Primary Production Consumption Research Articles

Secondary production of the central rangeland region of the United States.

Rangelands are the dominant land use across a broad swath of central North America where they span a wide gradient, from <350 to >900 mm, in mean annual precipitation. Substantial efforts have examined temporal and spatial variation in aboveground net primary production (ANPP) to precipitation (PPT) across this gradient. In contrast, net secondary productivity (NSP, e.g., primary consumer production) has not been evaluated analogously. However, livestock production, which is a form of NSP or primary consumer production supported by primary production, is the dominant non-cultivated land use and an integral economic driver in these regions. Here, we used long-term (mean length = 19 years) ANPP and NSP data from six research sites across the Central Great Plains with a history of a conservative stocking to determine resource (i.e., PPT)-productivity relationships, NSP sensitivities to dry-year precipitation, and regional trophic efficiencies (e.g., NSP:ANPP ratio). PPT-ANPP relationships were linear for both temporal (site-based) and spatial (among site) gradients. The spatial PPT-NSP model revealed that PPT mediated a saturating relationship for NSP as sites became more mesic, a finding that contrasts with many plant-based PPT-ANPP relationships. A saturating response to high growing-season precipitation suggests biogeochemical rather than vegetation growth constraints may govern NSP (i.e., large herbivore production). Differential sensitivity in NSP to dry years demonstrated that the primary consumer production response heightened as sites became more xeric. Although sensitivity generally decreased with increasing precipitation as predicted from known PPT-ANPP relationships, evidence suggests that the dominant species' identity and traits influenced secondary production efficiency. Non-native northern mixed-grass prairie was outperformed by native Central Great Plains rangeland in sensitivity to dry years and efficiency in converting ANPP to NSP. A more comprehensive understanding of the mechanisms leading to differences in producer and consumer responses will require multisite experiments to assess biotic and abiotic determinants of multi-trophic level efficiency and sensitivity.

Thermal niche diversity and trophic redundancy drive neutral effects of warming on energy flux through a stream food web.

Climate warming is predicted to alter routing and flows of energy through food webs because of the critical and varied effects of temperature on physiological rates, community structure, and trophic dynamics. Few studies, however, have experimentally assessed the net effect of warming on energy flux and food web dynamics in natural intact communities. Here, we test how warming affects energy flux and the trophic basis of production in a natural invertebrate food web by experimentally heating a stream reach in southwest Iceland by ~4°C for 2yr and comparing its response to an unheated reference stream. Previous results from this experiment showed that warming led to shifts in the structure of the invertebrate assemblage, with estimated increases in total metabolic demand but no change in annual secondary production. We hypothesized that elevated metabolic demand and invariant secondary production would combine to increase total consumption of organic matter in the food web, if diet composition did not change appreciably with warming. Dietary composition of primary consumers indeed varied little between streams and among years, with gut contents primarily consisting of diatoms (72.9%) and amorphous detritus (19.5%). Diatoms dominated the trophic basis of production of primary consumers in both study streams, contributing 79-86% to secondary production. Although warming increased the flux of filamentous algae within the food web, total resource consumption did not increase as predicted. The neutral net effect of warming on total energy flow through the food web was a result of taxon-level variation in responses to warming, a neutral effect on total invertebrate production, and strong trophic redundancy within the invertebrate assemblage. Thus, food webs characterized by a high degree of trophic redundancy may be more resistant to the effects of climate warming than those with more diverse and specialized consumers.

