to relatively high levels of direct herbivory and large blooms of drift algae, both of which can have important effects on the floral and faunal components of the community. Caging experiments were used to investigate these factors in a Thalassia testudinum bed in Biscayne Bay, Florida. Abundance of sea urchins, Lytechinus variegatus, and drift algae was manipulated within the cages. Naturally occurring levels of urchin grazing do not appear to affect the T. testudinum population. With experimentally increased urchin densities in the winter, seagrass shoot density and aboveground biomass decreased significantly. Similar effects were not detected in the summer, indicating that the impact of grazing on T. testudinum is lessened during this time of year. Shoot density was more vulnerable to grazing than aboveground biomass. This may be a result of grazing-induced increases in seagrass productivity, in which the remaining shoots produce more or longer leaves. In the winter, drift algal blooms form large mats that cover the seagrass canopy. Under the normal grazing regime these algal blooms do not have significant negative effects on the seagrass. With increased grazing pressure, however, there is a synergistic effect of grazing and drift algae on seagrass shoot density. At intermediate urchin density (10 per m(-2)), cages without algae did not undergo significant decreases in shoot density, while those with algae did. At the high density of urchins, the number of seagrass shoots in cages both with and without algae decreased, but the effect was more pronounced for cages with algae. Invertebrate abundance at the field site was low relative to other seagrass beds. There were no discernible effects, either positive or negative, of urchin and algae manipulations on the sampled invertebrate community.     

Scope for use of stable carbon isotopes in discerning the incorporation of forest detritus into aquatic foodwebs

Stable isotope analysis of carbon has been proposed as a means for discerning the incorporation of terrestrial forest detritus into aquatic foodwebs, and as such, has the potential to be used as a biomonitor of the aquatic effects of riparian deforestation. A synthesis of ¹³C/¹²C data from the literature indicates, however, that the scope for successful use of carbon isotope analysis in separating allochthonous and autochthonous food provenance is much more limited than was once thought. This occurs due the overlap in carbon isotope ratios between terrestrial forest detritus and those of both lotic attached algae and lentic filamentous attached algae. Only within rockyshored, oligotrophic lakes without macrophytes, and forest-fringed estuaries and lagoons, where the carbon isotope ratios for attached algae and forest detritus are significantly different, is there any likelihood of discerning the incorporation of allochthonous carbon into aquatic foodwebs using ¹³C/¹²C values alone.     

Coralline algal skeletal mineralogy affects grazer impacts

In macroalgal‐dominated systems, herbivory is a major driver in controlling ecosystem structure. However, the role of altered plant–herbivore interactions and effects of changes to trophic control under global change are poorly understood. This is because both macroalgae and grazers themselves may be affected by global change, making changes in plant–herbivore interactions hard to predict. Coralline algae lay down a calcium carbonate skeleton, which serves as protection from grazing and is preserved in archival samples. Here, we compare grazing damage and intensity to coralline algae in situ over 4 decades characterized by changing seawater acidity. While grazing intensity, herbivore abundance and identity remained constant over time, grazing wound width increased together with Mg content of the skeleton and variability in its mineral organization. In one species, decreases in skeletal organization were found concurrent with deeper skeletal damage by grazers over time since the 1980s. Thus, in a future characterized by acidification, we suggest coralline algae may be more prone to grazing damage, mediated by effects of variability between individuals and species.     

Production of lower-trophic organisms during incubation of unaltered deep-sea water

Incubation of unaltered deep-sea water and grazing experiment of nano- and micro- protozooplankton during incubation of deep-sea water were carried out to quantitatively characterize the planktonic structures of lower-trophic organisms and clarify the trophic pathways and controlling mechanisms involved. Phytoplankton biomass increased to 637 mg as carbon weight in a 500-l tank on Day 7 and was dominant in the planktonic structure of lower-trophic organisms. Nitrates in the incubation water was depleted after Day 7 and phytoplankton biomass decreased rapidly. On the other hand, bacteria, heterotrophic nano-flagellates and ciliates increased toward the end of incubation and were dominant in the later days of incubation. In grazing experiments on microbial organisms, bacterivory is more important for the carbon pathway in microbial food webs than herbivory when phytoplankton biomass is less than that of bacteria (low P/B conditions), while herbivory is more important than bacterivory when phytoplankton biomass is more than that of bacteria (high P/B conditions). Deep-sea water exhibited high phytoplankton productivity due to inherent high nutrients values. After depletion of nutrients, phytoplankton decreased (due also to enhanced nano- and micro-zooplankton grazing) and microbial organisms dominated. Thus, nutrients in the incubation water control the planktonic structure of lower-trophic organisms.