MARINE ECOLOGY PROGRESS SERIES Published January 24 Mar. Ecol. Prog. Ser. Particle aggregation during a diatom bloom. 11. Biological aspects Ulf Riebesell Alfred Wegener Institute for Polar and Marine Research, Colurnbusstr., D-2850 Bremerhaven. Germany ABSTRACT: For a 6 wk period covering the time before, during, and after the phytoplankton spring bloom, macroscopic aggregates (20.5 mm diameter) were repeatedly collected and water column properties simultaneously measured at a fixed station in the Southern North Sea. Distinct changes in aggregate structure and composition were observed during the study. Predominantly detrital aggregates dur~ngthe early phase of the study were followed by diatom-dominated algal flocs around the peak of the bloom. Mucus-rich aggregates containing both algal and detrital components and with large numbers of attached bacteria dominated the post-bloom interval. The phytoplankton succession within the aggregates closely reflected the succession in the water column with a time delay of a few days. Algal flocculation did not occur as a simultaneous aggregation of the entire phytoplankton community, but as a successional aggregat~onof selected diatom species. Although the concentrations of ~norganicnutrients diminished considerably during the development of the phytoplankton bloom, the termination of the bloom appeared to be mostly controlled by physical coagulation processes. The importance of biologically-controlled factors for physical coagulation is discussed. INTRODUCTION stickiness, proposed that physical processes alone could determine the time and extent of algal aggregation. Mass flocculation during diatom blooms as predicted Based on a physical coagulation model, Jackson pre- by Smetacek (1985) has since been documented both in dicted that the rate of aggregation strongly depends on the field (Kranck & Milligan 1988, Alldredge & Got- algal concentration. Above a critical maximum concen- schalk 1989) and in mesocosm expenments (Riebesell tration, physical coagulation would become the domi- 1989). Simultaneous aggregation of an entire phyto- nant force leading to a rapid flocculation of a large plankton community and its subsequent sedimentation portion of the algal biomass (Jackson 1990). The extent was shown to occur on tlme-scales as short as 24 h to which algal flocculation is determined by either (Alldredge & Gotschalk 1989). Desp~tethe broad evi- physically or biologically controlled processes remains dence for aggregate formation in association with phy- to be established. PI-esumably, a combination of both toplankton blooms, both the causes and regulating types of processes will drive aggregation under natural mechanisms are not well understood. Flocculation and conditions. subsequent sed~mentationof diatoms has been consid- Previous field investigations of a1ga.l flocculation ered to be an integral part of diatom life history have focussed on the aggregation event as such. The (Smetacek 1985, von Bodungen et al. 1986). Nutrient object of this study was to investigate particle aggrega- depletion was hypothesized to induce phytoplankton tion over a period covering the development and aggregation due to an assumed increase in stickiness of decline of a phytoplankton bloom, and to relate aggre- nutrient-depleted cells (Smetacek 1985, Logan & All- gation processes both to the state of the phytoplankton dredge 1989). A relationship between cell stickiness community and to the prevailing physico-chemical and the nutritional status of phytoplankton cultures was conditions of the environment. demonstrated by knrboe et al. (1990). The role of bacteria in bonding particles together in aggregates has been emphasized by Kranck & Milligan (1980) and MATERIAL AND METHODS Biddanda (1985).While bacterial bonding and the varia- tion of algal stickiness in response to nutrient depletion Sampling procedure. Investigations were conducted would imply that flocculation is largely controlled by in the German Bight (North Sea) from 18 April to biological factors, Jackson (1990),assuming a constant 1 June, 1989 at one station, which was sampled 10 O Inter-ResearchlPrinted in Germany 017 1-863019110069/0281/$ 03.00 282 Mar Ecol. Prog S times at intervals ranging between 3 and 11 d. Nutri- on video tape. Enumeration of organisms and fecal ent, chlorophyll a, and phaeopigment concentrations, pellets was performed on water samples preserved in particulate organic carbon and nitrogen content, bac- Lugol's iodine solution using the inverted microscope terio- and phytoplankton abundance, and the phyto- method of Utermohl(1958).Microscopic counting of the plankton species composition were determined on sea- organisms and fecal pellets contained in 5 individual water samples taken with 10 l Niskin bottles