A global estimate of terrestrial net secondary production of primary consumers

AbstractAimNet secondary production (NSP) emerges from the consumption of net primary production (NPP) by all heterotrophic organisms. There has been sporadic interest in the importance of NSP, but no global estimates have been produced. Here, we examine NSP and attempt a global estimate using contemporary NPP data combined with modern metabolic scaling theory for consumption rates. We distinguish between potential NSP, as the amount of secondary production that could be supported by NPP, and realized NSP, as the amount remaining after anthropogenic habitat disruption.LocationGlobal.Time period2000–2014.MethodsWe present a metabolic model of NSP, implementing a type II functional response for consumption rates, wherein search efficiency and handling time are calculated based on consumer mass and ambient temperature. We solve this model for each 0.05‐decimal‐degree pixel in the global terrestrial biosphere, using as data inputs NPP (MOD17A3) and land‐surface temperature (MOD11C3). We aggregate estimates within global land‐cover classifications (MCD12C1) to obtain cover‐specific and global estimates of NSP. We also correct our estimates based on declines in consumer abundance reported in Living planet report 2014.ResultsWe estimate that potential NSP is 4.74 Pg C/year globally (95% CI: 3.75–5.75). When we correct for global consumer population declines, realized NSP is estimated at 2.37 Pg C/year (95% CI: 1.86–2.89), a loss of 50% in the rate of carbon flux through secondary consumers.Main conclusionsOur estimates should not be viewed as the last word on NSP but are sufficient to suggest that the flux of carbon through consumers is of a similar magnitude to many other fluxes crucial to the global carbon cycle. We view this as a hypothesis to be tested that suggests NSP deserves significantly more attention in Earth systems, macroecology and biogeochemical research.

Distribution and Feeding of Herbivorous Zooplankton in the Laptev Sea

Sampling was conducted along the quasi meridional transect at 130° E from the Lena River estuary to northern deep-sea regions of the Laptev Sea in September 2015. The latitudinal zonality and the impact of river runoff are manifested in the temperature and salinity distribution, concentration of particulate organic matter, and the structure of plankton communities. The differences in the chl a concentration and primary production along the transect are insignificant. The feeding rate of mesozooplankton herbivores was assessed by a fluorescence technique. The total consumption of phytoplankton biomass and primary production are estimated based on the feeding rate, abundance of zooplankton species, and their diel migrations. The daily grazing impact of zooplankton on phytoplankton biomass increases from 2% on the inner shelf to 3% on the mid-shelf, 5% on the outer shelf, and 10% in the deep-sea part of the basin. The consumption of primary production also increases: 1, 4.5, 5.7, and 13.9%, respectively. In the fall, the consumption of phytoplankton does not compensate the energy demands for respiration. The latitudinal zonality of the Laptev Sea appears not only in the hydrophysical water parameters and the structure of plankton communities, but also in their functional characteristics.

Litter P content drives consumer production in detritus-based streams spanning an experimental N:P gradient.

Ecological stoichiometry theory (EST) is a key framework for predicting how variation in N:P supply ratios influences biological processes, at molecular to ecosystem scales, by altering the availability of C, N, and P relative to organismal requirements. We tested EST predictions by fertilizing five forest streams at different dissolved molar N:P ratios (2, 8, 16, 32, 128) for two years and tracking responses of macroinvertebrate consumers to the resulting steep experimental gradient in basal resource stoichiometry (leaf litter %N, %P, and N:P). Nitrogen and P content of leaf litter, the dominant basal resource, increased in all five streams following enrichment, with steepest responses in litter %P and N:P ratio. Additionally, increases in primary consumer biomass and production occurred in all five streams following N and P enrichment (averages across all streams: biomass by 1.2×, production by 1.6×). Patterns of both biomass and production were best predicted by leaf litter N:P and %P and were unrelated to leaf litter %N. Primary consumer production increased most in streams where decreases in leaf litter N:P were largest. Macroinvertebrate predator biomass and production were also strongly positively related to litter %P, providing robust experimental evidence for the primacy of P limitation at multiple trophic levels in these ecosystems. However, production of predatory macroinvertebrates was not related directly to primary consumer production, suggesting the importance of additional controls for macroinvertebrates at upper trophic positions. Our results reveal potential drivers of animal production in detritus-based ecosystems, including the relative importance of resource quality vs. quantity. Our study also sheds light on the more general impacts of variation in N:P supply ratio on nutrient-poor ecosystems, providing strong empirical support for predictions that nutrient enrichment increases food web productivity whenever large elemental imbalances between basal resources and consumer demand are reduced.

