Effects of Fertilization and Planktivorous Fish on Epilimnetic Phosphorus and Phosphorus Sedimentation in Large Enclosures

Abstract

1989. Effects of fertilization and planktivorous fish on epilimnetic phosphorus and phosphorus sedimentation in large enclosures. Can. 1. Fish. Aquat. Sci, 46: 1735-1 742.Enclosure experiments were conducted during 1986 and 1987 to test the hypothesis that predation-induced shifts in plankton size-structure affect epilimnetic total phosphorus (TP) through changes in sedimentation rates, both with and without external nutrient loading. Fertilization and/or planktivorousfish were applied to eight enclosures in a 2 x 2 factorial design. Epilirnnetic TP was higher with fish in the unfertilized enclosures and Bower with fish in the fertilized enclosures; that is, fertilization increased epilimnetic TP only in enclosures without fish, and much less than expected based on the amounts added. The fraction of TP sdirnenting per unit time, and the spring to summer decline in TP, was consistently lower with fish. Sedimentation rates were poorly correlated with epilirnnetic TP, but were more strongly correlated with TP decline rates and particulate phosphorus larger than 20 pm. Presence or absence of planktivorous fish can alter the trophic status of planktonic systems, and the impact of external nutrient loading by changing the size-distribution and biomass of plankton.En 1986 et 1987, des experiences ont 4t6 reali&es en enclos poker verifier i'hypoth2se que les modifications de la structure par taille du plancton dues 2 la predation influent sur le phospkore total (PP) de l'epilimwion en provoquant des changements dans les taux de s6dimentation, qu'il y ait ou non apport externe de substances nutritives. Dans ie cadre d'un plan d1exp6riences factoriel 2 x 2, on a mis des fertilisants et su des poissons planctivores dans huit enclose Le PP de I'epilimnisn 4tait plus eleve quand il y avait du poisson dans les enclos non fertilis4s et plus faible quand il y avait eu du poisssn dans les enclos fertilis6s; autrement dit, la fertilisation a fait accroitre le PT de If6pilimnion seulernent dans les enclos ne contenant pas de psisson et beaucoup rnoins que prevu d'apres les quantit6s ajout6es. La partie de PT qui se depose par unit4 de temps et la baisse de BT sbserv6e du psintemps A Pet6 etaient de faqon constante plus faibles lorsqu'il y avait du poisson. Les taux de sedimentation corr6Ies avec le PP de 1'4pilimnion, mais il I'4taient plus fortement avec les taux de diminution du PT et le phosphote particulaire d'un diarn2tre superieur 2 20 pm. La pr6sence ou ['absence de poissons planctonivores peut modifier le statut trophique des csmmunaut6s planctoniques et les r4percussions de I'apport externe de substances nutritives en changeant la repartition par taille et la bismasse du plancton. R educing phosphoms input has been used successfully to reverse lake eutrophication (VoIIenweider 1968: Edmcsndson 1969 Schindler 1975 Schindler , 1877 Hurley et al. 1986). However, the emphasis on external phosphorus loading as a determinant of the phosphoms content o f lakewaters and eutrophication has obscured the hportmce of food web inter...

Zooplankton effects on vertical particulate flux: Testable models and experimental results

“…Experimental studies are potentially the most powerful approach to determining zooplankton effects, but they are limited to lakes and to simple designs (ϩ/Ϫ macrozooplankton; Uehlinger and Bloesch 1987;Bloesch and Bürgi 1989;Sarnelle 1992). In several studies, zooplankton have been manipulated indirectly by establishing treatments with and without zooplanktivorous fish (Langeland et al 1987;Carpenter et al 1988;Mazumder et al 1989;Reinertsen et al 1990;Vanni et al 1997), an approach that may confound mechanistic interpretation because fish can affect phytoplankton abundance and species composition independently of their effects on zooplankton (Vanni and Layne 1997). Studies that include attempts to correlate sedimentation flux with zooplankton abundance from unmanipulated systems are also rare (Bloesch and Bürgi 1989;Elser et al 1995a).…”

Impact of planktonic food web structure on nutrient retention and loss from a late summer pelagic system in the coastal northern Baltic Sea

“…Lynch & Shapiro 1981, Hessen et al 1986, Horstedt et al 1988, Mazumder et al 1988. Consequently, sedimentary losses of particulate nutrients tend to increase due to the higher sinking velocities of larger cells (Bloesch & Burgi 1989, Mazumder et al 1989 and Increased contribution of zooplankton fecal material to the settling flux (Bloesch & Burgi 1989). Addition of visually feeding planktivorous fish reduces the numbers of larger herbivores, and leads to a biomass increase of nanoplankton, generally favored by zooplankton grazers (Hessen et al 1986, Mazumder et al 1988, Drenner et al 1989, as well as to a subsequent decline of sedimentation rates and retention of nutrients in the pelagic system (Mazumder et al 1992).…”

Benthic-Pelagic Links: Responses of Benthos to Water-Column Nutrient Enrichment

“…Many benthic consumers may also benefit from increases in phytoplankton biomass or production (Davies 1980, Welch et al 1988 by filter feeding or collecting newly sedimented phytoplankton (Jonasson 1972, Johnson et al 1989, Lindegaard 1994. The availability of phytoplankton to benthic consumers is modified by zooplankton grazing; reduced zooplankton biomass may result in increased sedimentation of high quality food to benthic consumers (Kajak 1988, Mazumder et al 1989b). These many potentially strong links between benthic and pelagic communities suggest that it may be misleading to focus only on responses of benthic or pelagic communities to resource perturbations (Threlkeld 1994) without assessing inter-habitat interactions.…”