Robert N. Head

STERYL CHLORIN ESTERS ARE FORMED BY ZOOPLANKTON HERBIVORY

Steryl chlorin esters (SCEs) were formed in laboratory feeding experiments when starved females of the copepod Calanus helgolandicus were allowed to graze on a culture of the diatom Thalassiosira weissflogii. They were found when the zooplankton had grazed for 48 hours and were also identified in fecal pellets subsequently left in seawater in the dark. The distribution contained the diatom sterols in approximately the same relative abundance as the free sterols in the substrate, as well as the most abundant copepod sterol, all esterified to the chlorophyll a degradation product, pyropheophorbide a. Hence, in studies aimed at using sedimentary SCE sterol distributions as indicators of phytoplankton community structure, cholesterol should not be considered since the cholesteryl ester of pyropheophorbide a was a significant component in the fecal pellet SCEs. The findings represent a step forward in unravelling the transformations undergone by chlorophyll a in aquatic environments, since the abundance and wide occurrence of sedimentary SCEs indicate that they are a significant preservational sink for the chlorophyll a biosynthesised in the photic zone.

Distribution and stability of steryl chlorin esters in copepod faecal pellets from diatom grazing

Abstract Steryl esters of the chlorophyll a transformation product pyrophaeophorbide a (SCEs), were formed when the copepod Calanus helgolandicus was allowed to graze on the marine diatom Thalassiosira weissflogii , confirming an earlier preliminary experiment. All of the animal and algal sterol components were found in the SCEs although there were distributional changes attributed to alteration prior to esterification to pyrophaeophorbide a , which resulted from utilisation by the copepod to produce cholesterol. This indicates that where there is insufficient cholesterol in the zooplankton diet in the natural environment, sedimentary SCE sterol distributions may not accurately reflect the original phytoplanktonic free sterol distribution. Although the concentration of SCEs dropped markedly as a result of pellet ageing, their abundance relative to the other major chlorins increased. This relative stability provides an explanation for a previous observation that SCEs in surface sediments are present in higher abundance than in the corresponding water column particulates and accounts for their significant abundance in the solvent extractable chlorins in sediments. Two novel products, tentatively proposed as the phytyl ester and the 24-methylcholesta-5,24(28)-dien-3β-yl ester of 13 2 -oxopyrophaeophorbide a were also observed in the pellets, although the widely occurring rearranged sedimentary chlorin 13 2 -hydroxychlorophyllone a (chlorophyllone) was only detected in aged pellets.

The hydrography and biology of a bloom of the coccolithophorid Emiliania huxleyi in the northern north sea

In June=July 1994 a study was made of a small bloom of the coccolithophorid Emiliania huxleyi in an area of the North Sea to the east of the Shetland Islands. Observations on the hydrography of the study area indicated the bloom was associated with Atlantic water and was confined to an area in which a stable shallow mixed layer had formed. There was no evidence to suggest association of horizontal physical structure with the bloom development. High cell densities of >1‐6 10 6 cells dm 3 , together with low concentrations of PIC (<50 m gd m 3 ) and detached liths (2‐3 10 4 liths cm 3 ) indicated that the bloom was studied at an early stage of development. Biochemical and physiological observations indicated active growth was taking place. The results presented are discussed in comparison with previous studies carried out in both oceanic and shelf seas.

The relationship between suspended particulate material, phytoplankton and zooplankton during the retreat of the marginal ice zone in the Bellingshausen Sea

Abstract The distribution, abundance and composition of suspended particulates, phytoplankton and zooplankton biomass were investigated for the marginal ice zone in the Bellingshausen Sea during the Austral spring of 1992. Marked changes were observed between the amount and composition of particulates under the sea-ice and those in open waters. Measures of phytoplankton abundance (chlorophyll) ranged from ∼0.05 μg l −1 under the ice to 3 μg l −1 in the open waters to the north. The high nutrient concentrations and low level of phytoplankton under the ice suggest that this region is typical of over-wintering conditions and provides a suitable background comparison to the development of more productive, recently ice-free waters, further north. Nearly all basic measurements of particulate material showed a gradient from south to north. However, the biochemistry of particulates (lipid in particular), a more sensitive measure of environmental growth conditions, showed the area as a whole to be broadly split into two; under ice (light limited) and open water (no light limitation). Total particulate carbon was almost entirely composed of inorganic carbon under the ice; waters away from the ice edge also contained significant levels of inorganic carbon. Hence, C:chlorophyll was estimated from POC or phytoplankton carbon. There was no evidence of nutrient limitation at any of the stations investigated. Integrated phytoplankton carbon for the upper 100 m ranged from 0.1 gC m −2 under the ice to 11.5 gC m 2 in the diatom dominated bloom in open waters. The mesozooplankton biomass in the same depth interval increased from 0.02 gC m −2 under the ice to 0.21 gC m −2 in open waters. However, zooplankton biomass in the upper 600 m changed little from under the ice out to open waters north of the ice edge; although the northernmost station, in the region of highest chlorophyll, had approximately three times more zooplankton carbon (0.67 gC m −2 than the other stations. There is some evidence, however, that the northern station may be associated with a frontal feature, and that conditions observed may not be solely related to previous ice edge processes.

