Grazing on intertidal microphytobenthos by macrofauna: is pheophorbide a a useful marker?

This study critically evaluates the use of pheophorbide a as a marker pigment for macro- faunal grazing intensity in intertidal sediments. Based upon evidence from planktonic studies pheophorbide a has recently been used to quantify grazing in the sediments; however, no accurate tests have been made of the application of these planktonic concepts to the benthos. Laboratory and field examinations used the cosmopolitan species Arenicola marina, Corophium volutator and Hydro- bia ulvae, as well as natural field assemblages from the Eden estuary, Scotland, to examine the rela- tionship between grazing and pigment composition. Faeces from the 3 species were examined using high performance liquid chromatography and showed no sign of pheophorbide a, but all species showed concentrations of 'chlorophyll a-like' (chl a) grazing products. In laboratory manipulations, a negative relationship was apparent between grazer numbers and the concentrations of pheophor- bide a and grazing products. In the field, pheophorbide a was rarely found but grazing products were present in consistently measurable amounts. When relevant physical and biological factors were regressed against grazing product concentration, a significant relationship was found with an ad- justed R 2 of 0.76. This multiple regression used 4 factors to predict chl a degradation concentration; however, the relationship was dominated by a positive relationship with chl a concentration and only 1 of the other 3 factors was directly related to grazing intensity. Neither pheophorbide a nor 'chl a- like' grazing products were found to be useful as marker pigments of macrofaunal grazing intensity in the intertidal system studied. This places doubt upon the use of pheophorbide a as a marker of macrofaunal grazing intensity in intertidal sediments in past papers, and discourages the use of any marker pigment in estuarine sediments without prior testing.