Reading Salinity in a Lagoon
A class samples a lagoon that opens to the sea through a 200 m mouth and receives freshwater from a creek at its head. On one pre-monsoon day they record surface salinity at five stations running from the mouth inland: 34, 31, 24, 12 and 3 parts per thousand. After the monsoon the same stations read 28, 22, 14, 6 and 1. The tidal range at the mouth is about 0.8 m, and on the survey day the two high tides were of clearly unequal height. Villagers report that the mouth closed completely in 2019 and again in 2023, and that salinity in the upper lagoon rose sharply in both years. The class must interpret the gradient and decide what actually controls it, knowing they have only two days of data and no current measurements.
1. Describe and explain the salinity pattern, and say which section of the lagoon is the main mixing zone.
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The pattern is a monotonic gradient from near-marine water at the mouth to nearly fresh water at the head, which is what a partially mixed lagoon fed by a creek should show. The sea is the salt source and the creek is the diluent, so salinity falls with distance inland and with freshwater inflow. The seasonal comparison matters more than the absolute values: every station freshened after the monsoon, but the largest proportional fall was in the middle stations, roughly 24 to 14, while the mouth fell only from 34 to 28 and the head stayed near fresh. That points to the middle section as the main mixing zone, where tidal seawater and creek water meet and dilute. The mouth is buffered because the sea reservoir is large; the head is buffered because it is already fresh. The villagers' observation that upper-lagoon salinity spikes when the mouth closes supports the same reading: cut the tidal supply and evaporation concentrates what remains.
2. How would you test whether evaporation or tidal flushing dominates the inner lagoon's salinity?
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The two processes leave different signatures and can be separated by a short, targeted programme. Tidal control should show salinity changing through the tidal cycle at a fixed station, a measurable tidal prism entering the mouth, and currents reversing with the tide; evaporation should show a slow rise in salinity during stagnant, hot, dry spells, strongest in shallow inner water, with little tidal signal. So measure salinity and temperature hourly over full tidal cycles at two or three stations, record currents or at least tidal height and mouth width, and estimate evaporation with a simple pan or meteorological formula. Then construct a rough water and salt budget for the inner lagoon. A decisive check is the closed-mouth years: if salinity climbed steadily while the mouth was shut and fell after it reopened, tidal flushing is the dominant control on the annual cycle, with evaporation as the modifier during closure.
3. The survey day showed two high tides of unequal height. What can and cannot be concluded from a single day?
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Unequal successive high tides indicate a mixed tidal regime, in which the two daily highs differ because the tide is not purely semidiurnal and is influenced by the Moon's declination and by local shape. What cannot be concluded from one day is the type of tide, because meteorological forcing such as wind setup, pressure change or river flood can distort a single day's curve, and neap–spring variation changes heights over a fortnight. A proper statement needs at least a 15-day record of hourly heights at the mouth, from which the number of highs per day, their inequality and the spring–neap range can all be read. The same caution applies to the salinity data: two days describe two days. The class should present the gradient as a strong pattern but the causal weight of tidal flushing versus evaporation as a hypothesis awaiting the tidal-cycle and budget evidence.