Particulate Organic Matter Composition in A Semi-Enclosed Marine System

Spatial and temporal changes in the biochemical composition of particulate organic matter in a semi-enclosed marine system (Stagnone di Marsala, Mediterranean Sea) were studied, on a monthly basis, from January to December 1994, in order to assess nutritional value of suspended particles for benthic suspension feeders. According to previous findings, the study site displayed a strong oligotrophy. Chlorophyll-a accounted for a very low fraction of the total suspended matter pool (0.1%), whereas at least 75% of POC was of detrital/heterotrophic origin. POC: PON ratio values indicate that bacterioplankton biomass accounted for a significant fraction of the total POC pool, displaying values comparable to those of the phytoplankton biomass (phytoplankton to bacterial biomass ratio was about (1). Temporal and spatial changes in the biochemical composition of particulate organic matter were rather limited and related to its sources, the main of which is represented by detrital particles released by the Posidonia oceanica (L.) beds. the comparison between our results and those encountered in other coastal lagoons indicates that the low abundance of suspension-feeding organisms observed in the study area is related to the “quality depression” of particles due to the dilution of high quality compounds (i.e., biopolymeric carbon) in a largely inorganic matrix. This result leads us to conclude that, to reach the same amount of high quality particulate food, a suspension feeder mollusc in the Marsala lagoon would need to filter a sea water volume around 3 times higher than in other Mediterranean coastal lagoons.     

Spatial and Temporal Changes of Suspended Matter in Relation to Wind and Vegetation Cover in A Mediterranean Shallow Coastal Environment

Seasonal and spatial changes in seston, (POC), particulate organic carbon, (PON) particulate organic nitrogen and chlorophyll-a concentrations were studied on a monthly basis in a Mediterranean shallow coastal area (Stagnone di Marsala, Western Sicily) in order to gather information on factors controlling particulate organic matter distribution and composition. Seston concentration and composition were connected to the main physicochemical and biological driving factors, such as temperature, salinity, dissolved oxygen, wind-speed and biomass of submerged vegetation. the Stagnone di Marsala is characterized by high temperatures with strong seasonality (range: 11-28°C), while values ranged from 33 to 45 salinity. Total suspended organic matter concentrations (by ignition loss) ranged from 2 mg l^-1 (in summer) to 12mgl^-1 (in winter) and chlorophyll-a concentrations from 0.02 to 2 μgl^-1. Despite a low POC/PON ratios (ranging from 5 to 11), the ratio of POC to chlorophyll (CHL-a) displayed very high values (annual average of 647). the data reported in this study, highlighting the oligotrophy of the Stagnone di Marsala area, indicate that the trophic state of the basin was controlled by different degrees of wind exposure (mean monthly wind velocity at exposed sites ranged between 4.2 and 6.7 m s^-1) and by gradients in vegetation cover. These two factors induced clear changes in the concentration and composition of the suspended particles, but played a different role in exposed and sheltered areas. Exposed areas with limited vegetation were characterized by large resuspension processes and wide temperature and salinity fluctuations caused by wind induced turbulence. in these areas, autotrophic biomass (as chlorophyll-a), due to phytoplankton and/or re-suspended microphytobenthos, appeared to play an important     

Spatial and Temporal Changes of Suspended Matter in Relation to Wind and Vegetation Cover in A Mediterranean Shallow Coastal Environment

Abstract

Seasonal and spatial changes in seston, (POC), particulate organic carbon, (PON) particulate organic nitrogen and chlorophyll-a concentrations were studied on a monthly basis in a Mediterranean shallow coastal area (Stagnone di Marsala, Western Sicily) in order to gather information on factors controlling particulate organic matter distribution and composition. Seston concentration and composition were connected to the main physicochemical and biological driving factors, such as temperature, salinity, dissolved oxygen, wind-speed and biomass of submerged vegetation. the Stagnone di Marsala is characterized by high temperatures with strong seasonality (range: 11–28°C), while values ranged from 33 to 45 salinity. Total suspended organic matter concentrations (by ignition loss) ranged from 2 mg l^?1 (in summer) to 12mgl^?1 (in winter) and chlorophyll-a concentrations from 0.02 to 2 μgl^?1. Despite a low POC/PON ratios (ranging from 5 to 11), the ratio of POC to chlorophyll (CHL-a) displayed very high values (annual average of 647). the data reported in this study, highlighting the oligotrophy of the Stagnone di Marsala area, indicate that the trophic state of the basin was controlled by different degrees of wind exposure (mean monthly wind velocity at exposed sites ranged between 4.2 and 6.7 m s^?1) and by gradients in vegetation cover. These two factors induced clear changes in the concentration and composition of the suspended particles, but played a different role in exposed and sheltered areas. Exposed areas with limited vegetation were characterized by large resuspension processes and wide temperature and salinity fluctuations caused by wind induced turbulence. in these areas, autotrophic biomass (as chlorophyll-a), due to phytoplankton and/or re-suspended microphytobenthos, appeared to play an important     

