Production To Higher Trophic Levels Research Articles

From cell size and first principles to structure and function of unicellular plankton communities

Here we review, synthesize, and analyse the size-based approach to model unicellular plankton cells and communities. We first review how cell size influences processes of the individual the cell: uptake of dissolved nutrients and dissolved organic carbon, phototrophy, phagotrophy, and metabolism. We parameterize processes primarily from first principles, using a synthesis of existing data only when needed, and show how these processes determine minimum and maximum cell size and limiting resource concentrations. The cell level processes scale directly up to the structure and function of the entire unicellular plankton ecosystem, from heterotrophic bacteria to zooplankton. Community structure is described by the size spectrum and by the emergent trophic strategies. We develop an analytical approximate solution of the size spectrum and show how the trophic strategies of osmotrophy, light- and nutrient-limited phototrophy, mixotrophy, and pure phagotrophy depend on the resource environment. We further develop expressions to quantify the functions of the plankton community: production, respiration, and carbon available to production of higher trophic levels, and show how the plankton community responds to changes in temperature and grazing from higher trophic levels. We finally discuss strengths and limitations of size-based representations of plankton communities and which additional trait axes will improve the description of plankton functional diversity.

A high-resolution time series of particulate matter fatty acids reveals temporal dynamics of the composition and quality available to zooplankton in a temperate coastal ocean

Particulate organic matter (POM) forms the base of the pelagic food web, but is a complex category of material that undergoes substantial changes in quantity and quality across different time scales. As the primary consumers of POM, zooplankton are influenced by these fluctuations, resulting in shifts in the food web pathways that contribute to the production of higher trophic levels. We measured POM fatty acids, which are critical nutritional components in marine food webs, along with a suite of associated biotic and abiotic environmental conditions at a temperate coastal site over four years. Using these data, we investigated the co-occurring patterns of prey quality, quantity, and size, to develop a holistic understanding of how the prey field for zooplankton varies over the seasonal cycle and inter-annually. The seasonal pattern of POM fatty acids corresponded to the succession of phytoplankton taxa, but displayed stronger relationships to size-fractionated Chl a than taxonomic observations. Times with high micro-Chl biomass (spring and fall blooms) had the greatest concentrations of high quality food, but fatty acid levels remained high throughout the summer when Chl a concentrations dropped and the size distribution shifted towards pico-Chl. The 18-carbon polyunsaturated fatty acids 18:3ω3 (α-linolenic acid) and 18:4ω3 (stearidonic acid) increased during this time and were correlated with pico-Chl. In addition, the important nutritional factor for zooplankton and fish, the fatty acid ratio DHA:EPA (22:6ω3/20:5ω3), peaked during the middle of summer separately from the peak in Chl a. These seasonal patterns resulted in tradeoffs among the abundance, size, and nutritional quality of prey for zooplankton. We validated the basal sources of several fatty acids used as trophic markers within food web studies and in particular note their power for resolving size-based trophic connections. Interannual variability, e.g., the occurrence of fall diatom blooms and the timing of community shifts, is also discussed. This work lays the foundation for future studies of the zooplankton community at this location and the incorporation of realistic prey conditions into zooplankton studies.

Cascading effects of algal warming in a freshwater community

Abstract Research on the effects of climate warming on ecological communities has focused on how temperature affects resource quantity. However, resource quality is also affected by warming, and changes in resource quality can have meaningful effects on the productivity of higher trophic levels. Aquatic communities in particular experience temperature‐mediated shifts in resource quality because the nutritional value of algae is highly sensitive to temperature. For example, the production of healthful omega‐3 polyunsaturated fatty acids (n‐3 PUFA) by algae often decreases with warming. Decreased levels of some algal PUFAs with warming have led to the hypothesis that global warming should lead to an overall decrease in productivity in aquatic communities. However, this hypothesis: (a) potentially oversimplifies the relationship between algal PUFAs and temperature, and (b) assumes that the nutritional requirements of consumers are not affected by temperature. Here, we test these assumptions using a freshwater community (Scenedesmus algae, Daphnia zooplankton, Chaoborus insects). Warming temperatures increased total algal PUFAs, but decreased algal cell size, resulting in no net effect of temperature on PUFA per algal cell. In contrast, quantities of algal neutral lipids decreased with warming. At the consumer level, Daphnia fed 12°C‐reared algae maintained higher population sizes than those fed 20°C or 28°C‐reared algae. However, the effect of algal food type diminished as Daphnia rearing temperature increased. The indirect effects of cold‐reared algae on the growth rate of Chaoborus predators were minor. These data highlight the importance of investigating the effects of temperature on both resource quality and on the nutritional needs of consumers. Our results suggest that at warmer temperatures, consumer nutritional requirements may be reduced. We caution against broad claims that the negative relationship between some algal PUFAs and temperature should result in overall declines in aquatic productivity with ongoing climate warming. A free Plain Language Summary can be found within the Supporting Information of this article.

