Three repeated experimental trials were done in summer months. Thirty-two cores (27 cm diameter, 14 cm depth) were collected from 50 m2 area of seagrass bed at 1 m depth on: June 28, July 12 and July 26, 2017 for trials 1-3, respectively. On each date, 16 cores were collected from seagrass habitat in pairs. Another 16 cores were collected from open sediment (OS) habitat. Extracted, paired cores were placed upright into an open-top plastic tub (49 x 33 x 42 cm) to produce eight tubs of each habitat.
Tubs were transported to Dauphin Island Sea Lab (~30-minute drive) filled with seawater (to core depth of 16 cm) pumped from Mobile Bay (20 km, east of site) and arranged in four blocks within an outdoor mesocosm. Each block contained two tubs of each habitat. After two days, a diatom-specific inhibitor (3 µM solution of germanic acid, i.e. Ge treatment) was randomly added to water, i.e. two tubs per block, one of each habitat type. Germanium (Ge) at high Ge/Si ratios (> 0.01) prevents formation of siliceous cell wall (Azam and Chisholm 1976). We added 3 µM solution and allowed two days for Ge incorporation.
Metabolism measurements:
Two days after, we quantified productivity and respiration from changes in oxygen content within 2-3 hour incubations of chambers and bottles following methods in Anton et al. (2009). Oxygen content was measured with a meter (HQ30d, Hach, Loveland, Colorado, USA) and, this was initial oxygen content for both chamber and bottle incubation. After incubation, we measured final oxygen content in bottles and chambers.
To compare rates between treatments, net community production (NCP) and respiration were assessed in mg O2 m–2 h–1 and mg O2 L-1 h-1 for benthic and water-column (WC) communities, respectively. Equations were:
WC NCP = (Fcb – Icb) t-1 (1)
WC respiration = (Fdb – Idb) t-1 (2)
Benthic NCP = [(Fcc – Icc) – (Fcb – Icb)] V t-1 A-1 (3)
Benthic respiration = [(Fdc – Idc) – (Fdb – Idb)] V t-1 A-1 (4)
where capital letters are for initial (I) or final (F) oxygen content (mg L-1) for clear (c) and dark (d) incubations (first letter in subscript) in chambers (c) or bottles (b) (second letter in subscript); t is incubation time (h), V is volume (L) and A is area (m2) of chamber. Gross primary productivity (GPP) was calculated as sum between NCP and absolute respiration for each tub.
To compare GPP between communities, control values were expressed in mg O2 m-2 h-1 after WC metrics of NCP and respiration were integrated over a 1 m depth (x 1000 L). System GPP was obtained by summing WC and benthic GPP.
Environmental measurements:
Salinity and temperature were measured at time of oxygen measurements using the same meter. Surface photosynthetic active radiation (PAR) (from environmental station 30°15.075' N, -88°04.670' E Dauphin Island, Alabama, USA; http://arcos.disl.org) was averaged over incubation duration and integrated over a 48 hour-period prior to incubations (Photosynthetic Photon Flux Density, PPFD). 48 hours reflects a short-term measure of light history.
Statistical analyses:
A series of two-way ANOVAs with trial and treatment as fixed factors were used to test for differences in producer biomass in both habitats and were used to test for differences in rates with and without diatom metabolism. Differences in rates were attributed to diatom metabolism and percent contribution to GPP was calculated based off mean GPP for each trial (n=4) with Ge rate as a proportion of control rate, expressed as a change from 100%.
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