Associations between cyanobacteria and indices of secondary production in the western basin of Lake Erie

AbstractLarge lakes provide a variety of ecological services to surrounding cities and communities. Many of these services are supported by ecological processes that are threatened by the increasing prevalence of cyanobacterial blooms which occur as aquatic ecosystems experience cultural eutrophication. Over the past 10 yr, Lake Erie experienced cyanobacterial blooms of increasing severity and frequency, which have resulted in impaired drinking water for the surrounding communities. Cyanobacterial blooms may impact ecological processes that support other services, but many of these impacts have not been documented. Secondary production (production of primary consumers) is an important process that supports economically important higher trophic levels. Cyanobacterial blooms may influence secondary production because cyanobacteria are a poor‐quality food resource and cyanotoxins may be harmful to consumers. Over 3 yr at 34 sites across the western basin of Lake Erie, we measured three indices of secondary production that focus on the dominant bivalve taxa: (1) growth of a native unionid mussel, (2) the size of young‐of‐year dreissenid mussels, and (3) the mass of colonizing animals on a Hester‐Dendy sampler. Associations between these indices and cyanobacterial data were estimated to assess whether cyanobacteria are associated with variation in secondary production in the western basin of Lake Erie. The results suggest cyanobacterial abundance alone is only weakly associated with secondary production, but that cyanotoxins have a larger effect on secondary production. Given recurring late‐summer cyanobacterial blooms, this impact on secondary production has the potential to undermine Lake Erie's ability to sustain important ecosystem services.

Benthic carbon is inefficiently transferred in the food webs of two eutrophic shallow lakes

Abstract The sum of benthic autotrophic and bacterial production often exceeds the sum of pelagic autotrophic and bacterial production, and hence may contribute substantially to whole‐lake carbon fluxes, especially in shallow lakes. Furthermore, both benthic and pelagic autotrophic and bacterial production are highly edible and of sufficient nutritional quality for animal consumers. We thus hypothesised that pelagic and benthic transfer efficiencies (ratios of production at adjacent trophic levels) in shallow lakes should be similar. We performed whole ecosystem studies in two shallow lakes (3.5 ha, mean depth 2 m), one with and one without submerged macrophytes, and quantified pelagic and benthic biomass, production and transfer efficiencies for bacteria, phytoplankton, epipelon, epiphyton, macrophytes, zooplankton, macrozoobenthos and fish. We expected higher transfer efficiencies in the lake with macrophytes, because these provide shelter and food for macrozoobenthos and may thus enable a more efficient conversion of basal production to consumer production. In both lakes, the majority of the whole‐lake autotrophic and bacterial production was provided by benthic organisms, but whole‐lake primary consumer production mostly relied on pelagic autotrophic and bacterial production. Consequently, transfer efficiency of benthic autotrophic and bacterial production to macrozoobenthos production was an order of magnitude lower than the transfer efficiency of pelagic autotrophic and bacterial production to rotifer and crustacean production. Between‐lake differences in transfer efficiencies were minor. We discuss several aspects potentially causing the unexpectedly low benthic transfer efficiencies, such as the food quality of producers, pelagic–benthic links, oxygen concentrations in the deeper lake areas and additional unaccounted consumer production by pelagic and benthic protozoa and meiobenthos at intermediate or top trophic levels. None of these processes convincingly explain the large differences between benthic and pelagic transfer efficiencies. Our data indicate that shallow eutrophic lakes, even with a major share of autotrophic and bacterial production in the benthic zone, can function as pelagic systems with respect to primary consumer production. We suggest that the benthic autotrophic production was mostly transferred to benthic bacterial production, which remained in the sediments, potentially cycling internally in a similar way to what has previously been described for the microbial loop in pelagic habitats. Understanding the energetics of whole‐lake food webs, including the fate of the substantial benthic bacterial production, which is either mineralised at the sediment surface or permanently buried, has important implications for regional and global carbon cycling.