Seasonal Fluctuations in the Nutritional Value of Particulate Organic Matter in A Lagoon

Water samples were collected on a fortnightly basis in the lagoon of S. Gilla (Sardinia, Mediterranean Sea) in order to study seasonal nutritional fluctuations of particulate organic matter. the lagoon is characterized by high quantities of suspended matter throughout the year. Thermohaline conditions had no effect on particulate matter quantity and composition, but the quantity as well as quality of suspended particles was drastically affected by the wind, the major effecter of sediment resuspension. As a result of sediment resuspension, seston was always richer in inorganic fraction. However, throughout the year of investigation, most particulate organic carbon was quite appealing for filter feeding communities, although the best POM quality was available during phytoplankton blooming. the phytoplankton pool of suspended matter was just a small fraction of the bulk, accounting for only 13% on average of particulate organic carbon. in terms of energy available in the seston, the highest amount was stored in organic matter heterotrophic fraction, whilst the smallest was to be found in living phytoplankton.     

Subsurface chlorophyll maximum in the tropical and subtropical western Pacific Ocean: Vertical profiles of phytoplankton biomass and its relationship with chlorophylla and particulate organic carbon

Vertical distribution of phytoplankton biomass in terms of carbon content (PC) and its relationship with chlorophylla and particulate organic carbon (POC) were examined together with phytoplankton growth rates in the tropical and subtropical western Pacific in 1979, where a prominent subsurface chlorophyll maximum (SCM) developed between 65 and 150 m. Fluorescence microscopy combined with image analysis was used for measurement of cell volume which was converted to PC. The SCM coincided consistently with subsurface maximum of PC, and the SCM primarily reflected in situ accumulation of phytoplankton biomass. The PC:chlorophylla ratio decreased with depth; the ratio was 1.8 times, on average, higher in populations at the SCM compared to those near the surface. This increase in relative cellular chlorophylla along with depth accentuated the magnitude of the SCM. The PC:POC ratio was substantially lower near the surface, 0.17 on average, and increased sharply around the SCM, with a mean value of 0.53. Thus suspended particles around SCM were richer in phytoplankton than those in the upper layers. A major part of PC was contributed by autotrophic eukaryotes both near the surface and at the SCM, and prokaryotic picoplankton comprised a relatively small proportion (6.3 to 14.9%) of PC. The high phytoplankton biomass around the SCM was suggested to be ascribed to in-situ growth of phytoplankton.Please address all correspondence and requests for reprints to Dr Furuya at his present address: Institute of Bioresources, Miè University, Kamihama, Tsu 514, Japan     

Origin and Distribution of Suspended Organic Matter As Inferred From Carbon Isotope Composition in A Mediterranean Semi-Enclosed Marine System

Abstract

The origin and distribution of suspended organic matter, the trophic features and the stable carbon isotopic composition of particulate organic carbon (POC) were studied monthly in a Western Mediterranean semi-enclosed basin. Sampling stations were selected as a function of wind-exposure and the degree of vegetation cover and then compared with an adjacent unvegetated site. the predominant vegetation was seagrass (Posidonia oceanica and Cymodocea nodosa) and Caulerpa prolifera. Water samples were analyzed for total suspended matter (inorganic and organic fractions), photosynthetic pigments (chlorophyll-a and phaeopigments), dissolved organic carbon, particulate organic carbon and their isotopic composition. Temperature and salinity were also measured at the same sampling sites within range of Mediterranean limits. the suspended organic matter concentration was 1.77 ± 1.55 mg l^?1; the chlorophyll-a concentration was low (0.35 ± 0.24 μg l^?1); the disolved organic carbon concentration was 2,140 ± 2,010 μg l^?1; the particulate organic carbon concentration was 212 ± 106 μg l^?1 and the isotopic composition was 18.77 ± 2.51%_°. There were significant temporal differences except for phaeopigments, POC and its POC isotopic composition, and there were no spatial differences other than for δ¹³C. This picture highlighted a general seasonal trend and trophical features similar to adjacent sea.

