Downward flux of organic matter and pigments in Lake Kinneret (Israel): Relationships between phytoplankton and the material collected in sediment traps

Sedimentation rates of organic materials and algal pigments were measured by near-bottom traps in Lake Kinneret (Israel) throughout 2006. Pigment composition in the epilimnion and in the traps was compared by trap-to-water indices (TWIs) based on the proportion of a given pigment to the concentration of β-carotene. The appearance of particular pigment in traps was associated with its ability to decompose and be transferred to the trap. The relative lability of pigments increased in the following order: β-carotene> lutein+zeaxanthin> Chlorophyll b> Chlorophyll a> echinenone> fucoxanthin> Chlorophyll c> peridinin. A seasonal variability, pertaining to the thermal stratification in the lake was observed in the TWI; it showed highest values during holomixis, and lowest after an establishment of anoxic conditions in the hypolimnion. Bacteriochlorophyll e was one of the dominant pigments in traps from mid-July to mid-September, when the green sulfur bacterium Chlorobium phaeobacteroides formed dense populations in the metalimnion. Using chlorophyllous pigments as a proxy for biomass, we evaluated the possible contribution of Ch. phaeobacteroides to the total sedimentation flux of all photosynthesizing organisms on an annual basis. The proportion of freshly fixed carbon in the downward flux changed seasonally, depending on the composition of the phytoplankton and thermal stratification, and was on average 22% of primary productivity at the lake centre.