Use of RNA:DNA ratios for assessing secondary production of planktonic food webs: effects of temperature, salinity, food and heavy metals

Copepods, the dominant component of the zooplankton community, function as a vital ecological link within marine food webs. This dissertation evaluates using RNA:DNA analysis of individual copepods (Acartia tonsa) as an indicator for secondary production under a variety of environmental conditions (temperature, salinity, food and heavy metals) in the laboratory and field. A method was developed to measure RNA and DNA of individual A. tonsa using a nucleic acid fluorescent dye. RNA:DNA and egg production (EP) were all significantly higher for fed copepods compared to starved copepods after 48 hrs. The relationship between RNA:DNA and EP was measured for A. tonsa cultured at three different temperatures and salinities to determine if RNA:DNA analysis is a useful indicator of secondary production under a wide range of environmental conditions. Wild A. tonsa were also sampled bi-monthly from Nueces Bay and their RNA:DNA measured as an index of secondary production in the field. RNA:DNA of wild A. tonsa, corresponded with high RNA:DNA and EP of well fed laboratory copepods and were inversely related to temperature. Secondary production of copepods is reduced in the presence of the dinoflagellate Karenia brevis, but it is unclear whether the effects are due to brevetoxin or poor nutritional quality. Fecundity, fecal production and RNA:DNA of wild A. tonsa were measured after being fed mono-algal and mixed-algal culture diets of K. brevis and Peridinium foliaceum for 48 hrs. Similar low values in all measured variables between 100 % K. brevis diet and starved copepods suggests A. tonsa does not graze K. brevis when offered as its sole food source. Significant differences in EP were found between mixed diets, but not RNA:DNA, suggesting EP is a more sensitive indicator of nutritional quality. Anthropogenic introductions of heavy metals also reduce zooplankton production. Cultured copepods were fed Thalassiosira spp. contaminated with either Cd or Cu, to determine if RNA:DNA and EP were affected by heavy metals. The present study found metals had no affect on EP or RNA:DNA. RNA:DNA analysis can be used as a proxy for zooplankton condition under known environmental parameters with strongest responses to change in food and temperature.