Effects of toxin-producing phytoplankton on copepods: feeding, reproduction and implications to the fate of toxins

Abstract

Phytoplankton growth and loss rates are governed by several factors and in many cases, when growth control fails, algal blooms are established. Such blooms can be formed by different phytoplankton species and some of them have harmful impacts on marine ecosystems. One of these negative impacts by some bloom-forming phytoplankton is the ability to produce toxic or inhibitory compounds with broad effects on aquatic organisms. Harmful effects can also reach humans if food or water contaminated with toxins is consumed. Copepods are one of the most numerous consumers of phytoplankton in marine environments. These invertebrates can also obtain their nutrition from other food resources such as single cell animals (e.g. ciliates), which could be an important alternative under bloom conditions. The aims of this thesis were to assess 1) to which extent toxic or inhibitory phytoplankton is consumed (or avoided) by copepods; 2) whether ciliates and other food types could contribute to the diets of copepods when toxic food is present; 3) if copepods accumulate toxins and excrete them with faecal pellets when toxic phytoplankton is ingested; and 4) the effect of toxic diets on the reproductive success of copepods. Based on some of the results obtained, the role of cellular mechanisms in the detoxification of toxins from the ingested toxic diets was investigated. In addition, the importance of the transport of toxins via copepods to higher consumers such as fish was evaluated. In some cases, toxic phytoplankton was not consumed by copepods. In other cases, copepods fed actively upon toxic phytoplankton. Reproduction was not sustained if toxic phytoplankton was the only food available, or when compared to an adequate diet. This was related to a decrease in the numbers of oocytes produced or in the degree of maturation of these cells. Toxins were detected in copepods and in their faecal pellets after ingestion of the toxic cyanobacterium Nodularia spumigena. However, the amount of toxins found in the copepods was far lower than the amount of ingested toxins, which could indicate that copepods might have metabolic mechanisms to get rid of these compounds. High metabolic activity (GSH transferase activity) was, however, detected in well-fed copepods and not related with the ingestion of toxic food. More sensitive studies (e.g. at the molecular level) are needed. Calculations indicated that part of the phytoplankton toxins detected in the copepods could be transferred to fish populations in the Baltic Sea. In some cases, this toxin transfer could be low, but in others it could considerable and threaten both fish and humans. Therefore, frequent monitoring of phytoplankton toxins in fish stocks is needed to assure safe consumption of fish by us