The community dynamics of epiphytic midge larvae across the pelagic–littoral interface: do animals respond to changes in the abiotic environment?

Abstract

In some littoral systems, abiotic heterogeneity results from the edge of vegetation towards the shore as macrophytes gradually impede mixing between pelagic and littoral water. During the summer of 1994, we investigated whether such abiotic heterogeneity influenced epiphytic midge larvae (Diptera:Chironomidae) inhabiting a stand of Scirpus americanus in Saginaw Bay, Lake Huron. After macrophytes became well established, gradients in six parameters of water quality (turbidity, alkalinity, pH, dissolved oxygen, conductivity, and chloride) formed horizontally from the macrophyte edge towards the shore. In outer portions of the stand where physico-chemical properties were influenced by the influx of pelagic surface waves, chironomid abundance increased to 1076 larvae ⋅ stem -1 . Diversity (H′) in these areas remained constant over the summer with grazing and filter-feeding taxa equally represented. Further into the macrophyte bed where water quality indicated highly reduced mixing, midge abundance never exceeded 27 individuals ⋅ stem -1 and larval biomass was reduced 2-4 fold. Loss of filter-feeding taxa resulted in a dramatic decline in diversity in these areas after formation of the abiotic gradients. This study suggests that pelagic-littoral water exchange may result in environmental heterogeneity that directly or indirectly influences epiphytic invertebrate community structure and function.Résumé : Dans certains systèmes littoraux, il se produit une hétérogénéité abiotique entre la bordure de la végétation et le rivage à mesure que le mélange des eaux pélagiques et des eaux littorales est entravé par la croissance des macrophytes. Durant l'été de 1994, les auteurs ont tenté de savoir si cette hétérogénéité abiotique avait une influence sur les larves épiphytes de moucherons (diptères : Chironomidae) qui s'abritent dans une colonie de Scirpus americanus de la baie Saginaw, dans le lac Huron. Une fois les macrophytes bien établis, des gradients horizontaux se sont formés pour six paramètres de la qualité de l'eau (turbidité, alcalinité, pH, oxygène dissous, conductivité et chlorure) depuis la bordure des macrophytes jusqu'au rivage. Du côté extérieur de la colonie, là où les propriétés physico-chimiques étaient modifiées par l'afflux des vagues de surface du secteur pélagique, l'abondance des chironomidés a augmenté pour atteindre 1 076 larves ⋅ tige -1 . Dans ces secteurs, la diversité (H′) est demeurée constante durant l'été, les espèces brouteuses et les espèces filtreuses étant également représentées. Plus loin dans l'herbier de macrophytes, où la qualité de l'eau indique que le brassage est extrêmement réduit, l'abondance des moucherons n'a jamais dépassé 27 individus ⋅ tige -1 , et la biomasse des larves a été réduite de 2 à 4 fois. La perte d'espèces filtreuses a entraîné une très grave diminution de la diversité à cet endroit après la formation de gradients abiotiques. Les résultats de l'étude semblent indiquer que l'échange entre les eaux littorales et les eaux pélagiques peut entraîner une hétérogénéité e...

Effects of plant community composition and exposure to wave action on invertebrate habitat use of Lake Huron coastal wetlands

“…Energy from waves push pelagic water well into littoral emergent marshes along large lake margins. Previous work has demonstrated that mixing gradients become established in littoral marshes shortly after Scirpus stems emerge above the water surface Suzuki et al 1995;Cardinale 1996;Cardinale et al 1997Cardinale et al , 1998. This mixing gradient includes changes in chemistry, rates of turnover of the water mass, and decreases in turbidity from the outer to inner marsh (Suzuki et al 1995;Cardinale et al 1997Cardinale et al , 1998.…”

“…Invertebrates associated with vegetation increased from 4671 to 44 496 individuals/m 2 from June to July at the outer Scirpus zone and from 3070 to 131 114 individuals/m 2 at the deepwater Scirpus zone. Pelagic water readily penetrated into the outer and deepwater zones during July bringing considerable seston and nutrients with it, but its penetration into the inner zones was dampened by plant stems with little pelagic water reaching the inner and nearshore stations (Suzuki et al 1995;Cardinale et al 1997Cardinale et al , 1998. The reduction of pelagic-water intrusion by plant stems from outer to inner Scirpus zones was not a major factor in June as Scirpus stems were starting to emerge from rhizomes in the sediments and had not yet reached the water surface.…”

“…1999). Our studies in Saginaw Bay marshes have begun to provide such data for Lake Huron coastal marshes Cardinale et al . 1997Cardinale et al .…”

“…Our working hypothesis has been that invertebrate communities are influenced by plant community and substrate type and, directly or indirectly, by exposure to wind, waves and ice scour along with inundation history of the site. Few data have been available to test this hypothesis Cardinale et al . 1997Cardinale et al .…”

“…The degree of mixing of pelagic water into the marsh creates gradients in water chemistry that correlates with periphyton productivity as well as invertebrate and fish-community composition from open water to shore Suzuki et al . 1995;Cardinale et al . 1997Cardinale et al .…”