High secondary production in a Coastal Plain river is dominated by snag invertebrates and fuelled mainly by amorphous detritus

Summary The 2‐years production dynamics of major taxa (family/order) and functional feeding groups was synthesised from detailed production analyses previously published for all major invertebrate species/genera (retained by a 100 μm sieve) found on snags (submerged wood) in a sixth‐order south‐eastern U.S. Coastal Plain river. The trophic basis of this production and annual consumption of six food types were quantified. Invertebrate production (148 and 203 g dry mass m−2 year−1 of snag surfaces for the 2 years) was among the highest ever measured for a specific habitat in non‐polluted or unregulated rivers and was still relatively high when converted to overall area of the river bed (48 and 70 g m−2 year−1). Snag production constituted 73% of total main‐channel production (production from snag and river bed combined). Chironomidae (40–44% over the 2 years), Ephemeroptera (14–21%) and Trichoptera (25–31%) together represented the majority (88–93%) of the putative production of primary consumers on snags. Functional group analysis showed that fine filterers (26–36% over the 2 years), gatherers (36–44%) and macro‐filterers (21–25%) represented the majority of total invertebrate production. Production of predaceous insects (mostly Plecoptera, Odonata and Megaloptera) represented only 5–7% of the total. Production of the three major taxa and three major functional groups was remarkably consistent over 2 years of monthly sampling, despite strong seasonal variation in discharge/light/temperature and production of individual genera and families. Seventy‐nine per cent of production by primary consumers was supported by microbially enriched amorphous detritus, the major component of seston originating primarily from floodplain forests. Invertebrates consumed ˜357 g AFDM m−2 (river bed) year−1 of seston, 325 g m−2 year−1 of amorphous detritus and 20 g m−2 year−1 of diatoms. Dividing seston consumption by seston transport indicated that ˜67% of seston was consumed over 100 km of river length. Consumption was higher than autochthonous net primary production, supporting other evidence that terrestrially derived dissolved organic matter, incorporated via the microbial loop, is the main trophic basis of this riverine food web. Macro‐filtering hydropsychid caddisflies were omnivorous with &gt; half their production from carnivory. Snag predators, including hydropsychids, consumed ˜60% (29 g m−2 year−1 of river bed) of invertebrate production (assumed to be from snags), leaving the remainder for emergence, downstream transport and consumption by fish.

Macroinvertebrate community structure, secondary production and trophic‐level dynamics in urban streams affected by non‐point‐source pollution

Summary1. Despite non‐point‐source (NPS) pollution being perhaps the most ubiquitous stressor affecting urban streams, there is a lack of research assessing how urban NPS pollution affects stream ecosystems. We used a natural experimental design approach to assess how stream macroinvertebrate community structure, secondary production and trophic structure are influenced by urban NPS pollution in six streams.2. Differences in macroinvertebrate community structure and secondary production among sites were highly correlated with stream‐water specific conductivity and dissolved inorganic phosphorus (DIP) concentrations. Macroinvertebrate richness, the Shannon diversity index and the Shannon evenness index were all negatively correlated with specific conductivity. These patterns were driven by differences in the richness and production of EPT and other intolerant taxa. Production of the five most productive taxa, tolerant taxa, non‐insect taxa and primary consumers were all positively correlated with stream‐water DIP.3. Despite the positive correlation between primary consumer production and DIP, there was no correlation between macroinvertebrate predator production and either total or primary consumer macroinvertebrate production. This was observed because DIP was positively correlated with the production of non‐insect macroinvertebrate taxa assumed to be relatively unavailable for macroinvertebrate predator consumption. After removing production of these taxa, we observed a strong positive correlation between macroinvertebrate predator production and production of available prey.4. Our results suggest that urban NPS pollution not only affects macroinvertebrate community structure, but also alters secondary production and trophic‐level dynamics. Differences in taxon production in our study indicate the potential for altered energy flow through stream food webs and potential effects on subsidies of aquatic insect prey to riparian food webs.

