Gut Pigment Content Research Articles

Diel Feeding Rhythm and Grazing Selectivity of Small-Sized Copepods in a Subtropical Embayment, the Northern South China Sea

Small marine copepods are key components of the pelagic food webs in Chinese coastal waters, but very few studies have addressed their trophodynamics, with even fewer studies addressing their diel feeding rhythms. In this study, the diel feeding rhythm and grazing selectivity of the copepod assemblage in Daya Bay during September 30 to October 2, 2014, were studied based on gut pigment analysis. Small copepods (body length < 1.5 mm) including Paracalanus parvus, Temora turbinata, Acrocalanus gibber, Temora stylifera, Euterpe acutifrons, and Acrocalanus gracilis, accounted for 73.9–100% of the total copepod abundance. The copepod assemblage generally exhibited a diurnal feeding pattern, characterized by a higher gut pigment content and ingestion rate during the daytime, consistent with variation in the ambient Chl α concentration. Fifty-five percent of the phytoplankton standing stock per day was consumed by the copepod assemblage, wherein diatoms, prymnesiophytes, and cyanobacteria were the main prey items with average contributions of 19.4–32.9% to the gut pigment contents. The copepod assemblage showed a strong feeding preference for prymnesiophytes, a weak feeding preference for diatoms, and avoidance of cyanobacteria. These results suggest a strong top-down control on phytoplankton community, especially on small groups from small copepods in the Daya Bay ecosystem.

Implications of Pyrosoma atlanticum range expansion on phytoplankton standing stocks in the Northern California Current

Coinciding with the marine heat wave in 2014 and a strong El Niño in 2016, blooms of the tropical-temperate pelagic colonial tunicate, Pyrosoma atlanticum, expanded into the Northern California Current (NCC). Given the high densities of P. atlanticum colonies observed in the NCC, and their potential for high grazing rates, we hypothesized that the species could have a substantial negative effect on the phytoplankton standing stock in the Northeastern Pacific Ocean. In the late winter, spring and early autumn of 2018, we measured abundances and length frequencies of P. atlanticum colonies off the coast of Oregon and estimated their spring grazing impact on the local phytoplankton standing stock. Abundances of P. atlanticum peaked in late winter (0.7 colonies m−3), dropped off in spring (0.42 colonies m−3), and declined to near-absent in early autumn (0.07 colonies m−3). We observed a doubling in the median size of P. atlanticum colonies from late winter to spring, followed by a decline in size in early autumn. With their high individual colony gut pigment contents and bloom densities, the overall spring grazing impact of P. atlanticum was substantial, reaching a maximum of 22% of the available phytoplankton standing stock.

Data on distribution, demographic structure and grazing of the dominant mesozooplankton species in the Yenisei estuary and adjacent shelf in early summer.

The data article refers to the paper “Distribution and grazing of the dominant mesozooplankton species in the Yenisei estuary and adjacent shelf in early summer (July 2016)” (Drits et al., 2020). The data were collected along quasi-longitudinal transect “Yenisei estuary – Kara Sea shelf” on 24–28 July 2016. Here we present data on the spatial and vertical distribution, demographic structure and gut pigment content of the dominant zooplankton species as well as the grazing impact on autotrophic phytoplankton in the three distinguished zones: freshwater zone, frontal zone of the Yenisei plume and marine shelf zone. The related article (Drits et al., 2020) considers the structure and functioning of zooplankton community in relation to environmental characteristics such as temperature, salinity, phytoplankton abundance, timing of ice retreat. Information presented in this article can be used by marine biologists for studies of structure and functioning of estuarine pelagic communities, ecology of zooplankton in the Siberian seas. Besides the data could provide a baseline for the assessment of the ecological role played by climate change events (e.g., increased precipitation, permafrost thawing, elevated river discharge) on the Arctic ecosystems.

