Estimating contributions of pelagic and benthic pathways to consumer production in coupled marine food webs (original) (raw)
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Productivity and carbon transfer in pelagic food webs
umu.diva-portal.org
Some of the major problems we face today are human induced changes to the nitrogen (N), phosphorus (P) and carbon (C) cycles. Predicted increases in rainfall and temperature due to climate change, may also increase dissolved organic matter (DOM) inflows to freshwater ecosystems in the boreal zone. N, P, C and light, are essential resources that most often limit phytoplankton (PPr) and bacterial production (BP) in the pelagic zone of lakes. PPr and BP not only constitute the total basal C resource for the pelagic aquatic food web, but also influence ecosystem function and biogeochemical cycles. In this thesis I studied how N, P, C and light affect the relative and absolute rates of PPr and BP, along a wide latitudinal and trophic gradient using published data, and in two in situ mesocosm experiments in a clear water oligotrophic lake. In the experiments I manipulated bottom-up drivers of production and top-down predation to examine how these factors interact to affect pelagic food web structure and function. The most important predictors of PPr globally (Paper I) were latitude, TN, and lake shape. Latitude alone explained the most variation in areal (50%) and volumetric (40%) PPr. In terms of nutrients PPr was primarily N-limited and BP was P-limited. Therefore bacteria and phytoplankton were not directly competing for nutrients. BP:PPr was mostly driven by PPr, therefore light, N, temperature and other factors affecting PPr controlled this ratio. PPr was positively correlated with temperature, but not BP, consequently, higher temperatures may reduce BP:PPr and hence the amount of energy mobilised through the microbial food web on a global scale. In papers II and III interaction effects were found between C-additions and top-down predation by young-of-the-year (YOY) perch. Selective predation by fish on copepods influenced the fate of labile C-addition, as rotifer biomass increased with C-addition, but only when fish were absent. Interaction effects between these top-down and bottom-up drivers were evident in middle of the food web, which is seldom examined in this type of study. Although the energy pathway from bacteria to higher consumers is generally longer than from phytoplankton to higher trophic levels, increased BP still stimulated the biomass of rotifers, calanoid copepods and YOY fish. However, this appeared to be mediated by intermediate bacterial grazers such as flagellates and ciliates. Light was an important driver of crustacean zooplankton biomass (paper IV), but the light:nutrient hypothesis was inadequate to predict the mechanisms behind the decrease in zooplankton biomass at low light. Instead, it appeared that reduced edibility of the phytoplankton community under low light conditions and reduced BP most strongly affected zooplankton biomass. Thus, the LNH may not apply in oligotrophic lakes where PPr is primarily N-limited, Daphnia is rare or absent and mixotrophic phytoplankton are abundant. N, P, C and light manipulations have very different effects on different parts of the pelagic food web. They influence the relative rates of PPr and BP, affect phytoplankton community composition, alter the biomass of higher trophic levels and change pathways of energy transfer through the pelagic food web. This thesis adds valuable information as to how major changes in these resources will affect food web structure and function under different environmental conditions and future climate scenarios.
