4.1 Résumé
Les chaetognathes, aussi connus sous le nom de « tigres du plancton » jouent un rôle important dans les communautés mésozooplanctoniques, en termes d'abondance et de biomasse. Bien que traditionnellement considérés comme étant strictement carnivores, des études récentes suggèrent l’usage de stratégies non -carnivores au sein du phylum. Les stratégies d'alimentation des chaetognathes de l’Arctique sont particulièrement intéressants comptes tenus de la forte saisonnalité dans les abondances et les distributions de proies connues (copépodes). Le présent chapitre traiter des stratégies d'alimentation saisonnières de deux principales espèces de l’Arctique : Eukrohnia hamata de la zone mésopélagique, ainsi que Parasagitta elegans de la zone épipélagique. Le contenu du tube digestif de spécimens récoltés au printemps, en été et en hiver au sud-est de la mer de Beaufort suggère de faibles taux de prédation chez toutes les espèces (0-0,27 proie ind. d-1). Toutefois, les taux de prédation de E. hamata et de P. elegans étaient plus élevés au printemps-été par rapport à l'automne-hiver. E. hamata mange des macroagrégats ne contenant pas de proies (˃500 µm), probablement de la neige marine, tout au long de l'année. Ce mode d'alimentation précédemment non répertorié a été confirmé pour E. hamata par des observations d'alimentation in-vitro dans la mer de Chukchi à l'automne, mais n’a pas été observé chez P.
elegans. Des quantités étonnamment élevées d’acides gras marqueurs de diatomées en
automne dans les spécimens de E. hamata suggèrent fortement que les diatomées ont été consommées directement, tandis que des ratios inférieurs à 18:1 (n-9)/(n-7), et valeurs inférieures de δ15N et δ13C comparativement au P. elegans ont tous suggéré son régime alimentaire plus omnivore. Nous suggérons que par la consommation de neige marine, les pelotes fécales du E. hamata pourraient contribuer davantage à la séquestration du carbone qu'on ne le pensait auparavant.
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4.2 Abstract
Chaetognaths, also known as the “tigers of the plankton” are important components of mesozooplankton communities, in terms of abundance and biomass. Although traditionally considered to be strict carnivores, recent studies suggest non-carnivorous strategies occur within the phylum. The feeding strategies of arctic chaetognaths are particularly interesting given the strong seasonality in the abundance of known food items, chiefly copepods. This chapter addresses the seasonal feeding strategies of two major Arctic species; the meso- pelagic Eukrohnia hamata and the epi-pelagic Parasagitta elegans. Gut contents of specimens collected in spring, summer and winter in the south-eastern Beaufort Sea suggested low predation rates in the two species (0 – 0.27 prey ind. d-1), but predation rates in E. hamata and P. elegans were higher in spring-summer compared to autumn-winter. E.
hamata ate non-prey macroaggregates (˃ 500 µm, probably marine snow) throughout the
year. This previously unreported feeding mode was confirmed for E. hamata by in-vitro feeding observations in the Chukchi Sea in the autumn. High levels of diatom fatty acid markers in E. hamata sampled in autumn, strongly suggested that diatoms were consumed directly, whilst lower 18:1 (n-9)/(n-7) ratios, δ15N and δ13C, compared to P. elegans confirmed its more omnivorous diet. We suggest that by containing algal-rich contents, the fecal pellets of E. hamata could contribute more to carbon sequestration than those of strict carnivores.
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4.3 Introduction
Chaetognaths, the so-called “tigers of the plankton” (Suthers et al. 2009), are semi-gelatinous zooplankters considered to be primary carnivores in the marine food web (Reeve 1970). Reported prey includes fish larvae, copepods and other chaetognaths (e.g. Alvarino 1965, Sullivan 1980, Brodeur & Terazaki 1999). In some locations, chaetognath populations could control copepod standing stocks (e.g. Sameoto 1972, Williams & Collins 1985). The fecal pellets of chaetognaths are said to be large and carbon-rich, and, considering the high abundances of chaetognaths in the ocean, these pellets could be important food for zooplankton in the water column or in the flux of carbon to the benthos (Dilling & Alldredge 1993, Giesecke et al. 2010).
