Influence Of Diatom In High Concentration On Copepod Reproduction And Development

Traditionally, diatoms have been considered as major phytoplankton components of spring bloom in marine ecosystem. The consumers of diatoms are mainly copepods, a dominant constitute of zooplankton that support the secondary production in most oceanic regions. However, numerous recent studies questioned the traditional concept that copepod production in marine ecosystem is primarily based on pelagic diatoms. Two hypotheses were put forward for the negative effect caused by diatom: (1) some essential nutrients, especially the polyunsaturated fatty acid (PUFA), which were necessary for copepod reproduction perhaps were insufficient in diatom cell. Evidences indicated that the PUFA content is directly correlated with the egg production and hatching of copepods, which is important for reproduction. (2) Diatom cells contain toxic or deleterious agents that could inhibit copepod eggs from hatching or causing malformed nauplii. Researches for diatom-copepod interaction provides a new point to re-evaluate the traditional view that diatom forms a major and sufficient food source in pelagic food web and influence the secondary production. The following questions should be answered for understanding the diatom-copepod interaction. (1) Do all abundantly occurring diatom species have some negative effects on copepod? or is it just a species-specific effect? (2) What is mechanism of the deleterious effect caused by diatom at high concentration? (3) If the deleterious effect occurs, is it a single species or mixed species effect? (4) Does the negative effect even occur in nauplii developing stages? Therefore, we conducted a research at laboratory conditions, studying interactions of different diatom species (Skeletonema costatum, Chaetoceros muelleri and Phaeodactylum tricornutum) and common copepod species (Calanus sinicus and Acartia bifilosa) sampled from Yellow Sea. Results showed that Some diatom species (Chaetoceros muelleri and Phaeodactylum tricornutum) induced deleterious effect on both egg production and hatching success. However, as a dominant diatom species in the Yellow Sea, Skeletonema costatum showed no negative effect,in high concentrations (1.0-2.0μg C ml-1). This indicates that the deleterious effect of diatom on reproduction for copepod is species-specific. The possible mechanism of the deleterious effect induced by diatom mainly include: (1) The particle size of a diatom species is not within the range for copepod ingestion. (2) Some essential nutrients necessary for reproduction of copepods are insufficient in diatom cells. Fatty acid composition analysis showed that for copepod reproduction, some saturated fatty acid (14:0) is necessary, and the ratio of polyunsaturated fatty acid 20:5ω3(EPA) Vs 14:0 (20:14) showed negative correlation with egg production and viability. Phytoplankton could release secondary metabolites, which prevented eggs from hatching at high concentration (2.0μg C ml-1). The influence of excellular agents prefers the difference of species to concentration of phytoplankton. In our study, egg production of Calanus sinicus were kept at a normal level (8.58+0.40 egg female-1d-1) after mixed diatom diets (Skeletonema costatum, Chaetoceros muelleri and Phaeodactylum tricornutum) were ingested. Similar results were shown from Acartia bifilosa after fed with mixed foods (green algae-diatoms) were fed. It is indicated that the negative effect caused by a single diatom species can be diminished and/or removed by feeding with mixed food. In addition to production process, the negative effect was also shown in nauplii developing stages. Compared to green algae, diatom could induce negative effect onthe development of copepods (Calanus sinicus and Acartia bifilosa) under the same food level (1.0μg C ml-1). The diatom resulted in higher mortality and failure to develop complete life cycle. From our experiments, we conclude that the traditional concept of pelagic food web linking diatom, copepod, and fish is still tenable, although some diatom species can induce negative effects on copepod. The possible reasons are mainly: (1) It is difficult to determine which diatom species is harmful for all copepod species. (2) The inhibitory effect of single diatom species on production for copepods can be reduced or eliminated by feeding copepod with mixed diets, even mixed diatoms, which can provide chance for copepod to take diatom in suitable size, and then sufficient nutrition. (3) Diatom in high concentration can release some harmful agents that could cause hatching failure for copepods. However, this phenomenon rarely occurs in the field, except for diatom bloom that is dominated by one or two diatom species.