Where does the water in your pond come from - rainwater, tap water, groundwater?

Is your pond natural, lined, plastic?

A better description of your pond may help people answer.

Except when freshly-filled, and assuming it is not a concrete pond, once "aged", the pond pH is going to refelect your local soil conditions, plus some influence from what is in it. It's contents are likely to make it less alkaline rather than more. Concrete ponds, and also ones surrounded by concrete paving etc., are always going to be alkaline - new ones will be very alkaline.

The pH of the pond is not alkaline, as such, your surface water, and presumably your soil, is.
You obviously have the facility to measure pH, but the natural plants in the general area ought to give a reasonable indication of soil pH. Blue clay is normally (always?) alkaline.

Hi,

Another factor that can influence pH is oxygen/carbon dioxide.

First thing in the morning when plants have been respiring and generating CO2 the pH will be lower. Once they've been photosynthesising for a few hours, carbonic acid levels drop and oxygen will push the pH back up.

This effect is more noticeable in soft water, as minerals in hard water keep the pH buffered in the alkaline range.

By testing in the morning and the evening, you might identify a trend but as has been said, minerals in the clay lining are likely to be a factor.