Calcification response of reef corals to seasonal upwelling in the northern Arabian Sea (Masirah Island, Oman)

Tropical shallow-water reefs are the most diverse ecosystem in the ocean. Its persistence rests upon adequate calcification rates of the reef building biota, such as reef corals. The calcification mechanism of reef corals performs best in oligotrophic environments with high seawater saturation states of aragonite (  sw ), leading to an increased vulnerability to anthropogenic ocean acidification and eutrophication. The calcification response of reef corals on rapid changes in  sw and seawater nutrient concentrations is currently under discussion in coral science. Here we present Porites calcification records from the northern 15 Arabian Sea upwelling zone and investigate the coral calcification response to low  sw and high nutrient concentrations due to seasonal upwelling. Calcification rate was determined from the product of skeletal extension and bulk density. Skeletal Ba/Ca and Li/Mg proxy data allowed to identify skeletal portions calcified during upwelling and non-upwelling seasons, respectively, and to reconstruct growth temperatures. With regard to sub-annual calcification patterns, our results demonstrate compromised calcification rate during the upwelling season. This is due to declining extension rate, which we attribute to light 20 dimming caused by high primary production. Interestingly, skeletal density shows no relationship with temporally low  sw during upwelling


Introduction
Tropical coral reefs are the most diverse aquatic ecosystem on the planet (Hughes et al., 2017). Their basic building blocks are 30 the calcareous (aragonite) skeletons of symbiotic scleractinian corals (reef corals) and sustained precipitation of skeletal carbonate (calcification) is fundamental for maintaining their structure and function (Perry et al., 2012). Optimum calcification of reef corals is found in oligotrophic water masses of temperatures between 21 °C to 29.5 °C and a saturation state of seawater with respect to aragonite >3.3 (Ωsw) (Kleypas et al., 1999). Anthropogenic greenhouse gas emissions threaten reef coral calcification by increasing sea surface temperature (SST) and by causing ocean acidification. In addition, land use, discharge 35 of wastewater and fish farming turn the near-shore shallow-marine environments towards more eutrophic conditions. The responses of reef coral calcification to the rapidly changing environment are highly variable and remain a matter of intense research (Cornwall et al., 2021;Guan et al., 2020;Hall et al., 2018).
Coral calcification rate (g cm -2 yr -1 ) is the product of linear extension rate (cm yr -1 ) measured along the axis of maximum growth and bulk density (g cm -3 ) (Dodge and Brass, 1984). Linear extension rate in reef corals is linked to efficiency of 40 photosynthesis in symbiotic micro-algae, providing energy to the host for upward growth of the skeleton (Muscatin et al., 1981;Sun et al., 2008). Availability of light and water temperature are the main drivers controlling photosynthetic efficiency and thus positively related to the extension rate (Al-Rousan, 2012;Logan and Tomascik, 1991;Lough and Barnes, 2000). At certain taxon-specific threshold temperatures, however, extension rate declines rapidly due to thermal stress for the microalgae symbionts (Cantin et al., 2010). Due to sub-annual variations in extension rate, skeletal portions of high and low density 45 are formed, with high-density bands (HDBs) coinciding with low extension rate (and vice versa for low-density bands = LDBs) (DeCarlo and Cohen, 2017;Highsmith, 1979;Klein and Loya, 1991;Knutson et al., 1972). In addition, aragonite saturation of the calcifying fluid (cf) determines bulk skeletal density (Mollica et al., 2018). cf is approximately five times higher than the aragonite saturation of the external seawater (sw) and long-term changes in sw due to ocean acidification lead to declining cf D'Olivo et al., 2019). Intra-annually, corals are, however, able to maintain relatively stable levels 50 of cf largely independent of short term variations in sw by upregulating their internal pHcf and DICcf pool DeCarlo et al., 2018;McCulloch, 2017, Ross et al., 2019a).
Near-shore coral reefs are increasingly exposed to anthropogenic eutrophication induced by land use, fish farming and sewage disposal (Lapointe and Clark, 1992;Chen et al., 2019;Chen and Yu, 2011). Eutrophication can have both beneficial as well as detrimental effects on coral growth, depending on the kind of available nutrients and their concentration (Tomascik and 55 Sander, 1985;Tomascik, 1990). In general, reef corals are highly adapted to oligotrophic waters with micro-algae symbionts to allow an efficient use of essential nutrients (Muscatine and Porter, 1977). This enables outcompeting other fast-growing biota on a reef whose growth is inhibited by the undersupply of nutrients (Vermeij et al., 2010;Barott and Rohwer, 2012).
Strong eutrophication disturbs this adaptive advantage, leading to harmful algal blooms followed by reef coral mass mortality (Al Shehhi et al., 2014) and reef destruction due to the increasing abundance of bioeroders (Hallock, 1988). However, moderate 60 increases of certain nutrients such as ortho-phosphate (PO4 3-) have been shown to promote linear extension rate Dunn et al., 2012;Koop et al., 2001). The opposite effect is reported for nitrate (NO3 -), even though the calcification response is less pronounced compared to PO4 3- (Koop et al., 2001). In general, increasing eutrophy is considered to cause reef corals to sacrifice skeletal density for increased extension rate ("stretching modulation of skeletal growth"), which can either lead to enhanced, constant or reduced rates of calcification (Carricart-Ganivet and Merino, 2001;Carricart-Ganivet, 65 2004;D'Olivo et al., 2013;Manzello et al., 2015).
Understanding how coral calcification responds to rapid changes in seawater nutrient conditions and sw is critical for more accurate predictions on the persistence of reef habitats under the influence of global change. Tropical upwelling areas are well suited for this kind of research as the upwelling deep water masses are rich in nutrients and low in sw. This allows these regions to serve as natural laboratories to investigate the calcification response of reef corals to these multiple environmental 70 stressors that are likely to affect global coral reefs in the near future (Camp et al., 2018;Wizemann et al., 2018). Published calcification data of the major reef-building coral Porites spp. growing within regions affected by seasonal upwelling are generally sparse and only available from three Pacific reef sites (Manzello et al., 2014;Mollica et al., 2018).
In this study, we report the first calcification data (calcification rate, extension rate, skeletal bulk density) of six Porites coral specimens from the so far unexplored region of the northern Arabian Sea upwelling zone (Masirah Island, Oman). Three out 75 of the six samples that were considered representative for the site under study were selected for further geochemical analysis (Li/Mg, Ba/Ca). This facilitated the establishment of a detailed sub-annual chronology yielding monthly resolved records of calcification. This approach allowed us for an unprecedented comparison of sub-annual calcification performance between the upwelling and non-upwelling season. In this way, this study improves the general knowledge on seasonal and annual patterns of reef coral calcification under exposure to variable sw and nutrient concentrations, thereby contributes to more accurate 80 predictions on the persistence of reef habitats under the influence of global change.

