Role of nearshore benthic algae in the Lake Michigan silica cycle

Abstract

Si cycling is linked with processes from global carbon sequestration to community composition and is especially important in aquatic ecosystems. Lake Michigan has seen dramatic fluctuations in dissolved silica (dSi) over several decades, which have been examined in the context of planktonic processes (diatom blooms), but the role of benthic organisms (macroalgae and their epiphytes) in Si cycling have not been explored. To assess significance of nearshore benthic algae in Si dynamics, we assembled dSi data from an offshore site sampled since the late 1980's, and sampled off three Milwaukee beaches during 2005-19. Using colorimetric assays and alkaline digestion, we measured dSi, biogenic silica in particulate suspended material (pSi) and biogenic silica in benthic macroalgae (Cladophora) and epiphytic diatoms (bSi). Offshore, dSi increased about 1 μM per year from 25 μM in the late 1980's to nearly 40 μM in 2019. Nearshore dSi fluctuated dramatically annually, from near zero to concentrations similar to offshore. Both Cladophora and its epiphytes contained significant bSi, reaching up to 30% of dry mass (300 mg Si g dry mass-1) of the assemblage in summer. Microscopic analyses including localization with a Si-specific-stain and X-ray microanalysis showed bSi in epiphytic diatom cells walls, but the nature and localization of Si in macroalgae remained unclear. A simple model was developed estimating Si demand of algae using the areal macroalgal biomass, growth rates inferred from P-content, and bSi content, and comparing Si demand with dSi available in the water column. This indicated that 7-70% of the dSi in water overlying nearshore benthic algal beds could be removed per day. Key elements of the Si cycle, including which organisms sequester bSi and how rapidly Si is recycled, remain unclear. This work has implications for coastal marine waters where large macroalgal biomass accumulates but bSi content is virtually unknown.

Fig 1. Dissolved silica concentrations (dSi) at Fox Point, Lake Michigan during the periods 1988–1991, 2007–2009 and 2018–9 through the top 60 m of the 100 m water column.

Dots indicate sampling points.

Fig 2. Dissolved silicate, suspended biogenic silica and biogenic silica in Cladophora assemblages in samples collected at 1 m depth at three Milwaukee-area beaches during 2005–2019.

Fig 3

A. Light microscopy image of Cladophora filament with encrusting diatom epiphytes. B. Epifluorescence microscopy image of PDMPO-labeling of bSi (pale green) accumulation in epiphytic diatoms on surfaces of Cladophora filaments. C. Scanning electron microscope (SEM) images of Cladophora filaments with typical dense late-summer diatom epiphyte load. D. SEM elemental mapping of Si (blue dots) on Cladophora and epiphytes—elemental spot analysis over Cocconeis diatom cells yielded signals for C ~0.6 cps(eV) and Si >7.5 cps(eV) compared to Cladophora, C ~0.6 cps(eV), Si <0.1 cps(eV). Scale bars are as marked.