Physical, chemical and biological features of a cyclonic eddy in the region of 61°10'N 19°50'W in the North Atlantic

Abstract The second leg (CD61) of a two cruise investigation of coccolithophore biogeochemistry in the NE subarctic Atlantic provided the opportunity to make a detailed study of a cyclonic eddy in the vicinity of 61°N 20°W. The eddy field in the NE Atlantic is thought to be particularly important with regard to the physics of this region, and may influence the resulting chemical and biological properties of subarctic Atlantic waters. This eddy was ca. 50 km in diameter, moved at ca. 1.5 km d −1 to the north of east, with a geostrophic circulation around the feature of ca. 25 cm s −1 and probably extended as far as the ocean floor, where it may have interacted with the bottom topography. The horizontal salinity, nitrate and biological gradients between adjacent waters and the eddy were less marked in the present study than in a previous investigation of a cyclonic eddy in the vicinity of 48N 22°W (Mittelstaedt, 1987), possibly due to the surface waters of the eddy mixing with surrounding waters. Satellite image sequences clearly link this feature with those studied in a mesoscale coccolithophore bloom studied in the same region on a previous cruise (CD60). Rates of primary production within the eddy were almost twice the mean values reported for Ocean Weather Station India (OWSI) at this time of year, but were similar to those noted during studies at the MLML site to the SE of the eddy location. Other biological rate measurements also indicated that the NE sub-polar Atlantic in mid-summer is more active than previously thought. Despite the extensive coccolithophore bloom studied immediately previously by CD60, there was no measurable coccolithophore calcification in the waters within the eddy in the present study. This is consistent with phytoplankton taxonomic data, which demonstrates that coccolithophore abundance was almost one hundred fold lower at this location on CD61 relative to CD60 and that lith and coccolithophore abundances were grestest in the water column beneath the mixed layer, suggesting sinking. These observations suggest that the decline of the bloom had occurred in the period between the two cruises.

Discrimination against 4-methyl sterol uptake during steryl chlorin ester production by copepods

To investigate possible reasons for the discrepancy between the abundance of 4-methyl components in free sterols and steryl chlorin esters (SCEs) which is commonly observed in sediments, experiments in which the copepod Calanus helgolandicus grazed on the dinoflagellates Prorocentrum micans and Alexandrium tamarense were carried out. With P. micans all the algal sterols were esterified as SCEs in the faecal pellets, but there was clear discrimination against the uptake of 4-Me sterols. This discrimination was also evident with A. tamarense. To investigate changes in the faecal pellet SCE and the free sterol distributions during pellet ageing in the P. micans experiment, portions were allowed to stand for up to 29 days in seawater in the dark. Although degradation of SCEs occurred, their esterified sterol distribution remained unchanged. The free sterols were degraded more rapidly than the SCEs and changes in the distribution were observed. This provides further laboratory evidence that sedimentary SCE sterols are more robust markers of phytoplankton communities than the corresponding free sterol distributions. Comparison of the SCE abundance in sterilised and unsterilised pellets indicated that the degradation was a result of microbial activity.

Steryl esters of pyrophaeophorbide b: a sedimentary sink for chlorophyll b

The production of steryl chlorin esters (SCEs) of both pyrophaeophorbide a and pyrophaeophorbide b has been demonstrated in a laboratory experiment in which the copepod Calanus helgolandicus grazed on the prasinophyte Tetraselmis suecica. Both algal sterols were converted into SCEs a and b with equal efficiency, indicating that in this case the SCE sterols were directly representative of the substrate. The sterols of the animal were also converted to SCEs, although with some change in the distribution as a result of cholesterol demand by the copepod. The results show that during herbivory chlorophyll b undergoes the same biotransformation to SCEs as reported for chlorophyll a and predict that SCEs b should occur in sediments beneath a phytoplankton population containing a significant contribution of chlorophyll b.

Phytoplankton and mesozooplankton distribution and composition during transects of the Azores Subtropical Front

Two transects of the Azores Current centered on 221W were carried out, in August 1998 and April 1999, to investigate relations between hydrography, the composition and distribution of particulate material as potential food for zooplankton, and the mesozooplankton communities in the surface 200 m. Small flagellates and picoplankton generally dominated the phytoplankton biomass during both cruises. However, large diatoms, in particular Rhizosolenia stolterfothii, were significant contributors to total phytoplankton biomass south of the Azores Front in April. Mesozooplankton size fractionated biomass was higher in April than August, reflecting higher abundance of a number of genera, e.g., Calocalanus, Calanus, Clausocalanus, Ctenocalanus, Oithona, Corycaeus, and Oncaea. The proportion of mesozooplankton biomass in the size fractions 200–500, 500–1000 and 1000–2000mm was relatively uniform on both transects. The mesozooplankton communities were dominated by calanoid copepods, and the majority of significant species were most abundant to the north of the Azores Current. Neither mesozooplankton biomass or species composition was related to the concentrations of particulate matter. The results are discussed in relation to the food requirements of the mesozooplankton. The dominance of the phytoplankton standing stock by small cells suggests that other food sources in addition to phytoplankton must be important in the diet of mesozooplankton in the region of the Azores Front. r 2002 Elsevier Science Ltd. All rights reserved.

A technique for the in situ assessment of the vertical nitrogen flux caused by the diel vertical migration of zooplankton

Abstract It is argued that if herbivorous zooplankton exhibiting diel vertical migration generally feed only above the thermocline at night, but continue to excrete during the day when they are below the thermocline, then there should be a diel variation in their mean body nitrogen content. Thus measurement of the body nitrogen content of migrating zooplankton at different times of the day may provide a technique for the in situ assessment of the rate at which they remove nitrogen from the surface mixed layer. We show that if the inter-individual variation in body nitrogen content is high and sample size is low, then detection of the predicted diel change will tend to be obscured. However, in a simple laboratory experiment, a significant reduction in the mean body nitrogen content of the copepod Calanus helgolandicus was found between fed individuals (mean 12·91 μg N, n=42, SD =1·63) and those starved for 18 h (11.8911 μg N, n=30, SD= 1·29; t-test, t69=2·92, P=00047), suggesting that the technique may be applied in the field.