Bioconversion of fatty acids at the basis of marine food webs: insights from a compound-specific stable isotope analysis

Polyunsaturated fatty acids (PUFA) are essential compounds that can limit the productivity of primary consumers in aquatic food webs, where the efficiency in energy transfer at the plant-animal interface has been related to food quality in terms of fatty acids (FA). At this interface, copepods play a pivotal role both as consumers of primary production and as a food source for higher trophic levels. Understanding the role of grazing copepods in the transfer of FA is therefore essential for our knowledge on the overall functioning of marine ecosystems. The harpacticoid copepod Microarthridion littorale grazed for 9 d on C-13 labelled diatoms and bacteria in the laboratory and was then subjected to FA-specific stable isotope analysis. The objective of this analysis was to inspect the copepod's ability to bioconvert short-chain FA (SC-PUFA, = 20 carbons) and the FA involved in the potential bioconversion pathways. Diatoms and bacteria were chosen as test diets because of their different FA composition, i.e. docosahexaenoic acid (DHA; 22: 6 omega 3) was absent in the bacteria, and eicosapentaenoic acid (EPA; 20: 5 omega 3) was <5% of the total FA weight of bacteria. The presence of labelled DHA in copepods feeding on bacteria showed that this PUFA must have been converted from other FA, possibly EPA. The FA composition of copepods in the laboratory was different from that of field copepods, which suggests the availability of more food sources in the field than those offered in the experiment. The weight proportion of C18 FA decreased in copepods feeding on either bacteria or diatoms relative to field copepods, while the proportion of both EPA and DHA increased. In contrast to planktonic calanoid copepods that have limited ability to bioconvert FA, benthic harpacticoid copepods apparently developed the ability to elongate FA and to exploit niches with poor quality food. Moreover, by improving the quality of the food they graze upon, especially in terms of EPA and DHA, harpacticoid copepods upgrade the nutritive value of food available to the higher trophic levels in marine food webs.

Long-term nutrient enrichment decouples predator and prey production

Increased nutrient mobilization by human activities represents one of the greatest threats to global ecosystems, but its effects on ecosystem productivity can differ depending on food web structure. When this structure facilitates efficient energy transfers to higher trophic levels, evidence from previous large-scale enrichments suggests that nutrients can stimulate the production of multiple trophic levels. Here we report results from a 5-year continuous nutrient enrichment of a forested stream that increased primary consumer production, but not predator production. Because of strong positive correlations between predator and prey production (evidence of highly efficient trophic transfers) under reference conditions, we originally predicted that nutrient enrichment would stimulate energy flow to higher trophic levels. However, enrichment decoupled this strong positive correlation and produced a nonlinear relationship between predator and prey production. By increasing the dominance of large-bodied predator-resistant prey, nutrient enrichment truncated energy flow to predators and reduced food web efficiency. This unexpected decline in food web efficiency indicates that nutrient enrichment, a ubiquitous threat to aquatic ecosystems, may have unforeseen and unpredictable effects on ecosystem structure and productivity.

The zooplankton consumption of primary production and an assessment of the waterbody trophic state on the basis of its structural and functional characteristics

The patterns of the seasonal dynamics of species composition, abundance, biomass, and ration of zooplankton in the Curonian Lagoon of the Baltic Sea have been considered. It is shown that zooplankton can consume up to 17–21% of the phytoplankton primary production. Of this, 7–8% of the primary production is transformed into secondary production and can be used by invertebrates and fishes. The structural and functional characteristics of zooplankton (index and coefficient of the trophic state, the number of dominating species, and the Shannon index by biomass) can be used to assess the waterbody trophic state.