Zooplankton functional group responses to environmental drivers off the west coast of Vancouver Island, Canada

Zooplankton diversity and biomass are integral to marine ecosystem functioning. Functional trait-based approaches enable an examination of ecosystem dynamics in which species groups are defined by their functional roles (i.e. functional groups) within the ecosystem, rather their taxonomic identity. Changes in the relative and absolute biomass of zooplankton functional groups may affect the productivity of higher trophic levels via change in secondary production rates and the efficiency of energy transfer. Traits from 55 mesozooplankton species were assembled from the literature and applied to a 35-year time series (1980–2016) for the southern shelf area off the west coast of Vancouver Island, Canada. Eight functional groups were identified by clustering species on the basis of functional trait similarity. A time series of biomass anomalies was calculated for each functional group, after which links between functional group biomass anomalies and several environmental variables were examined. Generalized additive modeling (GAM) identified local sea surface temperature as the most important driver of biomass anomaly patterns for three groups: Doliolids, Omnivore-Herbivores, and the Active Ambush Omnivores. The Southern Oscillation Index (SOI) was also a main driver for three groups: Egg-Brooding Carnivores, Broadcast Carnivores, and Cruise Carnivores-A. The Ocean Niño Index (ONI) was significant for Active Ambush Omnivores and Egg-Brooding Carnivores, whereas the Pacific Decadal Oscillation (PDO) was a driver of Broadcast Carnivores and Cruise Carnivores-A. The Doliolids group also varied with surface salinity, and Cruise Carnivores-B with the North Pacific Gyre Oscillation (NPGO). The relative contribution of several zooplankton functional groups to the overall biomass showed no temporal trends but some groups had increased or decreased abundance contributions. This suggests functional redundancy within groups, as smaller or larger species may dominate based on environmental conditions. Our analysis demonstrates that functional group biomass anomalies vary with environmental variables, where conditions may favor certain functional groups, therefore dictating how the ecosystem is functioning.

Changes in higher trophic level productivity, diversity and niche space in a rapidly warming continental shelf ecosystem

There is long-standing ecological and socioeconomic interest in what controls the diversity and productivity of ecosystems. That focus has intensified with shifting environmental conditions associated with accelerating climate change. The U.S. Northeast Shelf (NES) is a well-studied continental shelf marine ecosystem that is among the more rapidly warming marine systems worldwide. Furthermore, many constituent species have experienced significant distributional shifts. However, the system response of the NES to climate change goes beyond simple shifts in species distribution. The fish and macroinvertebrate communities of the NES have increased in species diversity and overall productivity in recent decades, despite no significant decline in fishing pressure. Species distribution models constructed using random forest classification and regression trees were fit for the dominant species in the system. Over time, the areal distribution of occupancy habitat has increased for approximately 80% of the modeled taxa, suggesting most species have significantly increased their range and niche space. These niche spaces were analyzed to determine the area of niche overlap between species pairs. For the vast majority of species pairs, interaction has increased over time suggesting greater niche overlap and the increased probability for more intense species interactions, such as between competitors or predators and prey. Furthermore, the species taxonomic composition and size structure indicate a potential tropicalization of the fish community. The system and community changes are consistent with the view that the NES may be transitioning from a cold temperate or boreal ecoregion to one more consistent with the composition of a warm temperate or Carolinian system.