from l, 10, aggregates from each sampling date was conducted on 20, and 30 m depths. intact aggregates under the inverted microscope. In During each station, SCUBA divers hand-collected cases where aggregates were too dense to be analysed 20 to 40 individual aggregates at approximately 10 m in their existing form, they were gently squeezed depth in 1 m1 polypropylene syringes. For sampling between slide and cover-slip. For the purpose of scan- convenience, the largest aggregates present at the time ning electron microscopy (SEM),individual aggregates of collection, which were generally > 0.5 mm in dia- were dehydrated through an alcohol series, critical- meter, were chosen. In order to avoid destruction of the point-dried in liquid CO2,and platin-palladium coated. aggregates during sampling, the forward ends of the They were then examined on an IS1 D 130 SEM. syringes were cut off and the resulting open cylinder For bacterial enumerations, 1 m1 of sample water was sealed with a serum stopper immediately after from each depth and 3 individual aggregates from each sampling. In the laboratory, only those aggregate sampling date fixed in 2 % glutaraldehyde were fil- samples were used for further analysis which still con- tered onto 0.2 Lm Irgalan black-stained Nuclepore fil- sisted of a single, intact aggregate. An attempt was also ters. After staining with DAPI or acridine orange, the made to sample surrounding water free of aggregates. abundance of free bacteria in the water samples and of However, due to the hlgh concentration of minute bacteria attached to the outer surface of aggregates aggregated particulate material during most of the was determined using an eplfluorescence microscope study period, in situ sampling of aggregate-free sur- (Hobbie et al. 1977). Numbers of bacteria attached to rounding water was not possible. the aggregate surface were determined by focussing Laboratory analyses. Dissolved inorganic nutrient on the outer aggregate surface and counting attached concentrations (Si, NO:, NH:, were analysed on bacteria in this thin layer in 30 random fields on each a Technicon Autoanalyser I1 using standard methods. aggregate. Since bacterial abundance in the water Nutrient samples were fixed with HgC12 (final concen- column was determined as number per volume, and in tration 0.01 %) immediately after collection. For the aggregates as number per surface area of aggregate, a determination of chlorophyll a and phaeopigment con- comparison in absolute numbers is not possible for this centrations, 500 m1 of sampling water were filtered data set. The evaluation of the data will therefore be onto 25 mm Whatman GF/C glass fiber filters. After confined to a relative comparison. extraction in 90% acetone (filters were homogenized Enrichment factors were calculated by dividing the with glass beads), the samples were analysed using concentration of a given organism in aggregates standard fluorometnc methods (Parsons et al. 1984). (number per volume), by its concentration in the water For the determination of the particulate organic carbon column. The approximate aggregate volume was esti- (POC) and nitrogen (PON) content, 1 1 of sampling mated by sizing the intact aggregates under an water was filtered through precombusted, 25 mm inverted microscope. Since surrounding water samples Whatman GF/C glass fiber filters and stored at -30 "C (i.e. water free of aggregates) could not be collected, until analysis. After the filters were treated with a drop regular water column samples containing both freely of 1N HC1 to remove inorganic carbon, POC/PON were suspended cells and aggregated material lsrupted measured on a Carlo Erba l500 CHN Analyser. during sampling and handling were used. Therefore, For particle aggregates, the chlorophyll a and the enrichment factors presented herein underestimate phaeopigment content as well as the POC/PON con- the actual enrichment of organisms in aggregates rela- tent were analysed using the same methods as tive to the aggregate-free water. Relative enrichment described above. For each sampling date, the factors of individual diatom species were obtained by chlorophyll a and phaeopigment content of 6 individual dividing the relative abundance of a given species in aggregates were measured. Two replicate sets of 3 to 6 aggregates (percent of total diatom cell number), by its aggregates (depending on aggregate size) were used relative abundance in the water column. for the determination of POC/PON. Aggregates were filtered with a minimum of surrounding water (gener- ally less than 0.2 m1 per aggregate). RESULTS Light microscopic analyses of both seawater samples and particle aggregates were carried out qualitatively Inorganic nutrient concentrations recorded over the on live material