A case for biological zeros

Abstract

Most ecological work on rocky shores entails the quantification of species distributions and abundances relative to some convenient and accurate datum point. The most commonly used are tide Ievels and chart data. The problem arises when tides are virtually absent as in most of the Mediterranean Sea or when accurate chart data are not available. In the Maltese Islands there is only one datum point located in the Grand Harbour, Valletta. A number of bench marks and trigonometric stations of various orders are found scattered all over the Islands, but these are not always located very close to the shore so that levelling work from these stations to a particular study site may become quite a laborious task which may exceed the capabilities of most ecologists. Furthermore, some of these marks are old and location data for them are untraceable, while others have turned out to be inaccurate. In fact a project is currently underway by the Mapping Unit of the Planning Authority to recalibrate these stations and to establish new bench marks. A further complicating factor is that different charts make use of different datum points. Thus, while Admiralty Charts use a zero point (Chart datum) which is the level of lowest astronomical tide and the level to which all bathymetric soundings are referred, all heights shown on the official Government of Malta survey sheets use a datum point which is 0.5859m above the Admiralty Chart datum and which is taken to be mean sea level (MSL) for the Maltese Islands. Additionally, there is also a Public Works Department (PWD) datum which is 0.4100m above the AdmiraIty Chart datum. It is not always clearly stated which datum points charts and maps are based on.

This state of affairs has resulted in field workers having to resort to some other datum point with which to relate all their data, including the so called “biological zero”. However, this also has its own set of problems.