Reservoir water quality data

Potable source-water reservoirs are the main water supplies in many
urbanizing regions, yet their long-term responses to cultural
eutrophication are poorly documented in comparison to natural lakes,
creating major management uncertainties. Here, long-term discrete data
(June 2006–June 2018) for classical eutrophication water quality
indicators, continuous depth-profile data for dissolved oxygen (DO), and
an enhanced hybrid statistical trend analysis model were used to evaluate
the eutrophication status of a potable source-water reservoir. Based on
classical indicators (nitrogen - N and phosphorus - P concentrations and
ratios; phytoplankton biomass as chlorophyll a, chla; and trophic state
indices), the reservoir was eutrophic to hypereutrophic and
stoichiometrically imbalanced. Anoxia/hypoxia occurred for 7-8 months
annually system-wide, even throughout the water column for days to weeks
in some years; and elevated total ammonia (up to ~900 µg tNH3 
L-1) in surface waters from late summer/fall through late winter/early
spring suggested substantial internal legacy nutrient loading. These
surprising DO and tNH3 phenomena may characterize many reservoirs in
urbanizing areas, and the associated cascade of negative impacts may
increasingly affect them under global warming. Total organic carbon (TOC),
seasonally influenced by phytoplankton biomass, commonly exceeded 6 mg L-1
which is problematic for potable water treatment, and significantly
trended up over time. Wet-year inflow dilution influenced an apparent
decreasing trend in nutrients within the hypereutrophic upper reservoir,
which receives most tributary inputs. Nevertheless, significant
reservoir-wide trends (increasing TP, phytoplankton chla, TOC) and mid-
and/or lower region trends (increasing TN, tNH3, decreasing TN:TP ratios)
suggest that water quality degradation from eutrophication has worsened
over time. These findings support broadly applicable recommendations to
strengthen protection of potable source-water reservoirs in urbanizing
watersheds: (i) Protective numeric water quality criteria are needed for
TOC as well as TN, TP, and chla; (ii) Continuous diel data capture more
realistic DO conditions than traditional sampling, and can provide
important insights for water treatment managers; and (iii) Assessment of
reservoir eutrophication status to track management progress over time
should emphasize classic indicators equally as statistical trends, which
are highly sensitive to short-term meteorological forcing.