Abstract
The impact of environmental variables and processes of nutrient enrichment on phytoplankton community at Epe lagoon was studied for 18 months (November 2012–April 2014). Two cyanobacterial bloom incidences were observed in the two dry seasons during this period. Rainfall pattern regulated nitrogen limitation which triggered cyanobacterial bloom development and influenced the bloom duration. Nutrient-laden influx from agricultural practices accounted for the increase in nitrate, phosphate and the relatively low silicate. Hypoxic conditions during the two bloom episodes reflected the vulnerability of this lagoon ecosystem. The cyanobacterial blooms were dominated by potentially toxic species; Anabaena circinalis, A. flos-aquae, A. limnetica and A. spiroides. A comparison of ambient nutrient ratios with the Redfield ratio (N/P/Si = 16:1:16) showed clear temporal variations that coincided with phytoplankton dynamics and the bloom regime. Low N/P ratios were recorded during the first 8 months, and there was a shift to higher ratios during the subsequent 8 months, followed by low N/P ratios in the last 2 months. Higher silicate values coincided with decreased cyanobacterial biomass, cyanobacteria bloom collapse and proliferation of diatoms during the first annual cycle. Cyanobacterial abundance exceeded the alert level 1 and almost reached alert level 2 which call for continuous environmental monitoring and management of coastal waters.
Highlights
Phytoplankton form the base of aquatic food webs (Sommer 1989; Arrigo 2005) and are important biological component of marine ecosystems and biogeochemical cycles (Malone 1977; Litchman 2007; Litchman et al 2015)
Total suspended solids and total dissolved solids were between (4–3166 mg/L) and (30–5290 mg/L), respectively. pH was 6.1–7.7, indicating acidic conditions, while salinity was in the range of 0–5‰ (Fig. 2)
biological oxygen demand (BOD) and Chemical oxygen demand (COD) were in the range of 2–15 mg/L, 0.5–13 mg/L and 15–160 mg/L, respectively (Fig. 3)
Summary
Phytoplankton form the base of aquatic food webs (Sommer 1989; Arrigo 2005) and are important biological component of marine ecosystems and biogeochemical cycles (Malone 1977; Litchman 2007; Litchman et al 2015). Their excessive growth can cause significant threat to biodiversity and ecosystem functioning as in the case of toxic algal blooms (HABs) (Paerl and Huisman 2008). Toxic cyanobacterial blooms can potentially serve as signs or ecological indicators of changes in aquatic ecosystem integrity (Jeppesen et al 1997; Sanchis et al 2002; Willame et al 2005; Carmichael 2008; Peretyatko et al 2010). The relationships among hydrologic discharge (flushing and residence time), vertical/horizontal salinity and thermal gradients, rainfall and drought, wind and tidal mixing determine the frequency, severity, spatial and temporal extent of bloom events in coastal ecosystems (Paerl 2006)
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