Spatial differences in δ¹³C showed that the source of suspended organic matter was different between stations as that between sources and wind-hydrodynamic constraints. In     

Pelagic-benthic coupling in a subtidal system of the North-Western Mediterranean

Suspended particulate matter, zooplankton, and macrobenthos dynamics were investigated in a shallow area of the Ligurian Sea (north-west Mediterranean) characterized by wide temporal variability over an annual cycle. As indicated by multivariate analyses, the seasonal dynamics can be summarized as follows: (1) a late winter-early spring phytoplankton bloom followed by high zooplankton and macrobenthos densities during the spring months; (2) low-quality particulate suspended matter in summer, and an increase in the importance of zooplankton taxa with a wide range of feeding strategies, a decrease in macrofaunal abundance, and an increase in deposit-feeders and predators; and (3) a second phytoplankton bloom in autumn, followed by an increase in copepod density and a low macrofaunal abundance. In conclusion, pelagic and benthic communities in the coastal area of the Ligurian Sea mainly seem to be controlled bottom-up. Our results suggest that the quality of the particulate organic matter may play an important role in determining the temporal changes of both plankton and benthic assemblages, while the direct influence of other environmental features (such as sediment grain size) is relevant only for some macrobenthic taxa (e.g. crustaceans).     

Origin and Distribution of Suspended Organic Matter As Inferred From Carbon Isotope Composition in A Mediterranean Semi-Enclosed Marine System

The origin and distribution of suspended organic matter, the trophic features and the stable carbon isotopic composition of particulate organic carbon (POC) were studied monthly in a Western Mediterranean semi-enclosed basin. Sampling stations were selected as a function of wind-exposure and the degree of vegetation cover and then compared with an adjacent unvegetated site. the predominant vegetation was seagrass (Posidonia oceanica and Cymodocea nodosa) and Caulerpa prolifera. Water samples were analyzed for total suspended matter (inorganic and organic fractions), photosynthetic pigments (chlorophyll-a and phaeopigments), dissolved organic carbon, particulate organic carbon and their isotopic composition. Temperature and salinity were also measured at the same sampling sites within range of Mediterranean limits. the suspended organic matter concentration was 1.77 ± 1.55 mg l^-1; the chlorophyll-a concentration was low (0.35 ± 0.24 μg l^-1); the disolved organic carbon concentration was 2,140 ± 2,010 μg l^-1; the particulate organic carbon concentration was 212 ± 106 μg l^-1 and the isotopic composition was 18.77 ± 2.51%_°. There were significant temporal differences except for phaeopigments, POC and its POC isotopic composition, and there were no spatial differences other than for δ¹³C. This picture highlighted a general seasonal trend and trophical features similar to adjacent sea.

Spatial differences in δ¹³C showed that the source of suspended organic matter was different between stations as that between sources and wind-hydrodynamic constraints. In     

Comparison of the sedimentation of a diatom spring bloom and of a subsurface chlorophyll maximum

The seasonal pattern of sedimentation was determined over a 8-mo investigation period covering the productive season at a permanent station in the Southern Kattegat (Denmark) in 1990. The phytoplankton succession was characterised by a 4-mo long subsurface maximum associated with the pycnocline which was entirely dominated by the dinoflagellate Gyrodinium aureolum. The bulk sedimentation of organic matter took place during this period and accounted for >60% of the annual particulate organic carbon (POC) and particulate organic nitrogen (PON) sedimentation. The spring bloom period contributed 60% of the sedimentation of intact phytoplankton cells, but only 20% of the POC and PON sedimentation. A minor fraction of the sedimenting matter from the subsurface phytoplankton maximum consisted of intact phytoplankton (<20%), suggesting that the phytoplankton was processed by heterotrophs and that it was mainly products from this activity which contributed to thevertical flux of organic matter. The variation in oxygen concentration below the pycnocline coincided with the pattern of sedimentation with a delay of 3 to 6 wk.     