Mesozooplankton connect the microbial food web to higher trophic levels and vertical export in the New Zealand Subtropical Convergence Zone

The Subtropical Convergence Zone (SCZ) in the Chatham Rise region east of New Zealand supports productive fisheries and benthic ecosystems and is an important CO2 sink. The SCZ trophic system is underpinned by a microbially-dominated food web, requiring an efficient link between pico-sized producers and larger consumers to support the necessary trophic transfers. However, the role of mesozooplankton (>200 μm body size) as intermediaries in this trophic transfer is not well understood. This study investigated the trophic role of mesozooplankton in subantarctic (SA) and subtropical (ST) waters of the SCZ, south and north of the Chatham Rise, in summer 2000. Mean summer biomasses of mesozooplankton were low in SA and ST waters (181 and 137 mg dry weight m−2, respectively) relative to other seasons. Mesozooplankton biomass was dominated by copepod nauplii, copepodites and small adults (Oithona, Calocalanus, Clausocalanus) and pre-adult euphausiids, while pelagic tunicates (salps and pyrosomes) were rare. Copepod gut pigment contents showed high weight-specific phytoplankton grazing, and feeding incubations showed rapid consumption of ciliates by all sizes of copepods, indicating omnivory as an important trophic mode. Irrespective, consumption by mesozooplankton had only minor impacts on water column phytoplankton and microzooplankton biomass, and production, and nutrient regeneration. Linear Inverse Ecosystem Models (LIEMs) were used to investigate the overall role of mesozooplankton in carbon (C) cycling among autotrophs, heterotrophs, and export fluxes, based on measured plankton rates and biomasses from summer 2000 in SA and ST waters. LIEMs indicated a dominant role for protistan zooplankton (ciliates and heterotrophic nanoflagellates) in consumption of primary biomass and production (71–97%) and a strong protistan – mesozooplankton trophic link. The summer mesozooplankton were shown to be important in transferring energy from the microbially-dominated food web to sinking fluxes and to higher trophic levels (~12 mg C m−2 d−1 to each), the latter equivalent to ~0.42 of mesozooplankton C consumed by higher trophic levels annually in the Chatham Rise SCZ.

Sex-specific grazing dynamics of the recently described copepod, Lovenula raynerae, in an ephemeral pond in the Eastern Cape province of South Africa

This study investigated the temporal changes in the sex specific ingestion rates of the recently described calanoid copepod, Lovenula raynerae, which numerically and by biomass dominates the plankton community in ephemeral ponds in the Eastern Cape province of South Africa. Ingestion rates of the copepod were investigated on two occasions, one week after emergence and two weeks thereafter, employing the gut fluorescent technique. Results of the study indicate the absence of any diel patterns in feeding for both sexes on both sampling occasions (p > 0.05 in both cases). Moreover, there was no significant differences in gut pigment contents between adult male and females during either of the surveys (p > 0.05). Daily rations of the adult copepods during the two studies were equivalent to <2% (range 0.8 to 1.8%) body carbon per day suggesting that they are consuming alternative carbon sources to meet their daily carbon requirements. The main findings of this study suggest that L. raynerae can be considered as a generalist omnivore.

Food availability on the shore: Linking epilithic and planktonic microalgae to the food ingested by two intertidal gastropods

Research on the interaction of primary producers and consumers is crucial for understanding trophic transfer in intertidal food webs. This study explores the association between epilithic and planktonic microalgae, and gut contents of two targeted intertidal gastropods, the periwinkle Echinolittorina radiata (splash zone) and the limpet Cellana toreuma (mid-intertidal zone). With the application of gut fluorescence technique and metabarcoding, this study investigates the quantity and composition of two different sources of microalgae (epilithic and planktonic) and the food ingested by the gastropods. The results suggest the following findings: 1) The planktonic microalgae have higher compositional similarity to the gut contents of grazing gastropods. 2) Increased gut pigment content in C. toreuma is observed with increasing abundance of epilithic and planktonic microalgae. However, there was no such pattern observed for E. radiata. This difference could be attributed to potentially divergent foraging behaviours of the two species that inhabit different shore heights.