The presence of algal cells (Alvarino 1965, Alvarez-Cadena 1993, Marazzo et al. 1997, Kruse et al. 2010), green detritus and phaeophytin (Philp 2007, Grigor et al. 2015) in chaetognath guts hints at non-carnivorous feeding. However, such vegetal remains are generally interpreted as originating in the guts of digested animal prey. Based on various aspects of chaetognath nutrition, notably in-vitro ingestion of seawater and active digestive processes in gut cells in the absence of visible gut content, Casanova et al. (2012) suggested that dissolved and fine particulate organic matter were major food sources for chaetognaths. Several studies on the polar chaetognaths Eukrohnia hamata, Parasagitta elegans and
Pseudosagitta maxima have reported on ˃90 % of individuals lacking visible gut prey (e.g.
Sameoto 1987, Øresland 1995, Froneman & Pakhomov 1998, Brodeur & Terazaki 1999, Bollen 2011, Giesecke & Gonzalez 2012, Grigor et al. 2015). For many polar zooplankters, the ability to switch between carnivorous and non-carnivorous feeding (e.g. detritivory or filtering) is an important adaptation to seasonal cycles in food availability (e.g. Auel et al. 2002, Hirche et al. 2003, Norkko et al. 2007, Søreide et al. 2008). Could the polar chaetognaths utilize similar seasonal adaptations, even though they do not obviously exhibit adaptations to filter phytoplankton?
Gut content analysis provides information on recent feeding but the resulting interpretation of trophic relationships can be flawed if sampling or handling protocols damage the gut,
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induce regurgitation or defecation, or if feeding is biased by the concentration of predator and prey in the sampling gear (Baier & Purcell 1997 and references therein). Additional and likely less-biased longer-term information on trophic dynamics can be gained from the analysis of fatty acids and stable isotopes. Some fatty acids specific to a given prey typically persist in predators for weeks to months with little or no breakdown or transformation, and can be useful trophic markers (Dalsgaard et al. 2003, Arim & Naya 2003, Grigor et al. 2015). Nitrogen isotopes are routinely used to infer trophic levels due to a step-wise increase in 15N/14N ratio (δ15N) with every ascending trophic level (Minagawa & Wada 1984, Michener & Schell 1994). Carbon isotopes can be used to infer the source of primary productivity, given close similarities in the 13C/12C ratios (δ13C) of an animal and its food (Hobson & Welch 1992).
This study aims at understanding how food resources are utilized year-round by co-existing
Eukrohnia hamata and Parasagitta elegans in the Canadian and Alaskan Arctic. In the south-
eastern Beaufort Sea, gut contents are examined to reveal diets from November 2007 to August 2008. In addition, we compare the fatty acid and stable isotope signatures of E.
hamata and P. elegans in the north-eastern Chukchi Sea (Alaskan Arctic) and Baffin Bay
(Canadian Arctic), based on collections made in autumn 2014.
4.4 Method
4.4.1 Study areas
Sampling surveys were carried out on-board CCGS Amundsen between from 2007 to autumn 2008 in the Beaufort Sea, and in autumn 2014 in the Chukchi Sea and Baffin Bay (Figure 4.1). 127 stations were sampled at weekly (or higher) resolution, in the Amundsen Gulf (69- 72°N, 120-131°W) between November 2007 and August 2008 (Figure 4.1a and Appendix E- 1). 6 stations were sampled in the north-eastern Chukchi Sea (71-76°N, 144-168°W) in September 2014 (Figure 4.1b and Appendix E-2). 3 stations were sampled in the Baffin Bay region (67-72°N, 61-73°W) in early October 2014; single stations in Scott Inlet Fjord, in Gibbs Fjord and southern Baffin Bay (Figure 4.1c and Appendix E-2).