Sources and biochemical composition of suspended particulate material in a submarine cave with sulphur water springs

Sources, biochemical composition and nutritional value of suspended particulate material were investigated from February 1994 to February 1995 in a submarine cave (Grotta Azzurra, Capo Palinuro, southwestern Italy) with hot sulphur springs and associated mats of chemolithoautotrophic bacteria in its innermost dark part (Snow Hall). Concentrations of total suspended material (TSM), particulate inorganic material (PIM), organic carbon (POC), organic nitrogen (PON), chlorophyll a (Chl a), phaeopigments (Phaeo), carbohydrates (TCH), proteins (TPR) and lipids (TLI) were measured at four stations along an outside–inside transect, in order to address whether the quantity and quality of suspended particles varied over time with increasing distance from the entrance of the cave and estimate the relative contribution of chemosynthesis versus photosynthesis in supplying POC to cave heterotrophs. The abundance and biochemical composition of suspended material available to filter-feeders varied over time, but no significant quantitative or qualitative differences were detected along the outside–inside transect. Concentrations of TSM and of its different compounds (PIM, POC, PON, Phaeo, TCH, TPR, TLI) were homogeneous among the four stations or fluctuated without any consistent trend, with no apparent ageing and degradation of organic material in the innermost dark station. Conversely, concentrations of Chl a significantly decreased from outside to inside the cave at all sampling periods. It is suggested that suspended POC in Grotta Azzurra consists of mixed assemblages of particles coming from advection of photosynthetic material from the open sea and local inputs of carbon by sulphur-oxidizing bacteria. Based on POC/Chl a ratios, the relative contributions of chemosynthesis versus photosynthesis in supplying POC to benthic heterotrophs was estimated to be 31 and 69%, respectively. Received: 9 December 1996 / Accepted: 14 January 1997     

The quantity,composition and distribution of suspended particulate matter in the Gulf of California

The distribution and composition of suspended particulate matter in the sea is very complex and not well understood. In this study, 3 different approaches were used to estimate the quantity and quality of suspended particulate matter in 34 samples from the euphotic zone of 9 stations in the Gulf of California. The results from electronic, microscopic and chemical analyses showed that most parameters measured were significantly correlated, e.g. the total particulate volume from particles of 2 to 150 diameter, as obtained from the Coulter Counter, proved to be significantly related to such parameters as seston, particulate nitrogen, particulate carbon, phytoplankton carbon and chlorophylla. It can be concluded from this study that the Coulter Counter can be a very useful instrument to determine, with little effort, the size, distribution, and volume of particulate suspended matter in the sea. These data can then be used to calculate some important biological parameters which are necessary to establish meaningful models of phytoplankton production. More detailed studies are necessary to prove the above mentioned relationships in depth, space and time.Contribution from the Scripps Institution of Oceanography, University of California, San Diego. This work was completed under the Scripps Tuna Oceanography Program supported by the Bureau of Commercial Fisheries under Contract No. 14-17-0007-963. Ship time was provided from National Science Foundation funds.     

Suspended particulates in the Irish Sea and feeding conditions for fish larvae

The size distributions (2 to 160 m equivalent spherical diameter) of suspended particulate material sampled on two cruises along a transect in the Irish Sea in 1988 are described in relation to hydrographic conditions, chlorophylla concentration and carbon to nitrogen ratios. Particulates were more abundant and larger size modes in the distribution were more evident, in the upper mixed layer of stratified water than in areas where the water column was fully mixed. The detrital content was estimated at 52% of total particulate matter above the thermocline in stratified regions and at around 97% at mixed water sites. In stratified regions the predominance of larger sized phytoplankton and lower levels of detritus is argued to support a more direct and efficient transfer of energy to fish larvae via larger sizes of copepods. Conversely, in mixed areas of high detrital loading the smaller size spectrum of particulates incorporates a less efficient transfer of energy through bacterial cycling and smaller copepods.     

Is kelp detritus a good food for suspension feeders? Effects of kelp species, age and secondary metabolites

The food quality of detrital particles derived from three species of kelps was evaluated in a laboratory feeding experiment utilizing two species of suspension feeders, the serpulid polychaete Pseudochitonopomaoccidentalis and the mussel Mytilustrossulus. Fresh and aged kelp particles were also evaluated, and growth in all treatments was compared to growth on ad libidum phytoplankton rations. Fresh particles from Laminariagroenlandica, aged particles from Agarumfimbriatum and Alariamarginata, and mixed phytoplankton promoted the highest growth rates in both consumers. Growth was inversely related to total polyphenolic concentration in the fresh kelp particles. The increase in quality of both Agarumfimbriatum and Alariamarginata particles with age corresponded with a rapid loss of polyphenolic secondary metabolites and an increase in total nitrogen. Received: 26 November 1996 / Accepted: 9 January 1997     

