Abstract
Water quality is a fundamental determinant of aquaculture success, shaping the growth, health, and survival of the cultured species. In traditional pond irrigation systems, tidally driven variability alters the physicochemical conditions, creating risks of stress, reduced productivity, and mortality. This study quantified the tidal–diurnal controls on water quality dynamics in Sawohan Village, Sidoarjo, Indonesia, across a freshwater–brackish– saline gradient. Water quality was monitored at three ponds during spring and neap tides, both during the day and night, yielding 48 sampling events. The measured parameters included pH, salinity, dissolved oxygen (DO), alkalinity, nitrate, nitrite, and ammonia. Data were analyzed using descriptive statistics and two-way ANOVA. The tidal phase and sampling time significantly affected several parameters (p < 0.05). The pH showed the lowest value of 7.47 in brackish ponds during nighttime spring tides, while salinity ranged from <0.5 ppt during neap tides to 19.0 ppt in the saline pond during daytime spring tides. DO was consistently lower at night than during the day and occasionally fell below the 5 mg.L-1 aquaculture threshold during nighttime neap tides. Alkalinity remained within 116.88–150.33 mg.L-1, with higher values at night, particularly during the spring tides. Nitrate concentrations frequently exceeded national and aquaculture guideline values, whereas nitrite and ammonia approached or exceeded their respective thresholds in brackish and saline ponds, especially during nighttime neap tides. Overall, these results demonstrate that tidal–diurnal interactions strongly influence nitrogen dynamics and water quality stability in traditional pond irrigation systems and should be explicitly considered in aquaculture water quality assessment and monitoring.