Diel changes of food sources and their contributions to nutrition of Orientomysis mitsukurii in a sandy shore environment

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 26:229-244 (2017) - DOI: https://doi.org/10.3354/ab00690 Diel changes of food sources and their contributions to nutrition of Orientomysis mitsukurii in a sandy shore environment Kazutaka Takahashi1,*, Akira Kuwata2, Takeo Suzuki3, Tatsuki Toda3, Keiichiro Ide2 1Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan 2Tohoku National Fisheries Research Institute, 3-27-5 Shinhama-cho, Shiogama, 985-0001, Japan 3Faculty of Engineering, Soka University, 1-236 Tangi-cho, Hachioji, Tokyo 192-8577, Japan *Corresponding author: akazutak@mail.ecc.u-tokyo.ac.jp ABSTRACT: Diel changes in feeding habits and the relative contributions of daytime and nighttime diets to the nutrition of the mysid Orientomysis mitsukurii in a sandy shore environment were determined by field sampling and additional laboratory experiments. O. mitsukurii was distributed just above the bottom during the daytime but a subpopulation swam up into the water column at night. Nighttime swimming activity was prevalent in small individuals but progressively decreased at larger body sizes, while the gut pigment contents of all mysid size classes consistently increased at night. Stomach content analysis revealed that O. mitsukurii individuals of all size groups ingested sedimented particulate organic matter (POM) containing large quantities of clay minerals during the daytime, whereas they mainly foraged on planktonic diatoms in the water column at night. Estimated in situ algal ingestion rates at night accounted for approximately 80-95% of the daily carbon assimilation. O. mitsukurii also exhibited high efficiency in grazing on planktonic diatoms in laboratory experiments. Conversely, daytime sedimented POM carbon accounted for only a minor proportion (5-20%) of the daily carbon assimilation. The utilisation patterns of the different food sources were confirmed by carbon and nitrogen stable isotopic signatures in the benthic community. Our results suggest that efficient utilisation of primary producers, such as planktonic diatoms, by O. mitsukurii is beneficial for maintaining the species’ high abundance from an ecological transfer efficiency perspective, and that sedimented POM is an alternative food source for securing the basic energy required for their survival, particularly when phytoplankton availability is low. KEY WORDS: Mysidae · Omnivore · Nighttime feeding · Vertical migration · Sandy shore environment · Detritus · Diatoms Full text in pdf format PreviousCite this article as: Takahashi K, Kuwata A, Suzuki T, Toda T, Ide K (2017) Diel changes of food sources and their contributions to nutrition of Orientomysis mitsukurii in a sandy shore environment. Aquat Biol 26:229-244. https://doi.org/10.3354/ab00690 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 26. Online publication date: December 04, 2017 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2017 Inter-Research.

Feeding, egg production, and respiration rate of pteropods Limacina in Arctic seas

The feeding, egg production, and respiration rate of the dominant pteropod Limacina helicina have been studied in Russia’s Arctic seas. The sinking rates of fecal pellets and dead individuals have been measured to estimate their role in vertical carbon flux. As has been shown, the rate of ecophysiological processes taking place in the pteropods is higher than that of copepods, the main consumers of phytoplankton. The gut pigment content in Limacina (3084 ng ind–1 as a maximum) was two orders of magnitude higher than in copepods. The egg production rate in Limacina even without feeding reached 4000 eggs ind–1 versus 350–450 egg ind–1 typical of the dominant copepods even with excess food. A close correlation between the pteropod feeding rate and individual body weight was observed for Limacina rather than a correlation with food concentration. The experimentally estimated sinking rate of Limacina fecal pellets was 270 m day–1, higher than for most copepods. The sinking rate of dead pteropods reaches 2000 m day–1. According to the literature, discarded mucous feeding nets sink at a rate of 80 to 1080 m day–1. Evidently, pteropods play a significant role in biogeochemical cycles by accelerating sedimentation. High rates of all studied processes suggest that Limacina are an important component of plankton communities and play the most important role in trophodynamics at sites of their accumulation.