Meso-zooplankton structure and functioning in the western tropical South Pacific along the 20th parallel south during the OUTPACE survey (February–April 2015)

Abstract. The western tropical South Pacific (WTSP) is one of the most understudied oceanic regions in terms of the planktonic food web, despite supporting some of the largest tuna fisheries in the world. In this stratified oligotrophic ocean, nitrogen fixation may play an important role in supporting the plankton food web and higher trophic level production. In the austral summer (February–April) of 2015, the OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) project conducted a comprehensive survey of 4000 km along 20∘ S, from New Caledonia to Tahiti, to determine the role of N2 fixation on biogeochemical cycles and food web structure in this region. Here, we characterize the zooplankton community and plankton food web processes at 15 short-duration stations (8 h each) to describe the large-scale variability across trophic gradients from oligotrophic waters around Melanesian archipelagoes (MAs) to ultra-oligotrophic waters of the South Pacific gyre (GY). Three long-duration stations (5 days each) enabled a more detailed analysis of processes and were positioned (1) in offshore northern waters of New Caledonia (MA), (2) near Niue Island (MA), and (3) in the subtropical Pacific gyre (GY) near the Cook Islands. At all stations, meso-zooplankton was sampled with a bongo net with 120 µm mesh size to estimate abundance, biomass, community taxonomy and size structure, and size fractionated δ15N. Subsequently, we estimated zooplankton carbon demand, grazing impact, excretion rates, and the contribution of diazotroph-derived nitrogen (DDN) to zooplankton biomass. The meso-zooplankton community showed a general decreasing trend in abundance and biomass from west to east, with a clear drop in the GY waters. Higher abundance and biomass corresponded to higher primary production associated with complex mesoscale circulation in the Coral Sea and between 170–180∘ W. The taxonomic structure showed a high degree of similarity in terms of species richness and abundance distribution across the whole region, with, however, a moderate difference in the GY region, where the copepod contribution to meso-zooplankton increased. The calculated ingestion and metabolic rates allowed us to estimate that the top–down (grazing) and bottom–up (excretion of nitrogen and phosphorous) impact of zooplankton on phytoplankton was potentially high. Daily grazing pressure on phytoplankton stocks was estimated to remove 19 % to 184 % of the total daily primary production and 1.5 % to 22 % of fixed N2. The top–down impact of meso-zooplankton was higher in the eastern part of the transect, including GY, than in the Coral Sea region and was mainly exerted on nano- and micro-phytoplankton. The regeneration of nutrients by zooplankton excretion was high, suggesting a strong contribution to regenerated production, particularly in terms of N. Daily NH4+ excretion accounted for 14.5 % to 165 % of phytoplankton needs for N, whereas PO43- excretion accounted for only 2.8 % to 34 % of P needs. From zooplankton δ15N values, we estimated that the DDN contributed to up to 67 % and 75 % to the zooplankton biomass in the western and central parts of the MA regions, respectively, but strongly decreased to an average of 22 % in the GY region and down to 7 % in the easternmost station. Thus, the highest contribution of diazotrophic microorganisms to zooplankton biomass occurred in the region of highest N2 fixation rates and when Trichodesmium dominated the diazotrophs community (MA waters). Our estimations of the fluxes associated with zooplankton were highly variable between stations and zones but very high in most cases compared to literature data, partially due to the high contribution of small forms. The highest values encountered were found at the boundary between the oligotrophic (MA) and ultra-oligotrophic regions (GY). Within the MA zone, the high variability of the top–down and bottom–up impact was related to the high mesoscale activity in the physical environment. Estimated zooplankton respiration rates relative to primary production were among the highest cited values at similar latitudes, inducing a high contribution of migrant zooplankton respiration to carbon flux. Despite the relatively low biomass values of planktonic components in quasi-steady state, the availability of micro- and macronutrients related to physical mesoscale patterns in the waters surrounding the MA, the fueling by DDN, and the relatively high rates of plankton production and metabolism estimated during OUTPACE may explain the productive food chain ending with valuable fisheries in this region.

Compensatory grazing by Daphnia generates a trade‐off between top‐down and bottom‐up effects across phytoplankton taxa

AbstractTrade‐offs among traits that shape different components of fitness determine the potential for species to coexist in communities and the regulation of ecosystem productivity by consumers vs. resources. We compared functional and structural traits of 14 species of freshwater phytoplankton to test the influence of life history (exponential growth rate, r, and asymptotic density, K), cell volume, cellular fatty acid (FA) biochemistry, and stoichiometry (cellular C:N and C:P ratios) on grazing and growth rate of Daphnia consumers. Grazers exerted the strongest top‐down effects on algae with the highest C:N and C:P, and lower levels of FA unsaturation. In contrast, the grazer's growth rate was most associated with phytoplankton species’ FA unsaturation and chain length. Different traits were related to grazing rate and trophic transfer efficiency, and as a result, the grazing rate on a particular phytoplankton taxon was unrelated to the grazers’ growth rate. The best nutrient competitors (those with high C:N, C:P, and K, which previous experiments showed had lower minimum requirements for nutrients) were most heavily affected by grazing, but did not contribute the most to grazer growth. Our results indicate that Daphnia increase their food consumption rate in order to compensate for low stoichiometric food quality. Strong competitors with low nutrient demands may therefore be subject to the most intense grazing by individual consumers but not support greater production of higher trophic levels.

