Occurrence and seasonal dynamics of metalimnetic Deep Chlorophyll Maximum (DCM) in a stratified meromictic tropical lake and its implications for zooplankton community distribution

Abstract

Deep Chlorophyll Maxima (DCM) is an important feature of stratified lakes and oceans but very little is known about the phenomenon in tropical systems. Proximate factors accounting for DCM presence include light, thermal stratification and nutrients but biotic interactions such as zooplankton grazing can actively support DCM formation, structure and maintenance. We examined DCM characteristics in tropical Lake Bosumtwi at biweekly intervals between April and July of 2005 and 2006 and also assessed zooplankton herbivore interactions with the DCM. The onset and development of the seasonal DCM peak was based on stable water column, steep thermocline, availability of a nutricline and nutrient limitation in the epilimnion. Euphotic depth doubled during DCM formation and increasing transparency tripled DCM Chl a in the metalimnion just below the oxycline. DCM, however, disaggregated with seasonally induced deeper mixing. The range of variability in surface chlorophyll did not differ significantly between seasonal and aseasonal DCM period. There was about one‐fourth difference between mean surface and metalimnetic chlorophyll concentrations but this shift in resource location did not trigger a descent to deeper waters by herbivores. Herbivore grazing could not be linked to surface variations in water clarity and deep‐water irradiance needed for DCM formation and maintenance. DCM presence therefore was not enhanced by surface water grazing activities of zooplankton. This suppressed biotic influence indicated that DCM dynamics is driven by lake clarity, thermal and nutrient gradients which constitute key abiotic drivers. Direct consumption of the DCM by herbivorous zooplankton was not evident and the energy therein is possibly lost to the anoxic deep waters of the lake through sedimentation. The sinking organic matter could be returned to the pelagic food web via the microbial loop represented by bacteria, ciliates and heterotrophic/mixotrophic flagellates inhabiting the lake's deep layers.

Limited importance of primary production in the deep chlorophyll layer for macro-zooplankton in an oligotrophic karst lake: A whole-lake 15N experiment

“…Based on their 13 C experiment Wilkinson et al (2014) inferred that the DCL contributed only minimally to the crustacean zooplankton diet, and Sanful et al (2017) studying natural populations, reached a similar conclusion. However, Francis et al (2011), using natural-abundance isotopic analyses, concluded that the DCL was a very important food source in oligotrophic lakes.…”

Abiotic Drivers of a Deep Cyanobacteria Layer in a Stratified and Eutrophic Lake

“…(2017) conducted a field study on Lake Ontario and determined significant dependencies of DCM magnitude and location on the temperature profile of a lake; Sanful et al. (2017) demonstrated euphotic depth was the primary driver of DCM formation and maintenance in another field study. Somavilla et al.…”

“…Huisman et al (2006) demonstrated through numerical simulations that there exists a minimum turbulence level in the surface layer of the ocean in order to achieve a stable DCM. Scofield et al (2017) conducted a field study on Lake Ontario and determined significant dependencies of DCM magnitude and location on the temperature profile of a lake; Sanful et al (2017) demonstrated euphotic depth was the primary driver of DCM formation and maintenance in another field study. Somavilla et al (2019) connected deep chlorophyll phenomena to surface blooms in an oceanic environment: field data demonstrated that cyanobacterial biomass was prevalent below the diurnal mixed layer, toward a DCL, when the net heat flux at the water surface became positive on a seasonal timescale.…”

Spatial Variations of Subsurface Chlorophyll Maxima During Thermal Stratification in a Large, Deep Subtropical Reservoir

“…Physical (light availability and mixing dynamics), chemical (nutrient supply), and biological (microbial food web and grazing rates) factors contribute to regulating the nature and variations of the SCM in lakes (Abbott et al, 1984;Cullen, 2015;Leach et al, 2018;Sanful et al, 2017;Sommer et al, 2012). Water clarity and underwater light are regarded as the overarching drivers of SCM variation (Hamilton et al, 2010).…”