Temporal variability in copepod gut pigments over the central western continental shelf of India

The Indian Western continental shelf (IWCS) is amongst the most productive regions of the world, being noteworthy for upwelling (south-west monsoon) and downwelling (north-east monsoon) that tunes the water biogeochemistry. The present study provides baseline information on temporal variation of in situ copepod gut pigments from IWCS. The copepods were collected between November 2011 and October 2013 and gut pigment contents and composition were estimated using the gut fluorescence method. Results revealed that copepods procured high gut pigment content in monsoon that coincided with ambient water pigment credited to discrete upwelling. Fluorometric analyses of copepod orders revealed presence of gut chlorophyll a (Chl a) throughout the study with highest gut Chl a (0.31 ± 0.25 ng copepod−1; N = 21) and total gut pigments (2.01 ± 2.15 ng copepod−1; N = 21) recorded in Calanoida. Consecutively, Calanoida and Poecilostomatoida chiefly consumed autotrophic biomass that was evident from presence of canthaxanthin and astaxanthin as dominant gut pigments. Interestingly, the marker pigment of Cryptophyceae was present only in Calanoida during monsoon and post-monsoon. Collectively these results conclude that copepods predominantly showed omnivory with discrete temporal variability by grazing upon autotrophic biomass that in turn probably supports the fishery.

Feeding dynamics of the fiddler crab (Uca annulipes) in a non-tidal mangrove forest

To investigate the lack of tidal influence on the feeding dynamics of fiddler crabs, we used an in situ gut fluorescence technique to measure gut pigment content of Uca annulipes in the non-tidal mangrove habitat of the St Lucia Estuary. Measurements were taken over a 24-h cycle and in the two extreme seasons, austral summer and winter, to examine any diel and seasonal shifts in feeding. Three hour gut evacuation experiments were conducted to determine the gut evacuation rate and potential sexual differences in feeding. It was found that under lack of tidal fluctuations, U. annulipes feeding is influenced by diel rhythms. In summer, males displayed a bimodal pattern of feeding, becoming more active in the morning and late afternoon with a gut evacuation rate of 0.795h–1, whereas females remained generally inactive and displayed short bouts of feeding during the day with a gut evacuation rate of 0.322h–1. The summer grazing impact of U. annulipes on microphytobenthos was higher compared with winter. In winter both sexes were fairly inactive, but displayed a greater consumption efficiency (65% compared with 45% in summer). U. annulipes feeding dynamics in a non-tidal habitat are shown to vary seasonally, daily and among sexes.

Mummichog Fundulus heteroclitus responses to long-term, whole-ecosystem nutrient enrichment

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 492:211-222 (2013) - DOI: https://doi.org/10.3354/meps10495 Mummichog Fundulus heteroclitus responses to long-term, whole-ecosystem nutrient enrichment Konner C. Lockfield1, John W. Fleeger1,*, Linda A. Deegan2 1Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, USA 2The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA *Corresponding author. Email: zoflee@lsu.edu ABSTRACT: The effects of eutrophication on coastal plants and sessile animals are becoming well known, but responses of mobile species are less well studied. Here, we link variation in abundance, biomass, body size, growth rate, and resource utilization in mummichogs Fundulus heteroclitus >40 mm in length to experimental nutrient enrichment in Plum Island Sound, Massachusetts, USA. To mimic cultural eutrophication, dissolved fertilizer was released into replicate saltmarsh creeks on each rising tide throughout entire growing seasons. In the summer of the sixth year of enrichment, we released coded-wire tagged mummichogs into nutrient-enriched (n = 3733 fish) and reference (n = 3894 fish) creeks and recaptured them over the next 2 mo. We found increased abundance (by 37%), biomass (58%), body size (8%), and herbivory (115%, measured as photosynthetic gut pigment content) in nutrient-enriched creeks, although body condition was unaffected. However, individual growth rates were 43% lower in nutrient-enriched creeks. Nutrient enrichment stimulated primary production, causing a bottom-up enrichment of the food web which fostered increased biomass and body size. However, the reduction in growth rate indicates an adverse consequence of long-term nutrient enrichment. This negative effect occurred in the absence of increased hypoxia in these highly tidally (4 m amplitude) flushed study creeks. The mummichog is an important predator/grazer in salt marshes, and nutrient-induced alterations in biomass or resource utilization will directly or indirectly affect lower trophic levels, including benthic algae, thereby impacting the ecosystem-wide response to eutrophication. KEY WORDS: Fundulus heteroclitus · Mark and recapture · Eutrophication · Decimal coded-wire tags · Growth rate · Salt marsh Full text in pdf format PreviousNextCite this article as: Lockfield KC, Fleeger JW, Deegan LA (2013) Mummichog Fundulus heteroclitus responses to long-term, whole-ecosystem nutrient enrichment. Mar Ecol Prog Ser 492:211-222. https://doi.org/10.3354/meps10495 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 492. Online publication date: October 31, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