Mass encystment and sinking of dinoflagellates during a spring bloom

The decline of a spring bloom dominated by dinoflagellates and the mass sedimentation of dinoflagellate cysts was documented in a coastal area of the northern Baltic Sea, SW Finland in 1983. The exceptionally large spring phytoplankton bloom was observed in early May. After depletion of nitrate phytoplankton biomass declined rapidly. The bloom was followed by intense sedimentation of spherical cysts and of organic matter at the end of May. These cysts were presumably hypnozygotes of Peridinium hangoei Schiller. Sedimentation of dinoflagellate cysts was estimated to correspond to ca. 45% of the maximum sedimentation of particulate organic carbon at this time, although most of the dinoflagellate biomass disintegrated already in the water column and was deposited as organic detritus or washed away by advection. It is concluded that the life cycle strategies of the dominant vernal phytoplankton species have a major impact on the sedimentation of the spring bloom.     

Biomass of suspended bacteria over coral reefs

The biomass of bacteria suspended in water flowing over coral reefs at Lizard Island and Yonge Reef (Northern Great Barrier Reef) was estimated by measurement of muramic acid. Values ranged from 20 mg C m^-3 in the open water up to about 60 mg C m^-3 over the reef flat. Direct counts of total numbers of free bacteria were made for comparison. Values of around 2.0x10⁹ cells g^-1 muramic acid showed that there was a good agreement between direct counts and muramic acid content of free bacteria in the open water. In samples containing suspended particulate matter, ratios of direct counts to muramic acid concentration were lower, because bacteria on particles could not be counted. Thus, these ratios were used to indicate the proportions of bacteria attached to particles. Changes in the biomass and numbers of bacteria were determined in water masses identified either by a drogue or fluorescein, as they moved across the reefs. In the zone on the outside of the reef, the number of free bacteria decreased compared to open sea water, but total biomass increased, showing that particulate matter containing bacteria was thrown up into suspension. About 50% of bacteria were attached to particles. Water flowing over the reef flats contained much particulate material with bacteria attached. Bacteria constituted between about 5 and 20% of particulate organic carbon.     

Hydrographic and biological aspects of algal bloom in the Adriatic Sea in 1988

The interaction of hydrological, chemical and biological factors in a defined time may cause an excess bloom of phytoplankton in an ecosystem, which can persist over a long period or occur from time to time.

This phenomenon has been recorded from the northern Adriatic and coastal enclosed basins along the eastern Adriatic coast (Ka?tela Bay, Split) for a long time now.

During 1988 this phenomenon of increased production was recorded from almost the entire northern and middle Adriatic with some traces even in its southern part. Some preliminary results for that period point to the fact that this was caused very likely by exceptionally warm and dry weather, with no significant precipitations and wind. Such conditions affect horizontal and vertical stratification of water masses.

High photosynthetic radiation, increased quantities of alochthonous organic matter, either in the form of particulate or dissolved organic matter, either in the form of particulate or dissolved organic carbon (POC or DOC), influx of nutrients (via rivers, municipal sewage or upwelling) are the main causes of sudden bloom and rapid reproduction rate of phytoplankton species (Diatoms) to the large quantities to which the pollution of the entire coastal area is due.     

Excretion of ammonia by zooplankton and its potential contribution to nitrogen requirements for primary production in the Catalan Sea (NW Mediterranean)

Excretion of ammonia by mesozooplankton (>200 m zooplankton) and its potential contribution to the nitrogen requirement for phytoplankton growth has been estimated for different hydrographical situations along a transect across the Catalan Sea (Northwestern Mediterranean). The nitrogen excreted as ammonia was estimated from mesozooplankton biomass and specific excretion rates. Nitrogen requirements of phytoplankton were estimated by means of carbon fixation rates and C:N ratios of <200 m particulate organic matter. Minimum C:N ratios and maximum primary production, zooplankton biomass, phytoplankton nitrogen requirements, and nitrogen excretion of zooplankton occurred near the Catalan density front. On average, the nitrogen regenerated by the mesozooplankton accounted for 43% of the nitrogen requirements of the phytoplankton. The specific excretion rates of ammonia and the percentage of phytoplanktonnitrogen requirements supplied by excreted nitrogen were higher at coastal stations. In some coastal and frontal stations, the ammonia excreted exceeded the phytoplanktonnitrogen demand. Bacteria competing for nutrient supply and the possible uncoupling between rate processes and standing stocks of phyto- and zooplankton could explain the apparent excess of regenerated ammonia.     