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The Amundsen Gulf is a bridge between the south-eastern part of the Beaufort Sea to the Canadian archipelago. This 400-km long gulf has a width of 170 km and a maximum depth ~630 m, and sea-ice cover between October and early June (Barber & Hanesiak 2004, Geoffroy et al. 2011). Three water masses prevail in the region; the nutrient-poor Pacific Mixed Layer (PML, 0-50 m depth; salinity ˂ 31.6 psu), the Pacific Halocline (PH, 50–200 m depth; 32.4-33.1 psu), and lastly the Atlantic Layer (AL, ˃200 m depth; ˃34 psu) (Carmack & Macdonald 2002). The Chukchi Sea (Alaska’s northernmost shelf sea) is a relatively shallow environment characterised by hotspots of high primary productivity stimulated by the northward invasion of nutrient-rich Pacific water (Hopcroft et al. 2004, Weingartner et al. 2005). Sinking phytoplankton may only be partly exploited by zooplankton, supporting benthic animals (Hopcroft et al. 2004 and references therein). On the eastern side of the Canadian archipelago lies the semi-enclosed basin of Baffin Bay, with numerous fjords along the east coast of Baffin Island. A unique hydrocarbon seep and chemolithic community occurs in Scott Inlet Fjord (DFO 2015), but the plankton communities in these fjords are poorly studied.
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Figure 4.1 Bathymetric maps of the Arctic Ocean, showing the positions of sampling stations
(black circles) in (a) Amundsen Gulf, (b) north-eastern Chukchi Sea and (c) Baffin Bay (SIF = Scott Inlet Fjord; GF = Gibbs Fjord; SBB = southern Baffin Bay). Details of sampling stations are shown in Appendices E-1 and E-2.
4.4.2 Sampling in the Amundsen Gulf
The biomass of chlorophyll a (˃0.7 µm) was used as an indicator of algae blooming. At 54 stations (Appendix E-1), seawater samples were collected from 7-12 depths at using the CTD rosette equipped with twenty-four 12 1 Niskin-type bottles (OceanTest Equipment). Methods for the determination of chlorophyll a (chl a) from seawater subsamples are outlined in Chapter 3 of this thesis.
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At 10 stations, zooplankton were sampled using a large square-conical net with a 1 m2 opening area, 200 µm mesh and a 2-L rigid cod-end. At 72 stations, zooplankton were sampled using a Hydrobios® Multinet, comprising 9 nets with 0.25 m2 apertures, 200 µm meshes and 2-L rigid cod-ends. Individual nets open and close sequentially to sample zooplankton from pre-selected depth intervals (Appendix E-1). Samplers were deployed to a depth of 10 m above the seabed, cod-end(s) first (non-filtering), and then hauled back to the surface at a constant velocity of 0.5 m s-1. For Multinet sampling, the upper 60 m and lower 60 m of the sampled water column were divided into three 20-m depth layers, and the remainder was divided into three equal layers. Samples were fixed in 4 % buffered formaldehyde-seawater solution.
4.4.3 Sampling in the Chukchi Sea and Baffin Bay
Square-conical nets (1 m2 opening areas, 2-L rigid cod-ends) were deployed to collect chaetognaths for fatty acid and stable isotope analyses (Appendix E-2). At 7 stations, epi- pelagic chaetognaths were sampled by oblique trawls of 500/750 µm-mesh nets at 90 m. The ship speed was approximately 1 m s-1, and the cable angle was ~60º. At 6 stations, meso- pelagic chaetognaths were sampled in vertical tows of 200/500 µm-mesh nets (see Amundsen Gulf sampling protocol). A maximum of 30 Eukrohnia hamata or Parasagitta elegans individuals were randomly removed from each sample, and kept frozen at -80 ºC for further processing.
4.4.4 Abundance of zooplankton
A total of 70 Multinet collections in the Amundsen Gulf (typically 9 samples per haul), 3 to 11 per month from November 2007 to July 2008, were analysed to estimate the abundance of main zooplankton species. Formalin-preserved samples were sieved into two size fractions; 0.2-1 mm and >1 mm. From each fraction, known subsamples were taken until 300 copepods were removed, and all animals in the subsamples were counted and identified to the lowest possible taxonomic level, typically species or stage. Abundances of species present in the water column were calculated (where possible), summing abundances of appropriate taxa in the two size fractions.