The effect of trophic state and depth on periphytic nematode communities in lakes

The aim of this study was to investigate whether nematode community patterns are shaped by nutrient and light availability. Accordingly, nematode communities inhabiting periphyton were investigated at gradual water depths (50, 150 and 250 cm) in three Swedish lakes showing graded trophic states. It was hypothesized that: (1) nematode density correlates positively with increasing nutrient availability and negatively with increasing depth; (2) increasing nutrient availability favors species and feeding type richness; (3) increasing depth favors deposit-feeders; and (4) differences in the algal composition of the periphyton affect the diet of algal-feeders. Our results showed that the biomass of periphytic algae increased with nutrient availability and decreased with increasing depth. Nematode density also increased with increasing trophic state. Species richness decreased with increasing depth in the investigated oligotrophic lake, while the opposite pattern was determined in the other two lakes. Lake trophic state significantly affected the trophic structure of the nematode community, with more algal-feeders occurring in the eutrophic lake whereas chewers were found only in the meso- and eutrophic lakes. Water depth was also shown to influence nematode feeding type structure, as in all lakes the abundance of deposit-feeders was greater at increasing depth. While diatoms dominated the periphytic algal community at all lakes and depths, analyses of the gut pigment content of nematodes showed that their diet shifted toward green algae in the oligotrophic lake and in shallow zones of the mesotrophic lake.

Response of copepod grazing and reproduction to different taxa of spring bloom phytoplankton in the Southern Yellow Sea

The responses of copepod grazing and reproduction to the spring phytoplankton bloom were studied in the temperate shelf water of the Southern Yellow Sea in March–April, 2009. Two different algal blooms were found during the cruises. A diatom-dominated bloom at Station Z11, and a dinoflagellate-dominated bloom at Station Z4. The gut pigment contents indicated that different sized copepods exhibited different responses to different-species phytoplankton blooms. Large copepods (LC: body size larger than 1000μm) and medium copepods (MC: body size ranging from 500 to 1000μm), grazed actively on diatom blooms, but inactively on dinoflagellate blooms, although the chlorophyll-a concentrations of dinoflagellate blooms were twice as high as than those of the diatom blooms. For small copepods (SC: body size smaller than 500μm), however, there was no significant difference in gut pigment contents between the two different algal blooms. Among the three size groups, LCs were the major grazers on the diatom bloom, while SCs were major grazers on the dinoflagellate bloom. Grazing impacts of copepod assemblages on phytoplankton blooms were low, only being equivalent to 1%day−1, or less, of the chlorophyll-a standing stock. The egg production rates of a large copepod, Calanus sinicus, were on average, 11.3eggind.−1day−1, which was among the higher levels recorded in the study area, especially at the two stations where phytoplankton was blooming (21.8 and 14.9eggind.−1day−1 at Stations Z11 and Z4, respectively). However, C. sinicus could only obtain sufficient food to support this high reproduction from the diatom bloom, but could not if relying only on the apparently unpalatable dinoflagellate bloom. Our analysis of copepod grazing and reproduction suggests that, although the spring blooms do enhance the reproduction of copepods, the taxa changed during spring blooms from large diatoms to small dinoflagellates would change the pathway of primary production. This would restructure secondary-producers (e.g. copepods) community structure, and have important ramifications through various marine trophic levels in the Southern Yellow Sea.

