. The Biological bulletin. Biology; Zoology; Marine biology. 352 M. J. ANDERSON its content of Ca(OH)2 (and resulting pH at its surface), but which did not contain all of the other compounds in cement. Substrata were made by spreading non-toxic hy- drophilic epoxy cement (Megapoxy HT) on one side of quarry tiles (11 X 11 cm) and coating this with dry mix- tures of varying proportions of Ca(OH)2 and sand (the same type of sand used in making concrete substrata). The pH of seawater at the surface of concrete and of sub- strata made on tiles was measured after drawing up 2 ml of fluid into a micr
. The Biological bulletin. Biology; Zoology; Marine biology. 352 M. J. ANDERSON its content of Ca(OH)2 (and resulting pH at its surface), but which did not contain all of the other compounds in cement. Substrata were made by spreading non-toxic hy- drophilic epoxy cement (Megapoxy HT) on one side of quarry tiles (11 X 11 cm) and coating this with dry mix- tures of varying proportions of Ca(OH)2 and sand (the same type of sand used in making concrete substrata). The pH of seawater at the surface of concrete and of sub- strata made on tiles was measured after drawing up 2 ml of fluid into a micropipette being dragged slowly across each surface. It was found that seawater at the surface of substrata made with a mixture in the ratio of 3 parts sand to 1 part Ca(OH)2 by weight (hereafter called E3) closely matched the pH of seawater at the surface of concrete (average pH = ) for the relevant period of these experiments. Sub- strata were also made with epoxy and sand only (ES). The pH at the surface of these substrata did not differ from that of surrounding seawater (average pH = ). Tests for the effect of Ca(OH)2 on larval settlement could be made by comparing settlement on E3 with that on ES. Tests for the effects of concrete on larval settlement that were not caused by Ca(OH)2 could be made by com- paring settlement on concrete with that on E3. In addition, to test for the effects of a biofilm and its potential inter- action with the pH of surfaces, each of E3, ES and concrete surfaces were exposed to seawater to develop a biofilm. These surfaces were "aged" in the containers in the one- way flow-through system described below (see Laboratory experiment) for 20 days prior to the beginning of the lab- oratory experiment. The field experiment included an additional treatment of sandstone substrata along with concrete, E3 and ES as used in the lab experiment. This was to control for any effect of the epoxy resin and to include in the field ex- periment a
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