An individual-based model of pigment flux in lakes: implications for organic biogeochemistry and paleoecology

Abstract

Vertical fluxes of pigments are used in limnology to monitor phytoplankton abundance, herbivore grazing, ecosystem efficiency, and historical changes in production. However, significant pigment degradation can occur during algal sedimentation. We used an individual-based model of pigment flux to quantify the relative importance of production and degradation as controls of pigment sedimentation. Pigment deposition increased with production, sinking rate, and phytoplankton depth and declined as lake depth and the depth of oxygen penetration increased. Unexpectedly, pigment sedimentation rate was not sensitive to variation in photooxidation rates, even though bleaching accounted for the second greatest amount of pigment loss. Digestion by zooplankton caused the most pigment degradation, but grazing increased pigment deposition when digestive losses were less than those due to oxidation of pigments in ungrazed cells. The model suggests that algal production may be underestimated in sedimentation studies that do not consider variability in water column depth. Further, comparisons with paleoecological analyses suggest that some inferred increases in production during lake ontogeny may arise from changes in regulation of pigment fluxes rather than from increased algal production.Résumé : Les flux verticaux de pigments servent en limnologie à surveiller l'abondance du phytoplancton, le broutage par les herbivores, l'efficience de l'écosystème et les changements historiques dans la production. Toutefois, une dégradation importante des pigments peut se produire pendant la sédimentation des algues. Nous nous sommes servis d'un modèle individualisé des flux de pigments pour quantifier l'importance relative de la production et de la dégradation comme contrôles de la sédimentation des pigments. Le dépôt de pigments augmentait avec la production, le rythme de descente et la profondeur du phytoplancton, et baissait à mesure qu'augmentaient la profondeur du lac et la profondeur de pénétration de l'oxygène. De façon imprévue, le taux de sédimentation des pigments n'était pas sensible aux variations des taux de photo-oxydation, même si la décoloration constituait le deuxième facteur de perte de pigments, le premier étant la digestion par le zooplancton; toutefois, le broutage accroissait le dépôt de pigments quand les pertes digestives étaient inférieures à celles qui étaient dues à l'oxydation dans les cellules non broutées. Le modèle permet de penser que la production algale est sous-estimée dans les études de sédimentation qui ne tiennent pas compte de la variabilité de la profondeur dans la colonne d'eau. En outre, les comparaisons avec les analyses paléoécologiques indiquent que certaines augmentations supposées de la production pendant l'ontogénie des lacs pourraient être causées par des changements dans la régulation des flux de pigments plutôt que par une hausse de la production algale.

Spatial distribution of diatom and pigment sedimentary records in surface sediments in Doubtful Sound, Fiordland, New Zealand

“…Moreover, the mechanisms that affect the flux and deposition of diatom frustules and pigments vary. Algal pigments in lakes are degraded through photo-oxidation, recycling and alteration through digestion (Leavitt 1993;Cuddington & Leavitt 1999;Leavitt & Hodgson 2001). Spatial heterogeneity of fossil pigment concentrations in Doubtful Sound ( Fig.…”

“…Environmental variables that may have improved the model further are biological factors such as grazing pressure and microbial degradation (e.g. Welschmeyer & Lorenzen 1985;Sancetta 1989;Cuddington & Leavitt 1999). Zooplankton grazing is assumed to be minor at inner fjord stations (Goebel 2001) and has not been quantified in the more mixed part of the fjord (i.e.…”

“…Similarly, using spatially explicit sampling of the water column at discrete depths and comparing the profile of fresh algal pigments with sedimentary algal pigments would refine which pigments are more susceptible to degradation in a fjord environment. The differential degradation of plant pigments has been studied in freshwater lakes (Cuddington & Leavitt 1999); however, there are only a few studies that have investigated the lability of pigments in marine environments (Bianchi et al 2002), and no published studies that have quantified pigment degradation across fjord systems. The strong spatial gradients in fjords present a model ecosystem to investigate the influence of abiotic parameters (e.g.…”

“…However, in order to interpret sedimentary records accurately in palaeoecological studies, one needs an understanding of the spatial distribution of sedimentary proxy records in the study area. Spatial variability in diatom (Anderson 1990) and pigment deposition have been well documented for lakes (Cuddington & Leavitt 1999). However, little is known about the spatial distribution of diatom and pigment records in marine sediments and the environmental factors that govern this variation.…”

Can fish introductions alter nutrient cycles in previously fishless high-latitude lakes?

“…The change was mainly related to diatom pigments, which showed an increase, but this may have been due to an increase in the abundance of benthic species, such as S. pinnata, and therefore a further confirmation of the switch to benthic production, which enhances pigment preservation in the sediment (Cuddington and Leavitt, 1999). The increase in alloxanthin could indicate an increase in bacterial production after fish introduction, which stimulated the growth of potentially mixotrophic cryptophytes ('the microbial loop').…”