Diel variations in Centropages tenuiremis (Copepoda) feeding, spawning and its relationship with temperature

Diel rhythms in feeding and spawning were investigated in Centropages tenuiremis from Xiamen Bay in March to May, 2006. Circular statistics were used to determine the peak time of spawning. The results showed that the feeding activities of females were stably higher at night-time, and there was a remarkable earlier shift in spawning peak time with warmer seawater. Thus, the lag times between peak times of gut pigment content and spawning were shortened with the increase of temperature. It suggested that there was a direct effect of feeding rhythms on egg production variations in copepods, and the seawater temperature would work on the converting time and then influence the spawning peak time. So the effect of temperature cannot be ignored in the investigation of the effects of feeding on egg production.

Combining gut fluorescence technique and spatial analysis to determine Littorina littorea grazing dynamics in nutrient-enriched and nutrient-unenriched littoral mesocosms

Spatiotemporal distribution patterns in relation to feeding behavior of herbivorous gastropods have been studied extensively, but still knowledge about small-scale patterns is limited in relation to eutrophication. This experimental study aimed to describe the small-scale distribution of Littorina littorea in nutrient-enriched and nutrient-unenriched mesocosms in a merely atidal region and relate the distribution to food abundance and possible competing organisms, while checking simultaneously for feeding activities. The latter part was accomplished through the “gut fluorescence technique” GFT (which, to our knowledge, has not previously been used for benthic grazers) to estimate per capita grazing rates and the former part through monitoring of spatial heterogeneity of L. littorea and co-variation with sessile organisms (using semivariograms and cross-semivariograms, respectively). After 5 months of nutrient addition, the abundance and biomass of L. littorea had increased in enriched systems, which also had significantly higher total biomass of green algae. Gut pigment content was higher in L. littorea from enriched mesocosms, and gut depletion rate was higher in L. littorea from unenriched mesocosms. Spatial analysis showed that L. littorea exhibited generally random patterns (suggesting feeding activities) but sometimes (often in the morning) spatial patchiness (clumped distribution) in both enriched and unenriched conditions. There was mainly positive co-variation between L. littorea and biofilm, while different nutrient conditions exhibited contrasting co-variation between L. littorea and barnacles (positive co-variation in enriched and negative co-variation in unenriched mesocosms). The study offered insights into how feeding behavior and spatial distribution of a species may interact with community components differently under different nutrient regimes. The applied methodology can be useful for purposes of faster examination of grazing effects among different regions and also to compare grazing intensities and interactions between grazers and the benthic communities in disturbed (including pollution and nutrient enrichment) and non-disturbed systems, as well as in up-welling versus non-upwelling areas.

Feeding of biofilm-dwelling nematodes examined using HPLC-analysis of gut pigment contents

The natural feeding behaviour of the nematodes Chromadorina bioculata (Schultze in Carus 1857) and Chromadorina viridis (Linstow 1876) was studied in situ, within epilithic biofilms of the Garonne River (France). Based on their feeding-type characteristics and population dynamics, it was hypothesised that these species feed selectively on microphytobenthos (MPB) within the biofilm, and that among MPB groups, diatoms are preferred. High-performance liquid chromatography (HPLC) was used for separation, identification and quantification of pigments both in nematode guts and in the biofilm. This is the first time that nematode gut pigment contents were examined under natural conditions. Diatoms dominated the MPB which also comprised cyanobacteria and green microalgae. The comparison between chlorophyll a content in nematode guts versus in the biofilm showed that C. bioculata and C. viridis fed opportunistically (non-selectively) on MPB within the biofilm. Only diatom biomarker pigments were found in nematode guts suggesting that they could preferentially fed on diatoms among MPB groups. However, the non-detection of biomarker pigments for other microphyte groups could be also linked to HPLC detection limits. It was estimated that Chromadorina nematodes daily ingested on average 0.03–0.67% of the MPB standing stock. This grazing covered only a small part of their energetic requirements, suggesting that besides MPB they probably also fed on other biofilm food sources. Some considerations on the applicability of the HPLC gut pigment analysis technique for the examination of nematode feeding are also presented.

