Deep chlorophyll maxima across a trophic state gradient: A case study in the Laurentian Great Lakes

Abstract

Deep chlorophyll maxima (DCM) are common in stratified lakes and oceans, and phytoplankton growth within DCM often contributes significantly to total system production. Theory suggests that properties of DCM should be predictable by trophic state, with DCM becoming deeper, broader, and less productive with greater oligotrophy. However, rigorous tests of these expectations are lacking in freshwater systems. We use data generated by the U.S. EPA from 1996 to 2017, including in situ profile data for temperature, photosynthetically active radiation (PAR), chlorophyll, beam attenuation (c p), and dissolved oxygen (DO), to investigate patterns in DCM across lakes and over time. We consider trophic state, 1% PAR depth (z 1%), thermal structure, and degree of photoacclimation as potential drivers of DCM characteristics. DCM depth and thickness generally increased while DCM chlorophyll concentration decreased with decreasing trophic state index (greater oligotrophy). The z 1% was a stronger predictor of DCM depth than thermal structure. DCM in meso-oligotrophic waters were closely aligned with maxima in c p and DO saturation, suggesting they are autotrophically productive. However, the depths of these maxima diverged in ultra-oligotrophic waters, with DCM occurring deepest. This is likely a consequence of photoacclimation in high-transparency waters, where c p can be a better proxy for phytoplankton biomass than chlorophyll. Our results are generally consistent with expectations from DCM theory, but they also identify specific gaps in our understanding of DCM in lakes, including the causes of multiple DCM, the importance of nutriclines, and the processes forming DCM at higher light levels than expected.

Deep Chlorophyll Maxima in the Global Ocean: Occurrences, Drivers and Characteristics

“…The interface between the two layers generally sets the environmental conditions favorable for the DCM to develop and occasionally be maintained (Beckmann & Hense, 2007 ; Cullen, 1982 , 2015 ; Cullen & Eppley, 1981 ; Riley et al., 1949 ; Steele & Yentsch, 1960 ; Venrick et al., 1973 ). DCMs have been extensively studied since the 1970s (see Ardyna et al., 2013 ; Baldry et al., 2020 ; Cullen, 2015 ; Scofield et al., 2020 for recent and/or synoptic assessments, respectively, in the Arctic Ocean, using a global approach, in the North American lakes, and in the Southern Ocean). What primarily emerges from this abundant literature is that DCM features cover a wide range in term of characteristics (depth, magnitude, shape, Cullen, 1982 ; Hense & Beckmann, 2008 ; Lavigne et al., 2015 ; Uitz et al., 2006 ), prevailing phytoplankton community and dominant physiological processes.…”

Did the COVID-19 Lockdown-Induced Hydrological Residence Time Intensify the Primary Productivity in Lakes? Observational Results Based on Satellite Remote Sensing

“…While, the hydrological residence time is not the only factor that influence the primary productivity, other factors such as temperature, increased light exposure, oxygen level, initial nutrients levels, etc., also could cause an increase or decrease in the level of Chl-a concentration in the aquatic environments [12][13][14]. For instance, Castelao et al [15] using geostationary satellite data showed the seasonal development of coastal upwelling in which the maximum was peaked in the summer and minimum during the winter. In oligotrophic waters, maximum phytoplankton production often occurs near the top of a nutricline [16].…”

Towards a quantitative reconstruction of lake trophic state in temperate lakes using subfossil cladocera and diatoms: Composition of a training set from NE Poland

“…Interestingly, in 10 stratified lakes the maximum Chl-a values occurred in meta-or hypolimnion indicating forming of deep chlorophyll maxima (DCM) or deep chlorophyll layer (DCL). The primary production within DCM may contribute to large amount (~60%) of total algal production in the lake (Camacho, 2006;Scofield, Watkins, Osantowski, & Rudstam, 2020). The mechanisms responsible for DCL formation are various (Camacho, 2006;Scofield, 2020), but its depth increase with water transparency (Scofield, 2020).…”