An analysis of tide-gauge and atmospheric data reveals that a transient, unusual sea-level drop occurred in mid-April 1997 along coasts of the Nisyros island volcano, SE Aegean Sea. This event had an amplitude several times larger than the average tidal range (∼6 cm) and caused a transient shift of coastal biological zones and a biological anomaly, the signs of which survived for at least one year. This proves first, that the observed sea-level low was an effect of meteorological origin, not related to a clustering of seismicity which affected Nisyros and the wider area. Second, that the coastal biological zoning and the Biological Mean Sea Level (BMSL) are stable and characteristic in each site, and do not depend on short-wavelength fluctuations of the sea level. And third, that biological observations can be used to identify the detailed history of recent sea-level changes caused by tectonic, volcanic or even meteorological effects; the amplitude of such changes may be as low as a few tens of centimeters. An implication of the above is that in rocky, nearly tideless and low-energy coasts, BMSL (which generally corresponds to the Mean Low Waters during spring tides) approximates Mean Sea Level (MSL) with an accuracy similar to that obtained from ∼1 year long tide-gauge records, and therefore can define a geodetic datum suitable for all engineering projects.