Importance of zooplankton for the persistence of a deep chlorophyll layer: A limnocorral experiment

Abstract

A variety of interacting physical, chemical, and biological hypotheses have been proposed to explain the formation of deep chlorophyll layers (DCL). We used an experiment to test the importance of zooplankton grazing and nutrient transport as factors maintaining the DCL. In oligotrophic Yellow Belly Lake (Sawtooth Mountains, central Idaho), which has a DCL, we compared changes in the chlorophyll profiles in 17-m-deep limnocorrals with and without crustacean zooplankton.15 N ammonia and rhodamine dye were added to the epilimnion or metalimnion of the corrals to measure nutrient transport and diffusivity. In the limnocorrals with zooplankton, epilimnetic macrozooplankton biomass was 2ϫ higher and estimated grazing rates were 1.8ϫ higher than those in the metalimnion. After 11 d, chlorophyll levels in the zooplankton treatment declined 72% in the epilimnion but only 53% in the metalimnion, leading to the maintenance of the DCL. In the treatment without zooplankton, the epilimnetic chlorophyll increased 11% and the metalimnetic algal levels decreased 41%, resulting in the formation of an epilimnetic chlorophyll maxima. Biologically mediated movement of 15 N from the epilimnion and metalimnion was downward, into either the metalimnion or the hypolimnion. Turbulent movement measured with rhodamine was high in the limnocorrals, and presumably 15 N also moved into adjoining strata through this process. Grazing, however, coupled with a downward movement of nutrients via sedimentation into the lower strata appears to explain the persistence of the DCL.

The Great Salt Lake's monimolimnion and its importance for mercury bioaccumulation in brine shrimp (Artemia franciscana)

“…Atmospheric deposition of Hg to the Great Salt Lake is only moderately high (Peterson and Gustin 2008). However, algal sedimentation, combined with intensive Artemia grazing (Wurtsbaugh 1992) and defecation should rapidly transfer POC with Hg to the deep brine layer (Pilati and Wurtsbaugh 2003). The density of most algae (Reynolds 1997) is less than that of the very dense deep brine water (Naftz et al 2011), so these particles would normally not reach the sediments.…”

Physicochemistry and zooplankton of two karstic sinkholes in the Yucatan Peninsula, Mexico

“…The higher temperatures at superficial layers have been shown to benefit zooplankton growth and reproductive maturity (Lampert and Sommer, 2007); this factor and the basic values in pH seem to favour the abundance of dominant rotifers in the meso-eutrophic lake and the indicators of biodiversity (specially richness) in the oligotrophic lake. Additionally, higher concentration of chlorophyll-a favours the growth and survival rates of zooplankton (Pilati and Wurtsbaugh, 2003) and generates lower ionic content (Kalff, 2002): these factors seem to be related to the greater richness and diversity in the oligotrophic lake.…”

Implications of nitrogen‐rich glacial meltwater for phytoplankton diversity and productivity in alpine lakes

“…Avoidance of ultraviolet radiation has been proposed as a mechanism behind the development of the deep chlorophyll maximum in alpine lakes (Villafane et al 1999) but has been dismissed by others (Vinebrooke and Leavitt 1999). Others have suggested that deep chlorophyll maximum development is driven by the availability of nutrients below the thermocline (Moll and Stoermer 1982) or a combination of zooplankton grazing coupled with the downward movement of nutrients (Pilati and Wurtsbaugh 2003). Saros et al (2005a) demonstrated that, after nutrient depletion occurs in the epilimnion of alpine lakes, the deep chlorophyll maximum develops where nutrients are available in deeper waters as long as PAR is sufficient (i.e., 1% of surface irradiance).…”