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4.4.5 Gut contents
A total of 21 net collections in the Amundsen Gulf were used to analyze chaetognath gut contents. 4742 chaetognaths were included in gut content analyses (4078 Eukrohnia hamata with body lengths between 2 and 39 mm and 654 Parasagitta elegans with body lengths between 2 and 42 mm). 10 Pseudosagitta maxima also present in the samples (15-65 mm) were not analysed further. Each individual chaetognath was stained with a solution of Borax Carmine to highlight potential prey tissues (Sameoto 1987). Gut contents were observed under the stereomicroscope (maximum magnification = 11.5×). Prey items were identified to the lowest taxonomic level possible, although accurate identifications of prey species were often difficult due to their augmented state of digestion. Prey items in the mouths of chaetognaths were ignored, as they are a likely reflection of the concentration of predator and prey in the sampling gear, and therefore biased. The average number of prey per chaetognath gut (npc) was estimated for each collection for the two species. Daily predation rates (DPR; number of prey items consumed ind.-1 d-1) were calculated after Bajkov (1935) [Eq. (4.1)].
DPR =npc x 24
tdig (4.1)
Where tdig = digestion time in hours at a suitable temperature (here ~0 ºC). For Eukrohnia hamata, we used 11 h, determined for this species in the Southern Ocean (Giesecke &
Gonzalez 2012). For Parasagitta elegans, we used 10.2 h, determined for this species in Massachusetts (Feigenbaum 1982).
To detect the presence of different food items, such as algae, the color of lipid droplets and detritus in the guts of 2118 Eukrohnia hamata and 201 Parasagitta elegans was also described. A limited subset of individuals (3 E. hamata and 3 P. elegans from single collections in November, February, May and July) were re-analysed using Scanning Electron Microscopy (SEM), to detect other evidence of recent feeding only visible at these high magnifications.
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4.4.6 Fatty acids
To determine the fatty acid profiles of Eukrohnia hamata in autumn (2014), we analysed samples of 5 pooled individuals from 1 station in the Chukchi Sea and 3 in the Baffin Bay region. We also analysed samples of 5 pooled Parasagitta elegans individuals from 3 stations in the Chukchi Sea and 2 in the Baffin Bay region (Appendix E-2). For E. hamata, three samples were analysed per station. For P. elegans, 1-3 samples were analysed per station. Samples were freeze-dried and an internal standard (5β cholanic acid, 1 µg) was added. Total lipids were extracted using a mixture of dichloromethane and methanol (2/1; 5 ml; 3×15 minute ultrasonications). The total lipid fraction was dried with N2 and MeOH-H2O KOH (80/20; 5 %; 3 ml) and was then added to the extract before heating at 90 ºC for 2 hours. The fraction containing the non-saponifiable lipids was obtained by liquid-liquid extraction, dried over Na2SO4, and derivatised using BSTFA (70 ºC, 30 minutes) prior to gas chromatography–mass spectrometry (GC-MS) analysis. The saponification mixture was acidified with HCl (10N; 2 ml) and the fatty acids extracted with hexane. Fatty acids were methylated (BF3MeOH; 20/80; 80 ºC; 30 minutes) prior to identification and quantification of 20 fatty acid methyl esters previously reported in Grigor et al. (2015), plus 20:1 (n-7) by GC-MS. Results are given as relative percentages of the various fatty acids identified in specimens at each station. A higher 18:1 (n-9)/(n-7) ratio was used to indicate a greater tendency towards carnivory (Falk-Petersen et al. 1990, Wang et al. 2015). Higher proportions of the monounsaturated FAs Σ20:1+22:1 MUFA were taken to indicate a greater contribution of Calanus copepods in the diet (Falk-Petersen et al. 1987, Wang et al. 2015). A higher ratio of 16:1/16:0 was used to indicate a greater contribution of diatoms compared to flagellates (Nelson et al. 2001, Wang et al. 2015).