Protist Diversity and Seasonal Dynamics in Skagerrak Plankton Communities as Revealed by Metabarcoding and Microscopy.

Protist community composition and seasonal dynamics are of major importance for the production of higher trophic levels, such as zooplankton and fish. Our aim was to reveal how the protist community in the Skagerrak changes through the seasons by combining high‐throughput sequencing and microscopy of plankton collected monthly over two years. The V4 region of the 18S rRNA gene was amplified by eukaryote universal primers from the total RNA/cDNA. We found a strong seasonal variation in protist composition and proportional abundances, and a difference between two depths within the euphotic zone. Highest protist richness was found in late summer‐early autumn, and lowest in winter. Temperature was the abiotic factor explaining most of the variation in diversity. Dinoflagellates was the most abundant and diverse group followed by ciliates and diatoms. We found about 70 new taxa recorded for the first time in the Skagerrak. The seasonal pattern in relative read abundance of major phytoplankton groups was well in accordance with microscopical biovolumes. This is the first metabarcoding study of the protist plankton community of all taxonomic groups and through seasons in the Skagerrak, which may serve as a baseline for future surveys to reveal effects of climate and environmental changes.

Should phytoplankton be a key consideration for marine management?

Phytoplankton are an extremely important component of the functioning of ecosystems and climate regulation. Because concentrations of phytoplankton are highly patchy in both space and time, it is proposed that more consideration concerning the potential impact from human developments and activities on the service provision afforded by phytoplankton should be accounted for in marine management processes. The multiple species of primary producers provide important provisioning and regulating ecosystem services (ES) and form the basis of marine food-webs, supporting production of higher trophic levels (a provisioning ES), and act as a sink of CO2 (a climate regulation ES). Spatial and temporal patchiness in the production of phytoplankton can be related to patchiness in the provision of these ES. Patches of naturally high phytoplankton productivity should be afforded consideration within processes to assess environmental status, within marine spatial planning (including marine protected areas) and within sectoral licensing, with marine planning and licensing acting at scales most in harmony with scales of phytoplankton heterogeneity (meters to tens of kilometres). In this study, consideration of phytoplankton in marine management decision making has been reviewed. This paper suggests that potential impacts of maritime developments and activities on the natural patchiness of phytoplankton communities be included in management deliberations, and mitigation be considered. This affords opportunities for researchers to engage with management authorities to support ecosystems-based management. Doing so will assist in maintaining or achieving good environmental status and support further, reliant, ES.

Scaling copepod grazing in a coastal upwelling system: the importance of community size structure for phytoplankton C flux

Crustacean zooplankton, often dominated by copepods and euphausiids, are the major phytoplankton grazers in coastal upwelling systems. It has been argued that grazing by zooplankton is a size-dependent process, such that models incorporating the size structure of zooplankton are appropriate for describing herbivore C-transfer. Here, b ased on the size-spectrum theory and on gut-fluorescence experiments, conducted with numerically dominant copepods from two upwelling sites off the Chilean coast , we show that C-specific ingestion rates of copepods are size-dependent. We further show that the size structure of the copepod community, synthesized by the slope of the normalized size spectrum, determines the impact of grazing on phytoplankton. C-specific ingestion rates, depending on species size, were in the range of 0.02-8.40 (µg C x µg C -1 d -1 ). A modelled normalized biomass-spectra of a copepod community in the size range of 0.5 to 74.0 µg C showed that C-specific grazing impact can increase by a factor of 4 when small-sized species (0.1-10 µg C indiv -1 ), such as Paracalanus cf. indicus , Acartia tonsa , Oncaea spp. and Corycaeus spp., dominate the community in terms of biomass. By contrast, when larger-sized copepods dominate (10-100 µg C indiv -1 ), such as Calanus chilensis , Calanoides patagoniensis and Rhyncalanus nasutus , total zooplankton biomass may increase, but with a sharp decrease in the efficiency of C transfer via herbivores. Our findings indicate that processes affecting the size structure of zooplankton communities can substantially impact on phytoplankton C flux through the pelagic food web, thus controlling production of higher trophic levels.