Deep Sea Research Part II: Topical Studies in Oceanography, 2002
Three fates potentially consume primary production occurring on ocean margins: portions can be oxidized within the water column, portions can sediment to shelf/slope depots, and portions can be exported to the interior ocean. Zooplankton mediate all three of these processes and thus can alter the pathway and residence time of particulate organic carbon. As part of both US DOE-and NSF-sponsored studies on the Cape Hatteras and South Atlantic Bight (SAB) shelves, the role of microzooplankton in these processes was determined by measuring phytoplankton production and its consumption by microzooplankton. Grazing and growth rates were measured during 46 dilution incubation experiments using chlorophyll a (chl a) as a proxy for phytoplankton (prey) biomass. Chl a production and grazing were determined for the o200 mm phytoplankton community and also the o8 mm size class. Primary production at Cape Hatteras was determined using H 14 CO 3 À incubations during two Lagrangian drifter studies lasting several days in March and July 1996. From similar measurements during cross-shelf transects over larger spatial scales, primary production was also calculated for the Hatteras study area using a wavelength-resolved bio-optical model. Primary production during the Lagrangian studies was generally 0.5-1.0 gC/m 2 /d in March and 0.5-2.0 gC/m 2 /d in July. Modeled estimates of primary production for the larger Hatteras study region in March and July averaged 1.8 gC/ m 2 /d. Typically, o8 mm cells contributed one-half or more of primary production. Positive linear regressions described relationships between phytoplankton production measured as changes in chl a and its grazing by microzooplankton. In the dilution experiments conducted throughout the SAB and Hatteras shelves, microzooplankton grazed 65% of o200 mm chl a production, and 81% of o8 mm chl a production. These relationships were temperature-dependent: losses of chl a production in both size fractions to microzooplankton herbivory increased with increasing temperature. Higher grazing rates were found in the o8mm compared to the o200 mm size class. Model regressions were used to estimate the impact of microzooplankton grazing on H 14 CO 3 À-derived estimates of primary production in Cape Hatteras shelf waters. Integrated water column grazing removed 40% and 58% of o200 mm and o8 mm primary production, respectively, during the Lagrangian experiment in March, and 61% and 74% in July. Averaged over larger spatial scales using a bio-optical model, microzooplankton ingested 42% and 61% of primary production in March and July, respectively, with an overall mean of 52%. These data generally support the notion that, contrary to traditional paradigms about shelf ecosystems, small autotrophs contributed significantly to production, and that this carbon was actively incorporated into the microbial food web.
A Changing Paradigm of Pelagic Food Webs
International Review of Hydrobiology, 2008
The paper catalogues the development of knowledge that has increasingly challenged the conventional understanding (the paradigm) that the predominant pathway of energy flow in pelagic systems is founded on the primary production of phytoplankton, its consumption by herbivorous zooplankton and, in turn, the feeding of larger carnivorous animals, including fish of commercial value. Aquatic food webs are now recognised to be more diverse in their carbon sources, more complex in structure and more versatile in their function. Adjacent terrestrial catchments supply nutrients and, crucially, dissolved inorganic carbon, dissolved organic carbon and particulate organic carbon. Pivotal roles in the processing of autochthonous organic carbon are often fulfilled by pelagic microbes and bacterivorous nanoplankton and microplankton. Mesozooplankton consumes algae directly if sufficiently concentrated but is itself a limiting resource to pelagic fish, which either must be specially adapted to gather sufficient zooplankton or must switch to benthic or littoral foods. Opportunistic feeding by such fish, though primarily nourished in the inshore, continues to control abundant zooplankton. Mobility and dietary electivity of adult fish integrate the pelagic food web into that of the benthos. Except in very large lakes and in the open ocean, the influence of benthic food webs influences the flow of carbon within the pelagic and beyond, serving to invalidate the traditional paradigm of trophic relationships and demanding formulation of another.
Marine Biology Research, 2016
Estuarine systems are characterized by complex physicochemical and hydrological changes occurring across multiple scales, which determine spatiotemporal variation in distribution and abundance of consumers and their resources. However, little is known about the effects of these biophysical interactions on the interannual dynamics of estuarine food webs. In this work, a five-year stable isotope data set was used to investigate interannual variability in trophic links between basal production sources (seagrass, macroalgae, saltmarsh, particulate organic matter in suspension and in the sediment) and an omnivorous fish (Jenynsia multidentata) in a subtropical estuary emptying in the Southwestern Atlantic. The isotopic variability (δ 13 C, δ 15 N) of basal sources and the consumer was analysed seasonally on a mudflat from 2010 to 2014. Jenynsia multidentata showed significant interannual variation in their trophic links with primary producers. In most seasons and years, the consumer relied heavily on benthic-associated food resources, but shifted to pelagic food resources during certain seasons. A 'green tide' caused by a massive bloom of vicariant macroalgae occurred on the mudflat, but our findings suggested that the carbon-derived portion of this basal production source was not assimilated by J. multidentata. Instead, seagrass was the most assimilated benthic basal food source in most seasons and years. These results suggest that the intensity of benthic and pelagic trophic pathways sustaining estuarine consumers are not static, but change in response to intra-and interannual variation in the availability of basal production sources. Our findings reinforce the need to account for interannual trends in availability of resources when modelling estuarine food web dynamics.