4.4.7 Carbon and nitrogen
To determine the stable isotope signatures of Eukrohnia hamata in autumn (2014), we analysed individuals from 3 stations each in the Chukchi Sea and Baffin Bay region. We also analysed Parasagitta elegans samples from 4 stations in the Chukchi Sea and 2 in the Baffin Bay region (Appendix E-2). These samples each contained up to 5 individuals. For E.
hamata, 3-6 samples were analysed per station. For P. elegans, 1-5 samples were analysed
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using an ECS 4010 Elemental Analyser/ZeroBlank Autosampler (Costech Analytical Technologies). Carbon and nitrogen masses were determined to the nearest microgram. Gases produced (N2 and CO2) after oxidation/reduction and water removal were separated using an internal GC column. Isotope ratios were measured by on-line continuous-flow isotope ratio mass spectrometry (IRMS) with a Thermo Electron Delta Advantage spectrometer operating the continuous-flow mode (Thermo Electron ConFlo III). Five of each of the standards (USGS40 and USGS41, Qi et al. 2003) were analysed at the beginning and end of each run. One standard was run for every 12 samples to check for combustion and correct any instrumental drift; isotope ratio errors were ±0.006 or better.
δ13C values were calculated for samples as changes in sample ratios of 13C/12C from those in the international standard Vienna Pee Dee Belemnite (13C/12C). δ15N values were calculated as changes in sample ratios of 15N/14N from those in the standard AIR (15N/14N), as in [Eq. (4.2)].
δ13C or δ15N = [(𝑅
sample/𝑅standard) − 1]×1000 (4.2)
Where R = ratio of 13C/12C or 15N/14N
Trophic levels (TLs) of samples were calculated as changes in δ15N values from that of a typical food-web baseline (TL = 1 in Eq. 4.3). We used particulate organic matter (POM) as the baseline, in line with other arctic studies (e.g. Iken et al. 2005, Bergmann et al. 2009, Roy et al. 2015).
TL = 1 +(δ15Nsample−δ15Nbaseline) δ15N
enrichment per TL (4.3)
Where 1 = TL of POM. We used a δ15Nbaseline value of 6.8 ‰, previously reported for shallow water POM (˂50 m) in the North Water Polynya in late spring/summer of 1998 (Hobson et al. 2002). It was assumed that δ15N values increased by 3.8 ‰ with every ascending trophic level (Hobson & Welch 1992, Hobson et al. 2002, Connelly et al. 2014).
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4.5 Results
4.5.1 Amundsen Gulf
4.5.1.1 Phenology of algae blooms
Based on chlorophyll a biomass, a bloom of ice algae was first detected in late March in the PML (0-60 m), peaked at these depths in late April and early May (chl a biomass ˃ 5 mg m- 3), and succeeded by a surface bloom of phytoplankton (10 mg chl a m-3). In July, phytoplankton penetrated the PH (60-200 m), and phytoplankton biomass peaked in mid-July in the PML (˃ 16 mg chl a m-3; Figure 4.2).
Figure 4.2 Vertical distributions of chlorophyll a biomass (mg m-3) in the upper 200 m of the water column along the ship track from November 2007 to July 2008 in the Amundsen Gulf. Black dots indicate sampling depths. Details of sampling stations are shown in Appendix E- 1. Chl a data were provided by Michel Gosselin (Université du Québec à Rimouski).
4.5.1.2 Zooplankton community
A total of 119 zoo- and icthyoplankton species and 57 other taxa not identifiable to the species level were detected in the Amundsen Gulf collections from November 2007 to July 2008. Numerically, the mesozooplankton was dominated by the usual guild of copepods (Oithona
similis, Triconia borealis, Metridia longa, Calanus glacialis, Microcalanus pygmaeus, Microcalanus pusillus, Pseudocalanus elongatus, Pseudocalanus minutus and Calanus hyperboreus), which were present at all stations (Table 4.1). Eukrohnia hamata was also
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present at all stations in the Amundsen Gulf with abundances ranging from 4 m-2 in late March to 3318 m-2 in late July. Parasagitta elegans was less abundant than E. hamata at all stations in Amundsen Gulf (0 m-2 in December to 1927 m-2 in June).