Changes in the Biochemical Composition of Tetraselmis Suecia and Isochrysis Galbana During Growth and Decay

The variations of the biochemical composition of Tetraselmis suecica and Isochrysis galbana during growth and decay were determined. the content of chlorophyll a (Chl-a) of the cultures, as expected, slowly degraded into phaeopigments during decay, confirming that chlorophyll measurements do not always provide an accurate estimate of phytoplanktonic biomass and, consequently, may fail if used to measure the food availability of particulate matter for consumers. Measurements of total amounts of proteins, carbohydrates and lipids, related to the nutritional value of particles in terms of caloric content, are shown to provide information on the readily available food for consumers, particularly during the blooms. the protein/carbohydrate, C/N and POC/Chl-a ratios were used to evaluate the differences between these two species during the growth and the decomposition processes. A comparison between experimental and field conditions was undertaken to implement our understanding of the growth and degradation processes of particulate organic matter of phytoplanktonic origin in the sea and its role on natural systems, during and after phytoplankton blooms.     

Temporal and spatial variability in stable isotope ratios of SPM link to local hydrography and longer term SPM averages suggest heavy dependence of mussels on nearshore production

Temporal changes in hydrography affect suspended particulate matter (SPM) composition and distribution in coastal systems, potentially influencing the diets of suspension feeders. Temporal variation in SPM and in the diet of the mussel Perna perna, were investigated using stable isotope analysis. The δ¹³C and δ¹⁵ N ratios of SPM, mussels and macroalgae were determined monthly, with SPM samples collected along a 10 km onshore–offshore transect, over 14 months at Kenton-on-Sea, on the south coast of South Africa. Clear nearshore (0 km) to offshore (10 km) carbon depletion gradients were seen in SPM during all months and extended for 50 km offshore on one occasion. Carbon enrichment of coastal SPM in winter (June–August 2004 and May 2005) indicated temporal changes in the nearshore detrital pool, presumably reflecting changes in macroalgal detritus, linked to local changes in coastal hydrography and algal seasonality. Nitrogen patterns were less clear, with SPM enrichment seen between July and October 2004 from 0 to 10 km. Nearshore SPM demonstrated cyclical patterns in carbon over 24-h periods that correlated closely with tidal cycles and mussel carbon signatures, sampled monthly, demonstrated fluctuations that could not be correlated to seasonal or monthly changes in SPM. Macroalgae showed extreme variability in isotopic signatures, with no discernable patterns. IsoSource mixing models indicated over 50% reliance of mussel tissue on nearshore carbon, highlighting the importance of nearshore SPM in mussel diet. Overall, carbon variation in SPM at both large and small temporal scales can be related to hydrographic processes, but is masked in mussels by long-term isotope integration.     

Suspended particulate organic matter in a Mediterranean submarine cave

Submarine caves display a paucity of benthic density and biomass that may be related to low trophic resources. Analysis of organic carbon, organic nitrogen, carbohydrate, protein and lipid content of suspended particulate matter was made during the period July 1985–July 1987 in a Mediterranean cave (Marseille, France) in order to determine any differences in the particulate organic matter composition along an horizontal transect. Particulate organic matter content clearly declined from the entrance of the cave to the dark inner area. This impoverishment could not be explained by a simple decrease in a few organic compounds, but appeared to be related to the combination of a decrease in both the amount and the composition of the suspended particles. Three progressive levels of impoverishment were identified towards the dark inner area of the cave: (i) decreasing amounts of seston; (ii) decreasing organic content of particles; (iii) increasing proportions of the geopolymeric (i.e., humic) components in the remaining organic matter, indicating increased degradation. The cave appeared to be sharply divided into two distinct sections — a twilight outer section whose waters contained slightly lower amounts of particulate organic matter than the open sea, and a dark inner section, 8 to 10 m higher, separated from the outer section by a steep rise and with waters of low organic matter content. The water in the twilight section appeared to be in thermal equilibrium with the open sea, and that in the dark inner section displayed thermal stratification. These differences indicated the presence of two distinct water bodies with contrasting average residence times, estimated as 1 d in the outer twilight section and 8 d in the dark inner section. The joint action of sedimentation and degradation resulted in an abrupt depletion of particulate organic matter in the dark inner section accompanied by a decrease in the benthic fauna. The decline in benthic heterotroph populations is probably related to the abrupt transition to oligotrophic conditions.