Journal of Sea Research, 2018
A two-dimensional biomarker approach including fatty acids and stable isotopes of seston and copepods was applied to examine how the variability at the base of the food web affects trophic interactions between primary producers and copepod consumers over a sampling period of two years. We investigated how the composition of the seston affected feeding behaviour by analysing the fatty acid and stable isotope signals of the copepods Calanus helgolandicus, Acartia spp., Centropages spp. and Temora longicornis at Helgoland Roads, North Sea. Our results indicate that the relative contributions of autotrophic and heterotrophic fractions in the seston determined the stable isotope signal of the seston and hence the δ 15 N of copepods. Our findings show that the combination of stable isotope and fatty acid analyses provides an ideal tool to address the complexity of trophic relations in planktonic food-webs and to define relative trophic position and feeding preferences of e.g. copepods. Defining accurate baselines from bulk seston samples containing a mixture of auto-and heterotroph protist communities still remains a challenge when defining lower food-web dynamics in natural plankton communities.
Frontiers in Marine Science, 2021
Trophic contributions of diverse OM sources to estuarine and coastal food webs differ substantially across systems around the world, particularly for nekton (fish, cephalopods, and crustaceans), which utilize basal resources from multiple sources over space and time because of their mobility and feeding behaviors at multiple trophic levels. We investigated the contributions of putative OM sources to fish food webs and assessed the spatiotemporal patterns, structures, and trophic connectivity in fish food webs across four seasons from three closely spaced (10–15 km) sites: an estuarine channel (EC), a deep bay (DB), and an offshore (OS) region in Gwangyang Bay, a high-productivity, low-turbidity estuarine embayment off the Republic of Korea. While nearly all previous studies have focused on few representative species, we examined δ13C and δ15N values of whole nekton communities along with dominant benthic macro-invertebrates, zooplankton, and their putative primary food sources. The ...
Freshwater Biology, 2017
Lakes are among the most seasonally forced ecosystems on Earth. Seasonal variation in temperature and light produce cyclic patterns in water column mixing, nutrient supply and phytoplankton biomass. Diet responses of consumers to these patterns have rarely been quantified. Moreover, pelagic‐littoral coupling of dietary resources by mobile consumers is commonly considered to be static over annual cycles. This study quantifies littoral‐pelagic diet responses of multiple consumers to a strong shift in pelagic phytoplankton abundance over an annual cycle (September 2014 to August 2015) in a large (area 614 km2), oligotrophic, monomictic lake (Lake Taupō, New Zealand). Intra‐annual patterns in pelagic phytoplankton (chlorophyll a) and zooplankton were determined over multiple years. Major resource and consumer δ13C and δ15N were then collected over an annual cycle. Temporal patterns in food‐web structure were examined using convex hulls as a proxy of community trophic niche size. Diet wa...
PloS one, 2015
We examined stable carbon and nitrogen isotope ratios for a large variety of consumers in intertidal and subtidal habitats, and their potential primary food sources [i.e., microphytobenthos (MPB), phytoplankton, and Phragmites australis] in a coastal bay system, Yeoja Bay of Korea, to test the hypothesis that the transfer of intertidal MPB-derived organic carbon to the subtidal food web can be mediated by motile consumers. Compared to a narrow δ13C range (-18 to -16‰) of offshore consumers, a broad δ13C range (-18 to -12‰) of both intertidal and subtidal consumers indicated that 13C-enriched sources of organic matter are an important trophic source to coastal consumers. In the intertidal areas, δ13C of most consumers overlapped with or was 13C-enriched relative to MPB. Despite the scarcity of MPB in the subtidal, highly motile consumers in subtidal habitat had nearly identical δ13C range with many intertidal foragers (including crustaceans and fish), overlapping with the range of MP...
Journal of Fish Biology, 2022
Seasonal differences in the availability of resources potentially result in the food web architecture also varying through time. Stable isotope analyses are a logistically simple but powerful tool for inferring trophic interactions and food web structure, but relatively few studies quantify seasonal variations in the food web structure or nutrient flux across multiple trophic levels. We determined the temporal dynamics in stable isotope compositions (carbon, nitrogen and sulphur) of a fish community from a highly seasonal, temperate estuary sampled monthly over a full annual cycle. Sulphur isotope values in fish tissues discriminated among consumers