ECOLOGICAL STOICHIOMETRY IN RUNNING WATERS: PERIPHYTON CHEMICAL COMPOSITION AND SNAIL GROWTH

Mismatches between the elemental composition of primary consumers and their food suggest that the production of primary consumers may be limited by food mineral content. Although the consequences of these mismatches for consumers have been studied extensively for zooplankton and terrestrial herbivores, the relationship between periphyton elemental composition and primary consumer performance has been rarely explored. We manipulated periphyton elemental composition by growing stream periphyton under all combinations (2 × 2 factorial) of low and high inorganic N and P and offered low and high quantities to Elimia livescens snails. Periphyton quantity, as measured by ash-free dry mass and total carbon, was kept the same among nutrient treatments within a level of food quantity. We measured Elimia growth as change in blotted wet mass and shell-free ash-free dry mass. Periphyton chemical composition was described by measurements of percentages of C, N, P, ash and chlorophyll a. At low periphyton quantity, Elimia growth on periphyton grown under high P was 40–66% higher than on periphyton grown under low P. Higher P content of the periphyton grown under high P is the most likely explanation for the increased growth of Elimia. At high food quantity, periphyton chemical composition had no effect on Elimia growth. Snails had increased growth when offered more periphyton of lower quality, but generally not when offered an increased quantity of high-quality periphyton. Although snails removed more periphyton when offered a higher quantity of food, snails did not actively compensate for the lower quality food by increasing removal rates when the same amount of each food type was offered. The elemental composition of Elimia was not affected by food quality or quantity, consistent with ecological stoichiometry theory. We suggest that dissolved P was the ultimate cause of differences in Elimia growth at low food quantity, mediated through changes in periphyton chemical composition. Limitation of primary consumer growth by periphyton chemical composition may be widespread in oligotrophic streams, because periphyton production and food quality are typically low.

The Phytoplankton Zooplankton Link in the Lake Ontario Food Web

Monitoring in Lake Ontario in 1970 and 1982 demonstrated that the zooplankton community was dominated by microzooplankton, which suggested a longer, perhaps inefficient food chain. In this study, annual monitoring of the offshore region of Lake Ontario between 1986 and 1992 was used to determine if microzooplankton were still dominant despite recent changes in nutrient loading and species introductions. Microzooplankton accounted for 49.7% of the total summer zooplankton biomass while small edible phytoplankton accounted for 67.0% of the biomass during the summer. By direct in situ measurement using a Haney grazing chamber, rather than size grazing relationships, the relative impact of micro- and mesozooplankton grazers on phytoplankton production during the summer of 1995 was evaluated. Microzooplankton filtration rates (%/d) for 1995 were significantly higher than mesozooplankton filtration rates. Zooplankton consumed only 17.5% /d of the primary production with microzooplankton grazing representing 69.8% to 93.2% of this amount. Microzooplankton are clearly still dominant and their consumption of primary production in Lake Ontario is low. The major pathway of energy transfer can not be through the classical phytoplankton > large zooplankton > planktivore > piscivore food chain but rather through the phytoplankton > microzooplankton and presumably predacious zooplankton and fish. The longer food chain is a result of the introduction of a size-selective planktivore, the alewive, which has decreased the length and presumably lowered the consumption rate of the entire zooplankton community. This structural impact, a longer food chain, theoretically creates a higher factor of biomagnification of organic chemicals for top-level predators along with lower rates of energy transfer within the food web and suggests lower fish production than in a shorter food web.

Distribution of zooplankton in the Northeast Water Polynya during summer 1992

Abstract The distribution and abundances of zooplankton in the Northeast Water (NEW) Polynya during the summer of 1992 have been determined from samples collected with Bongo (this study) and MOCNESS (earlier analysis) plankton net tows. The initial hypothesis was that the troughts of the Greenland Shelf in the region of the NEW polynya would be conduits for the infusion of zooplankton species of Arctic and North Atlantic origin on to the shelf. Analysis of MOCNESS tows indicated instead that advection of zooplankton onto the shelf in the polynya region was limited, resulting in low abundances of the large herbivores in the polynya. The distribution of zooplankton determined from the 43 Bongo tows reported here strengthen this finding. Low abundances of the large herbivorous copepods and of other primarily herbivorous zooplankton taxa were observed throughout the ice-free region, in contrast to higher abundances observed further to the east in Westwind Trough or in the south in Belgica Trough. Cluster analysis was used to identify groups of co-occurring zooplankton taxa and groups of stations with similar relative compositions of zooplankton taxa. The community composition of the Belgica Trough was distinctly different from that observed in northern regions, with high abundances of open ocean copepods ( Calanus finmarchicus and C. hyperboreus ). Two types of communities were identified in the northern regions: one typical of the eastern basin (Westwind Trough) and one typical of the western basin (Norske Trough) of the northern trough. The relationship between the different zooplankton communities and the water mass characteristics was not conclusive; however, it did appear that zooplankton populations had been advected in the upper water column into the Westwind Trough from north of the study area. The low abundances of herbivorous taxa in the ice free region supported previous estimates that meso-macro zooplankton may not be significant consumers of primary production in the polynya.