A latitudinally banded phytoplankton response to 21st century climate change in the Southern Ocean across the CMIP5 model suite

Abstract. Changes in Southern Ocean (SO) phytoplankton distributions with future warming have the potential to significantly alter nutrient and carbon cycles as well as higher trophic level productivity both locally and throughout the global ocean. Here we investigate the response of SO phytoplankton productivity and biomass to 21st century climate change across the CMIP5 Earth System Model suite. The models predict a zonally banded pattern of phytoplankton abundance and production changes within four regions: the subtropical (~ 30 to 40° S), transitional (~ 40 to 50° S), subpolar (~ 50 to 65° S) and Antarctic (south of ~ 65° S) bands. We find that shifts in bottom-up variables (nitrate, iron and light availability) drive changes in phytoplankton abundance and production on not only interannual, but also decadal and 100-year timescales – the timescales most relevant to climate change. Spatial patterns in the modelled mechanisms driving these biomass trends qualitatively agree with recent observations, though longer-term records are needed to separate the effects of climate change from those of interannual variability. Because much past observational work has focused on understanding the effects of the Southern Annular Mode (SAM) on biology, future work should attempt to quantify the precise influence of an increasingly positive SAM on SO biology within the CMIP5 models. Continued long-term in situ and satellite measurements of SO biology are clearly needed to confirm model findings.

Interactions among members of the microbial loop in an estuary dominated by microzooplankton grazing

ABSTRACT: Modified dilution experiments were employed to characterize the microbial loop insurface waters at sites along an estuarine−offshore gradient in the northern Gulf of Mexico. Estu-arine surface waters were more variable than those offshore due to the strong influence of riverdischarge. Phytoplankton (chlorophyll a) and prokaryotes were both significantly higher in theestuary than offshore, but virus and heterotrophic nanoflagellate abundances did not differ acrossthe gradient. Grazing was detected in the majority of experiments, while viral lysis was onlydetected in 24% of phytoplankton and 12% of prokaryote experiments. Growth and grazing ratesfor both phytoplankton and prokaryotes were tightly coupled, except for locations within strongenvironmental transition zones. A large, but variable, percentage of phytoplankton (83 ± 149%)and prokaryote (70 ± 64%) production was grazed by microzooplankton across the estuary. Whilemicrozooplankton grazing is responsible for the removal of the majority of microbial production,thus supporting productivity of higher trophic levels, the role of viral lysis remains unclear.KEY WORDS: Grazing · Viral lysis · Estuaries · Nutrient limitation · Microbial loop

Phytoplankton Biomass and Composition in a River-Dominated Estuary During Two Summers of Contrasting River Discharge

Estuaries located in the northern Gulf of Mexico are expected to experience reduced river discharge due to increasing demand for freshwater and predicted periods of declining precipitation. Changes in freshwater and nutrient input might impact estuarine higher trophic level productivity through changes in phytoplankton quantity and quality. Phytoplankton biomass and composition were examined in Apalachicola Bay, Florida during two summers of contrasting river discharge. The 20 μm diatoms were an increasingly (20 to 70 %) larger component of phytoplankton biomass. Lower summer river discharges that lead to an areal contraction of lower (5–25 psu) salinity waters composed of higher phytoplankton biomass dominated by small ( 25 psu) salinity waters composed of relatively lower phytoplankton biomass and a higher percent contribution by >20 μm diatoms. A reduction in summer river discharge that leads to such a change in quantity and quality of estuarine phytoplankton available will result in a reduction in estuarine zooplankton productivity and possibly the productivity of higher trophic levels.

What triggers Acartia species egg production in a Mediterranean lagoon?

Environmental conditions triggering egg production and hatching success of copepod species are essential to better understand their population dynamics and consequently their contributions to the production of higher trophic levels. A monitoring station was sampled during 2 years in the Thau lagoon (Mediterranean Sea) to study temporal successions of 3 Acartia congeneric species (Acartia clausi, Acartia discaudata and Paracartia grani). Seasonal patterns defined the dominance of one Acartia species over the two others. There were two perennial species A. clausi and A. discaudata with maximum abundance in winter and in spring, respectively. The third species P. grani occurred in summer and was absent from the water column from February to April. Egg production and hatching success experiments were conducted at each sampling date to determine the different reproductive strategies of the Acartia species. Maximum egg productions (∼25–31 eggs fem−1 d−1) and mean hatching success (∼65%) were similar between species but occurred at different periods depending on environmental triggers. Model averaging based on field data showed that temperature prevailed over other variables and that Acartia responses were different regarding the species. Temperature was the main trigger for egg production of A. clausi (negative effect) whereas temperature and salinity should be considered jointly to predict A. discaudata egg production. P. grani egg production described an uni-modal relationship with temperature. The effect of temperature on hatching success explained strongly the three species dynamics, especially the production of P. grani resting eggs.

