Organic matter and pigments in surface sediments: possible mechanisms of their horizontal distributions in a stratified lake

Abstract

The analysis of spatial changes of organic matter and pigments (chlorophyll a, chlorophyll b, bacteriochlorophyll e, and phaeophytin a) in the uppermost bottom sediments was used to distinguish between sources of settled material in Lake Kinneret (Israel) during thermal stratification. The proportion of phytobenthic chlorophyll a decreased relatively to total chlorophyll a down to zero at 17 m. Bacteriochlorophyll e, derived from Chlorobium phaeobacteroides, was the most abundant pigment below the thermocline. Organic matter content in the sand-free sediment fraction and chlorophyll a to organic matter ratio were the lowest in the bottom area most extensively affected by shoaling internal seiches and where resuspension occurred. The proportion of resuspended matter sharply decreased with bottom depth in the hypolimnion. The importance of degraded materials of planktonic origin apparently increased towards the lake center. These distribution patterns could be achieved by dispersion of the rapidly sinking resuspended particles with fast metalimnetic jets and by focusing of the lighter planktonic particles by water motions in the turbulent benthic boundary layer. Our study showed that analysis of distribution of freshly settled organic components on the bottom may be a useful tool for understanding particle transport mechanisms in lakes.Résumé : L'analyse des variations spatiales de la matière organique et des pigments (chlorophylle a, chlorophylle b, bactériochlorophylle e et phéophytine a) dans la couche supérieure des sédiments du fond a été utilisée pour distinguer les sources des matières déposées dans le lac Kinneret (Israël) durant la stratification thermique. La proportion de chlorophylle a phytobenthique diminuait relativement à la chlorophylle a totale pour devenir nulle à 17 m. La bactériochlorophylle e, issue de Chlorobium phaeobacteroides, était le pigment le plus abondant sous la thermocline. La teneur en matière organique dans la fraction de sédiments exempte de sable et le ratio chlorophylle a à matière organique étaient les plus faibles dans la région du fond la plus touchée par les seiches internes qui balayent les hauts fonds et là où se produisait une remise en suspension. La proportion de matière remise en suspension diminuait abruptement en raison directe de la profondeur du fond dans l'hypolimnion. La quantité de matières dégradées d'origine planctonique s'accroissait apparemment vers le centre du lac. Ces profils de distribution pourraient être attribuables à la dispersion par des jets métalimnétiques rapides des particules remises en suspension qui descendent rapidement et à la concentration des particules planctoniques plus légères par les mouvements de l'eau dans la couche limite benthique turbulente. Notre étude a montré que la distribution des composantes organiques fraîchement déposées sur le fond peut aider à comprendre les mécanismes de transport des particules dans les lacs.[Traduit par la Rédaction] Ostovsky and Yacobi 1010 Fig. 7. Hypothetical scheme of bottom sediment formation in...

Exchange between littoral and pelagic waters in a stratified lake due to wind-induced motions: Lake Kinneret, Israel

“…Two major fractions of suspended material form the settling community. These are material entrained by floods of the Jordan River and autochthonous material due to the high productivity of the lake (Ostrovsky & Yacobi, 1999). Sediments in the northern and western part of the lake are richer in fine-grained detrital material (2-20 lm) because the main input is from the Jordan River (Serruya, 1971) and certainly constitutes a readily available source to the turbidity layer.…”

“…It is also interesting to note the spatial variability in the bottom friction velocities, which will result in localized resuspension of sediments from the bottom back into the water column. Ostrovsky & Yacobi (1999) E Fig. 17 Simulated residual currents in the surface layer obtained using a spatially uniform wind field as measured at Station T3 and applied over the entire lake surface.…”

“…organic matter and pigments as tracers found a decrease in the proportion of resuspended material in bottom sediments from the peripheral zone of the hypolimnion towards the lake interior. Ostrovsky & Yacobi (1999) suggested that the particles, once resuspended, may be transported horizontally towards the lake centre by fast-moving metalimnetic jets as shown in Figs. 6f, g and 14a, pumping the mixed inshore waters towards the lake interior.…”

“…The BBL in Lake Kinneret has a significant influence on the timing and location of the biogeochemical processes and may also serve as a catalyst for organic matter degradation, leading to enhanced accumulation of dissolved nutrients (Nishri et al, 2000). It has been suggested that suspended particles and nutrients are transported directly from the BBL into the metalimnion (Imberger, 1994;Ostrovsky and Yacobi, 1999). However, the mechanisms responsible for such an exchange were not evaluated in these studies.…”

Sedimentation of photosynthetic pigments during the bloom of the green sulfur bacterium Chlorobium phaeobacteroides in Lake Kinneret: spatial patterns

“…The increase of OM towards the lake periphery is probably associated with resuspension of settled particles, caused by water motions, and followed by focusing of light organic-rich particles that occurs in the direction from the lake periphery towards the center. These mechanisms were earlier discussed in more detail (Ostrovsky & Yacobi, 1999;Bloesch, 2004). Wind energy applied to the water surface is actually much lower in October than in June-September (Koren & Ostrovsky, 2002); therefore, the increase of resuspension in October, implied …”