Leaf litter processing and energy flow through macroinvertebrates in a woodland pond (Switzerland).

Energy generated by leaf litter processing and its flow through the associated macroinvertebrate community was quantified in a pond near Geneva (Switzerland). Annual density, biomass, and production on oak (Quercus robur) leaf litter were assessed for all macroinvertebrate taxa with emphasis on predators. Empirical energetic relations provided an energy budget for the macroinvertebrate community. On 1 m2 of pond bottom, the processing of 5641 kJ of oak leaf litter resulted in 8.5% of leachate (6 days), and after 1 year 32% of material remained; the other 59.5% was biologically (animal or microbial) converted, including 11.2% processed by shredders. The mean annual density of associated macroinvertebrates was 51374 individuals, mean biomass was 3.53 g (dry mass) and production was about 1451 kJ (or 65 g). Predator production was 170 kJ/m2, non-chironomid primary consumer production was 101 kJ/m2 (including 57 kJ from shredders) and chironomid primary consumer production was estimated at 1180 kJ/m2. Predators contributed to a high proportion of total biomass (39%) but to a smaller amount of production (12%) or density (6%). In this two-stepped food-chain mainly based on detritus, the transfer coefficient between first level (detritus + primary producers) and third level (secondary consumers) was high (2-2.5%) and indicated efficient conversion of energy. This high efficiency was partly related to the reutilization of fine particulate organic matter by the collectors. The production estimate measured on leaf litter was compared with two other predominant substrates (Typha latifolia stems and Chara sp.), and exhibited the highest value. This study shows how leaf litter can constitute a direct source for high secondary production and be an efficient energy source in freshwater ecosystems. It is also demonstrated that a woodland pond can support a high macroinvertebrate production as compared with other freshwater ecosystems.

Invertebrate Productivity in a Subtropical Blackwater River: The Importance of Habitat and Life History

Habitat and life history are critical elements in assessing the production dynamics of invertebrates and their role in aquatic ecosystems. We studied invertebrate productivity at two sites in a subtropical blackwater river (the Satilla) in the Lower Coastal Plain of Georgia, USA, and found that submerged wooden substrates, or snags, are heavily colonized by aquatic insects. We compared invertebrate productivity on the snag habitat with productivity in the sandy benthic habitat of the main channel, and the muddy benthic habitat of the backwaters. The size—frequency method was applied to individual taxa in order to determine total invertebrate productivity. Emphasis was placed on the importance of the length of larval life, or the cohort production interval, in determining biomass turnover rates. The diversity of taxa was much higher on the snag habitat than in either of the benthic habitats. Filter—feeding caddisflies (especially Hydropsyche spp.) and black flies (Simulium spp.) were the major consumers on the snag habitat. Several species of midges, mayflies, and beetles also were abundant. Total densities, standing stock biomass, and production were very high for primary consumers on snags. Annual production was 51.9 and 67.1 g°m—2°yr—1 (dry mass per surface area of snag, or effective habitat) for the two sites. Hellgrammites, dragonflies, and stoneflies were the major insect predators colonizing snags, and their production was 5.5 and 5.2 g@mm—2°yr—1 (effective habitat). Annual production/biomass ratios (P/B) were usually 5—10 for insects that had univoltine or bivoltine life cycles. Annual P/B estimates were very high for midges (&gt;100) and black flies (&gt;70), since length of larval life was estimated to be very short. The sandy—substrate benthos consisted almost exclusively of very small midges with oligochaetes of lesser abundance. Densities were quite high (&gt;20 000/m2), but biomass was very low (° 100 mg/m2 or less). Production of primary consumers was &gt;11 g°m—2°yr—1 with a very high estimate of annual P/B (166—227). The major predators were Ceratopogonidae (biting midges) larvae with an annual production of 1.6—2.6 g°m—2°yr—1. The muddy—substrate benthos consisted primarily of oligochaetes (Limnodrilus) and midges. Annual production was °7—10 g°m—2°yr—1 for primary consumers. The major predators were larger Tanypodinae midges. On a substrate surface area basis, standing stock biomass on snags was 20—50 times higher than in the sandy habitat and 5—10 times higher than in the muddy habitat. Production on snags was only 3—4 times higher than production in the benthic habitats, with higher annual P/B in the latter. The production estimates for the snag habitat are among the highest yet reported for lotic ecosystems, and it appears that production on snags is limited by available substrate. Habitat areas per length of shoreline were estimated so that we could approximate relative amounts of biomass and production for a stretch of river. Although the snag habitat accounted for only °6% of the effective habitat substrate over a stretch of river, it was responsible for over half of invertebrate biomass, and °15—16% of production. Taxa within each habitat were categorized to functional feeding groups, and habitat—specific functional groupings were evaluated using numbers, biomass, and production. Filtering collectors predominated on snags, and gathering collectors in benthic habitats. When corrected for habitat abundance, the distribution of biomass among filtering collectors, gathering collectors, and predators was very close. However, the distribution of production was °12% filtering collectors, 71% gathering collectors, and 17% predators. We suggest that production is the most meaningful parameter to consider in functional group analysis and that the use of numbers or biomass alone can sometimes result in misleading conclusions. As a middle order (5th—6th) stream, the distribution of production or biomass among functional groups in the Satilla River differs considerably from that predicted by the river continuum concept, predicting a high percentage of grazing consumers.