Secondary production of Chironomidae in a large eutrophic lake: implications for lake sturgeon production

Abstract. Most foodweb research in lentic systems has focused on pelagic primary (phytoplankton) and secondary (zooplankton) production as the primary energy sources for higher trophic-level production. Recent research has demonstrated that secondary production of benthic primary consumers can affect pelagic fish production and foodweb structure in lakes. We used the instantaneous growth method to calculate secondary production of chironomids in Lake Winnebago, Wisconsin (USA), where previous research has shown that lake sturgeon (Acipenser fulvescens) rely heavily on the benthos (chironomids) as a food source. We also used literature-derived data and the instantaneous growth method to calculate annual production of lake sturgeon to test whether chironomid production is sufficient to support the current lake sturgeon population in Lake Winnebago. We collected benthic samples with an Ekman grab at 4 profundal sites during ice-free conditions on 11 dates from spring 2008 through spring 2009. We measured in...

Mussel Production and Water Framework Directive Targets in the Limfjord, Denmark: an Integrated Assessment for Use in System-Based Management

Dinesen, G. E., K. Timmermann, E. Roth, S. Markager, L. Ravn-Jonsen, M. Hjorth, M. Holmer, and J. G. Støttrup. 2011. Mussel production and water framework directive targets in the Limfjord, Denmark: an integrated assessment for use in system-based management. Ecology and Society 16(4): 26. https://doi.org/10.5751/ES-04259-160426

A trophic pathway from biogenic methane supports fish biomass in a temperate lake ecosystem

Although some primary consumers such as chironomid larvae are known to exploit methane‐derived carbon via microbial consortia within aquatic food webs, few studies have traced the onward transfer of such carbon to their predators. The ruffe Gymnocephalus cernuus is a widespread benthivorous fish which feeds predominantly on chironomid larvae and is well adapted for foraging at lower depths than other percids. Therefore, any transfer of methanogenic carbon to higher trophic levels might be particularly evident in ruffe. We sampled ruffe and chironomid larvae from the littoral, sub‐littoral and profundal areas of Jyväsjärvi, Finland, a lake which has previously been shown to contain chironomid larvae exhibiting the very low stable carbon isotope ratios indicative of methane exploitation. A combination of fish gut content examination and stable isotope analysis was used to determine trophic linkages between fish and their putative prey. Irrespective of the depth from which the ruffe were caught, their diet was dominated by chironomids and pupae although the proportions of taxa changed. Zooplankton made a negligible contribution to ruffe diet. A progressive decrease in δ13C and δ15N values with increasing water column depth was observed for both chironomid larvae and ruffe, but not for other species of benthivorous fish. Furthermore, ruffe feeding at greater depths were significantly larger than those feeding in the littoral, suggesting an ontogenetic shift in habitat use, rather than diet, as chironomids remained the predominant prey item. The outputs from isotope mixing models suggested that the incorporation of methane‐derived carbon to larval chironomid biomass through feeding on methanotrophic bacteria increased at greater depth, varying from 0% in the littoral to 28% in the profundal. Using these outputs and the proportions of littoral, sub‐littoral or profundal chironomids contributing to ruffe biomass, we estimated that 17% of ruffe biomass in this lake was ultimately derived from chemoautotrophic sources. Methanogenic carbon thus supports considerable production of higher trophic levels in lakes.

Ecosystem Overfishing in the Ocean

Fisheries catches represent a net export of mass and energy that can no longer be used by trophic levels higher than those fished. Thus, exploitation implies a depletion of secondary production of higher trophic levels (here the production of mass and energy by herbivores and carnivores in the ecosystem) due to the removal of prey. The depletion of secondary production due to the export of biomass and energy through catches was recently formulated as a proxy for evaluating the ecosystem impacts of fishing–i.e., the level of ecosystem overfishing. Here we evaluate the historical and current risk of ecosystem overfishing at a global scale by quantifying the depletion of secondary production using the best available fisheries and ecological data (i.e., catch and primary production). Our results highlight an increasing trend in the number of unsustainable fisheries (i.e., an increase in the risk of ecosystem overfishing) from the 1950s to the 2000s, and illustrate the worldwide geographic expansion of overfishing. These results enable to assess when and where fishing became unsustainable at the ecosystem level. At present, total catch per capita from Large Marine Ecosystems is at least twice the value estimated to ensure fishing at moderate sustainable levels.