Salps in the Lazarev Sea, Southern Ocean: I. Feeding dynamics

Feeding dynamics of the Antarctic salps Ihlea racovitzai and Salpa thompsoni were studied in the Lazarev Sea in fall 2004, summer 2005–2006 and winter 2006. Pigment concentrations in the guts of both species were positively correlated with ambient surface chlorophyll a (chl a). No evidence was found for salp clogging even at dense surface concentrations of up to 7 μg chl a L−1. However, gut pigment concentrations had a lower range than ambient pigment concentrations, suggesting that salps increased retention times of ingested material in low-food environments. For medium-sized I. racovitzai and S. thompsoni, estimated individual daily rations reached 7–10 and >100% of body carbon in winter and summer, respectively. Daily respiratory needs of I. racovitzai and S. thompsoni accounted for 28 and 22% of daily carbon assimilation based on pigment ingestion rates in winter, and for 2 and 1% in summer, respectively. The grazing impact of the salp populations on the phytoplankton standing stock was negligible during all seasons due to generally low salp densities. Fatty acid trophic biomarkers in the salps suggest high year-round contributions of flagellates and modest contributions of diatoms to the salp’s diet. These markers showed low seasonal variability for I. racovitzai. The more pronounced seasonality of trophic markers in S. thompsoni were likely related to their generally deeper residence depth in winter linked to a seasonal alternation of sexual and asexual generations.

In vivo determination of Daphnia feeding rates using PAM fluorometry

We examined the utility and sensitivity of PAM fluorometry as a tool for estimatingin vivo gut fluorescence and phytoplankton ingestion rates by Daphnia.KEYWORDS: ingestion; PAM fluorometry; zooplankton; gut pigmentsThe quantity, composition and rates of crustacean zoo-plankton feeding are used to quantify the trophic role ofthese animals in aquatic food webs. The most widelyused field and laboratory methods measure: (i) rate ofchange of the prey field (Gauld, 1951) and (ii) concen-tration of phytoplankton in the zooplankton gut(Mackas and Bohrer, 1976). Each approach has advan-tages and disadvantages (see Ba˚mstedt et al., 2000) andthe choice of method must consider the trade-offbetween information gained and implementationdifficulty.Clearance rate estimations are attractive becauseanimals need not be sacrificed and they can yield infor-mation on selectivity in terms of composition of theprey field. Recent methodological advances haveemployed tools including the FlowCAM (Ide et al.,2008) and the PhytoPAM (Lu¨rling and Verschoor, 2003)to reduce time-consuming microscopic analyses.However, these methods still indirectly measure materialactually ingested and are prone to error associated withovercrowding and sloppy feeding. On the other hand,direct measures of gut contents such as the gut fluor-escence technique are practical and simple: ingestionrate is calculated as the product of gut transit time(GTT) and content. Ingested phytoplankton biomass isestimated with solvent extraction followed by fluorome-try and GTT estimated as the rate of change of gutcontent for animals incubated in filtered water. Dietcomposition can further be measured using pigment-specific analysis (e.g. HPLC; Kleppel and Peiper, 1984;Quiblier et al., 1994). This approach has been widelyused in marine studies but criticized regarding thecorrect interpretation of the photosynthetic pigmentdegradation products in the gut (Ba˚mstedt et al., 2000).Although the gut fluorescence method is relativelysimple and thus attractive, tissue requirements may behigh (e.g. HPLC analyses; Kleppel and Peiper, 1984).Furthermore, solvent treatment is prohibitive when theobjective is to relate feeding to physiological statemeasurements (e.g. enzyme activity). An ideal methodwould permit accurate repeated measurement offeeding activity on the same individuals (or subset) asused for physiological measurements. Recent studieshave pursued non-destructive gut fluorescence measure-ments and demonstrate that in vivo (as opposed toextracted) measures of gut fluorescence represent poten-tially rapid and reliable estimates. Such methods havebeen used to estimate gut pigment content and GTTon

Composition and structure of macrozooplankton and micronekton communities in the vicinity of free-drifting Antarctic icebergs