Estimate of biological production in some stream invertebrates

Abstract Annual production of invertebrates was investigated in two streams, the Hinau and Horokiwi, in the North Island of New Zealand. Estimates were based on the instantaneous growth rate method, with adjustments made according to the number of generations produced per year. Production of primary consumers (herbivores and detritus feeders) was 7.6–72.1 g.m‐2.y‐1, and of secondary consumers (predators) 0.8–11.9 g.m‐3.y‐1. In both streams the lowest production was found in tree‐shaded situations. Production: biomass ratios ranged from 5.3 to 7.9 for primary consumers, and from 3.7 to 6.5 for secondary consumers. These ratios fall within the range of results found by workers in other countries. Fish in the two streams studied feed mainly on the primary consumers and so may be competing for food with invertebrate secondary consumers.

Consumption of Primary Production by a Population of Kangaroo Rats (Dipodomys Merriami) in the Mojave Desert

The energy requirements for a population of kangaroo rats in the Mojave Desert were estimated indirectly. Energy expenditure for maintenance was 84.1 megacal ha—1yr—1. Total energy expenditure for growth was 1.4 megacal ha—1yr—1, half of which was secondary productivity. Secondary productivity was 0.8% of the total energy flow. This percentage of the total energy flow is lower than for nondesert rodents and may be due to the low productivity of the desert areas in which D. merriami is found. The diet of D. merriami contained an average of 75% seeds. Green herbage formed 35% of the diet during the winter quarter. Kangaroo rats showed a high preference index for Erodium cicutarium seeds, which were larger and had a higher caloric value than the seeds of other annual plants. The average assimilation efficiency was 0.87 for the natural diet. Estimates indicated that kangaroo rats consumed 97.8 megacal ha—1yr—1, 99% of which was from the primary production. Total net primary production (NPP) on the study area was only 1,400 megacal ha—1yr—1; of this, 900 megacal ha—1yr—1 was considered to be available to D. merriami. Kangaroo rats consumed 6.9% of the NPP. This is higher than for any other rodent populations that have been studied and is probably due to the large proportion of available production (AP) in the total primary production (64%). The population consumed 10.7% of AP, which is high when compared to most other rodent species. The high consumption efficiency for D. merriami may be an adaptation to areas of low production, or it may be simply the result of high densities of kangaroo rats in areas of low production. Dipodomys merriami consumed over 95% of the estimated Erodium production and 90% of the total Erodium production. It is estimated that the rats consumed a sufficient number of Erodium seeds in 1970 to reduce the density of that plant by more than 30% in 1971. The D. merriami population was on the brink of overexploitation of its food sources and was potentially food limited. It is probable that desert herbivores in general are food limited since productivity in the desert is so variable.