Responses in organic matter accumulation and processing to an experimental wood addition in three headwater streams

Summary1. Additions of large wood are being used to restore streams that have been subjected to channelization, wood removal or riparian timber harvest. This added wood potentially increases channel stability, habitat complexity and organic matter retention and improves habitat and productivity of higher trophic levels. However, few stream restorations monitor restoration effectiveness after project completion.2. We added 25 aspen logs (each 2.5 m length × 0.5 m diameter) to 100‐m reaches of each of three forested headwater streams in the Upper Peninsula of Michigan, U.S.A. These wood‐poor streams drain forests that were completely harvested of timber over a century ago and have been selectively logged for the past 50–60 years. An upstream unmanipulateds 100‐m reach in each stream served as the control.3. We evaluated responses in organic matter processing by measuring red maple leaf decomposition 1 year before and 2 years after wood addition. We also quantified coarse organic matter standing stocks in the main channel and in debris accumulations associated with large wood. In response to wood addition, we predicted both organic matter standing stocks and leaf decomposition rates would increase, thereby enhancing resource availability to higher trophic levels.4. Leaf decomposition rates did not change following wood addition. Temporal variation in rates among streams was mostly explained by differences in degree days, water velocity, scour/burial and water column inorganic nitrogen concentrations, but not large wood. Variation within streams across years was explained by differences in degree days, water velocity and shredder biomass.5. Contrary to our prediction, organic matter standing stocks did not increase significantly at the reach scale. However, the experimentally added wood retained c. 4% of total annual coarse benthic organic matter (CBOM) in the first year and an additional c. 15% in the second year, suggesting accumulation over time in the manipulated reaches. The CBOM held by the new logs may be more biologically available because it is less susceptible to burial and transport than material in the streambed.6. Some shredding macroinvertebrates responded to changes caused by the wood additions. In particular, the common caddisfly shredder, Lepidostoma sp., increased in abundance in leaf bags following wood addition, whereas the biomass of the winter stoneflies, Capniidae, declined in the first year.7. Considerable funds are spent to restore in‐stream habitat, but few restorations are monitored, particularly over long periods (>5 years). Our results show that longer‐term monitoring is needed to determine the efficacy of these restorations on ecosystem function; organic matter decomposition in our low‐gradient streams did not respond to a substantial increase in large wood after 2 years.

Responses in zooplankton populations to food quality and quantity changes after whole lake nutrient enrichment of an oligotrophic sub-alpine reservoir

To sustain production of higher trophic levels in oligotrophic systems it is important that the trophic transfer of energy and nutrients is efficient. The phytoplankton-zooplankton interface is of specific interest since nutritional constraints can decouple energy flow in this step. Increased nutrient loading to oligotrophic systems with initially low abundance of high quality phytoplankton could induce changes in seston composition that reduces the nutritional value for zooplankton. We carried out a whole lake enrichment experiment for five years in two ultraoligotrophic subalpine hydroelectric power reservoirs in Sweden. The first year was an untreated reference year. Phosphorus and nitrogen were added to Lake Stora Mjolkvattnet during the following four years, and upstream Lake Burvattnet was used as an untreated reference lake. The phosphorus content of seston in the experimental lake increased in the years of fertilization and seston phosphorus to carbon ratios (atomic) were non-limiting for zooplankton growth. Decreasing concentrations of phosphorus in the reference lake lead to low phosphorus to carbon ratios that probably affected zooplankton growth negatively. The seston fatty acid concentrations and phytoplankton composition indicated good food quality in both lakes. The phytoplankton increased in the experimental lake despite an increase in zooplankton biomass. Some changes in the relative contributions of plankton species occurred but the same species were present. The crustacean zooplankton community composition shifted towards smaller species during the latter years in the experimental lake, indicating increased predation pressure from fish. A major result of this whole-lake ecosystem fertilization experiment is that gentle fertilization can significantly boost phytoplankton production while food quality remains high, and plankton community composition is not substantially altered.