Recent warming in the Antarctic has led to increased production of icebergs; however, the ecological effects of icebergs on pelagic communities within the Southern Ocean have not been well-studied. We used a 10 m 2 MOCNESS to collect macrozooplankton and micronekton in the upper 300 m of the water column near free-drifting icebergs in the Atlantic sector of the Southern Ocean during three seasons: December 2005 (late spring), June 2008 (late fall) and March-April 2009 (late summer). Communities were dominated in all three seasons by Antarctic krill ( Euphausia superba) and salps ( Salpa thompsoni), which collectively comprised 60-95% of the community wet biomass in most cases. During our spring and summer cruises, mean biomass was elevated by 3.1-4.3x at a distance of 0.37 km from large icebergs vs. 9.26 km away. These differences were not statistically significant, and no trend in biomass with distance was apparent in samples from fall 2008, when total biomass was an order of magnitude lower. Biomass levels near icebergs during Dec 2005 and Mar-Apr 2009 were comparable to values reported from marginal ice zones, suggesting that waters around icebergs support macrozooplankton and micronekton communities comparable in magnitude to those in some of the most productive areas of the Southern Ocean. Sample variance also was significantly higher within 1.85 km of icebergs during Dec 2005 and Mar-Apr 2009, reflecting increased patchiness on scales sampled by the MOCNESS (20–40×10 3 m 3 filtered per sample). This pattern was not significant during Jun 2008. Large predatory medusae were observed within 1.85 km of icebergs and in Iceberg Alley, an area through which icebergs pass frequently, but were virtually absent in areas remote from icebergs. Small euphausiids showed an inverse distribution, with low densities in areas populated by large medusae. A shift in community composition from a near-iceberg assemblage dominated by herbivores to a carnivore-dominated community in Iceberg Alley may reflect a transition from bottom-up to top-down control with increasing distance and time. Body sizes of dominant species varied seasonally but did not show consistent trends with distance from icebergs. Concentrations of photosynthetic pigments in the guts of E. superba and S. thompsoni corresponded broadly to patterns in surface chlorophyll a concentrations and were comparable to maximum gut pigment concentrations measured in animals collected from highly productive marginal ice zones. Our results suggest that the macrozooplankton and micronekton assemblages near free-drifting icebergs can be quantitatively and qualitatively different from those in surrounding, iceberg-free waters, perhaps due to both bottom-up and top-down processes as well as physical forcing by the passage of a large object through the upper ocean.

Feeding dynamics of the invasive gastropod Tarebia granifera in coastal and estuarine lakes of northern KwaZulu-Natal, South Africa

Gut fluorescence and carbon budget techniques were applied to Tarebia granifera (shell height 10–12 mm) at the iSimangaliso Wetland Park, a UNESCO World Heritage Site. This snail has recently invaded a number of estuaries in northern KwaZulu-Natal, where it reaches densities of over 1000 ind. m −2 and becomes a dominant component of the benthic community. Its rapid establishment and spread have raised concerns about potential top-down impacts on the ecosystem. This study shows that T. granifera can utilize large amounts of microphytobenthos (MPB) in addition to detritus. In situ total available MPB pigment concentrations ranged from 11.6 to 110.5 mg pigm. m −2. T. granifera’s gut pigment content ranged from 54 to 1672 μg pigm. ind −1. Gut evacuation rates ( k) ranged from 0.36 to 0.62 h −1 ( R 2 range: 16.2–35.2, P < 0.05). Individual ingestion rates ranged from 6.6 to 30.4 μg pigm. ind. −1 d −1. T. granifera was estimated to consume from 0.5 to 35% of the total available MPB biomass per day, or 1.2–68% of the daily primary benthic production. The carbon component estimated from the gut fluorescence technique contributed 8.7–40.9% of the total gut organic carbon content. The average carbon daily ration contributed by microalgal biomass was ≈16% body carbon per day. Variability in the data was attributed to the complex feeding history of snails. Further studies are needed to validate these results and provide more information on the ecological impact of T. granifera on this wetland and other similar invaded ecosystems, both estuarine and freshwater.