| | | 19.12 Sources of settling material |
Despite the relatively high suspended biomass during summer situation, and the presence of suitable physical conditions, significant aggregate formation and export did not take place [295]. Contrary to expectations aggregation was not involved during mass sedimentation of diatoms during spring [295]. A positive correlation between sedimentation of detritus aggregates, sedimentation rates of biogenic matter and sedimentation velocities/specific loss rates of POC and phaeopigments is encountered. Aggregation of detritus may thus partly control the sedimentation dynamics, but it did not correlate with turbulent shear as expected according to classical coagulation theory. The aggregates are possibly fragile and form only during periods of low turbulence. The correlation between phaeopigments and aggregate sedimentation, the observed disintegration of faecal pellets into amorphous detritus, and the inclusion of pellets in the larger aggregates indicate that faecal matter constitutes an important component of sedimenting matter [295]. This points at the significance of the larger planktonic heterotrophs in the food web of the Gulf.
The food web composition in spring is characterised by a relatively high biomass of large phytoplankton and a dominance of protozoa (mainly ciliates), while mesozooplankton are almost absent. During midsummer a medium phytoplankton biomass is encountered with significant quantities of mesozooplankton (both copepods, cladocerans and rotifers). However, all other heterotrophic components increase their biomass as well, and a complicated food web prevails. The early autumn is characterised by low phytoplankton biomass and a heterotrophic system with decreasing biomass. Protozoa (ciliates) increase their importance relative to metazoa. Bacterial production is reduced.
The relative export of biogenic matter in summer is possibly low due to a high potential for retention, disintegration and decomposition of detritus by heterotrophic organisms. The high concentrations of mesozooplankton during summer result in a disintegration of the larger detritus particles and aggregates. Or they disintegrate or ingest faecal pellets by mechanisms such as coprophagy, coprorhexy or coprophagy (sensu [344]). Strong wind action may also affect the disintegration of aggregates. Furthermore, picoplankton and the microbial food web are important in the organic carbon flow and for regeneration. Ciliates were dominant among the large protozoa, and cladocerans as well as rotifers constitute a significant fraction of the metazoa. Contrary to copepods, these organisms produce faecal particles that easily disintegrate.
| | | 19.12 Sources of settling material |