Table 4.1 Composition of the mesozooplankton (30 most abundant taxa) sampled in the Amundsen Gulf from November 2007 to July 2008, based on data from 70 Multinet hauls. *Taxa could not be identified to species level.
Species / taxon n hauls present Mean number m-2 (± 1 SD) Date of peak abundance Oithona similis 70 30475±14290 27/01/08 Triconia borealis 70 17583±12191 17/04/08 Metridia longa 70 15402±10758 05/03/08 Calanus glacialis 70 10255±6702 27/01/08 Microcalanus pygmaeus 70 4818±3750 27/01/08 Microcalanus pusillus 67 4603±3260 19/04/08 Pseudocalanus elongatus 70 4085±3241 27/01/08 Pseudocalanus minutus 70 3431±2874 05/12/07 Cyclopina sp.* 51 1968±2308 10/06/08 Calanus hyperboreus 70 1962±1565 19/04/08 Radiolarians* 66 1053±2262 27/05/08 Scolecithricella minor 70 798±752 18/04/08 Triconia parila/notopus 18 787±1745 01/05/08 Frittilaria sp.* 18 648±3756 06/05/08 Limacina helicina 66 608±964 01/07/08 Clione limacina 68 606±563 03/03/08 Spinocalanus longicornis 66 540±758 27/01/08 Pseudocalanus acuspes 59 467±1305 27/01/08 Eukrohnia hamata 70 456±565 23/07/08 Microcalanus sp.* 18 381±2957 25/07/08 Pseudocalanus sp.* 27 307±830 01/07/08 Boroecia maxima 69 295±274 19/04/08 Bivalves* 27 282±938 23/11/07 Aetideopsis rostrata 55 277±443 19/04/08 Oikopleura sp. 43 273±660 01/07/08 Dimophyes arctica 50 273±386 05/03/08 Paraeuchaeta glacialis 70 253±519 28/06/08 Aglantha digitale 68 252±308 27/07/08 Gaetanus tenuispinus 57 212±260 18/04/08 Parasagitta elegans 68 173±339 10/06/08
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4.5.1.3 Visible prey items and predation rates
Prey organisms were detected in 0.9 % of Eukrohnia hamata and 0.8 % of Parasagitta
elegans in the Amundsen Gulf. Copepods comprised 97 % of total prey items in E. hamata
and all detected prey items in P. elegans. Identifiable prey taxa were Oithona similis, Calanus spp., a female Pseudocalanus spp. (Figure 4.3), and a chaetognath in one E. hamata individual. Multiple copepods (2 or 3) were detected in five chaetognaths. Daily predation rates (DPRs) were in the range 0-0.27 prey ind. d-1 for E. hamata, and 0-0.09 prey ind. d-1 for P. elegans. In general, predation rates were higher on dates in November and December
compared to dates in spring and summer. Average npc values were generally higher in chaetognaths from Multinet collections compared to square-conical net collections, but note that collections from the latter gear were often considerably larger (Figure 4.3).
Figure 4.3 Average number of prey per chaetognath gut (npc) in square-conical (S-C) net
and Multinet collections from November 2007 to August 2008 in the Amundsen Gulf. Numbers of individuals shown above data points. k is the number of collections. Inset bottom: photograph of a Parasagitta elegans specimen (12 mm) with a relatively large Pseudocalanus
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4.5.1.4 Lipid droplets and detritus
Lipid droplets occurred in the body cavities of 72 % of 3408 analysed Eukrohnia hamata. Amounts were substantial though not quantified. The fraction of the E. hamata population with oil in the body cavity peaked between mid-January and early-May (74-87 %), thereafter declining quickly to below 50 % in late-May and early-June. Low percentages were observed in December (51 %). Droplets were typically yellow in color between late November and early March (72-90 % of droplets), but from then until late July were typically green in color (58-92 % of droplets). In addition to occurring in the guts of only 4 % of 497 Parasagitta
elegans, lipid droplets were smaller and less abundant in this species. Whilst detritus (mainly
crustaceous debris) was observed in the guts of 12 % of Parasagitta elegans, 38 % of
Eukrohnia hamata contained detritus in their guts, with 79 % of these containing thick green