Arabian Sea climate and oceanography
The Arabian Sea is the northwestern part of the Indian Ocean between India and the Arabian Peninsula (Fig. 1). The regional climate of the Arabian Sea is characterised by a semi-annual alternation of the prevailing wind directions (Beal et al., 2013). 85 During northern hemisphere summer, strong winds of the southwest monsoon cross the Arabian Sea in the direction of the low pressure system above the Tibetan Plateau (Findlater, 1969). During winter, reversal of the atmospheric pressure system causes the northeast monsoon (Hastenrath and Greischar, 1991). Low wind speeds without preferred orientation typically occur during the two intermonsoon seasons (spring intermonsoon, autumn intermonsoon) (Beal et al., 2013;Lee et al., 2000). Surface wind fields are the driving force behind the upper hydrospheric structure and the seasonal variation of the oceanic surface current 90 system (Swallow and Bruce, 1966). During summer, southwest monsoonal winds cause a strong coastal current (Oman Coastal Current), which runs northward parallel to the coast of the Arabian Peninsula and induces rigorous upwelling (Currie et al., 1973;Currie, 1992, Smith andBottero, 1977). Increased nutrient supply during southwest monsoonal upwelling is associated with an increased primary productivity in the euphotic zone (Anderson et al., 1992;Bauer et al., 1991;Quinn and Johnson, 1996) (Fig. 2). Compared to equatorial upwelling regions of the eastern Pacific, the northern Arabian Sea upwelling is 95 characterized by a high phosphate to nitrate ratio (Kleypas et al., 1999). But although nutrient supply is linked to upwelling, concentrations of PO4 3and NO3remain on a high level throughout the year, because of a prevailing iron limitation of the primary production (Mother Earths Iron Experiment) (Smith, 2001). Southwest monsoonal upwelling furthermore causes a drop in surface water pH, causing seawater aragonite saturation (Ωsw) to decrease temporally from 3.5 -4 during non-upwelling season to 3 during the upwelling season, which is well below critical values assumed to be required for coral growth (i.e., Ωsw 100 >3.3) (Kleypas et al., 1999;Omer, 2010;Takahashi et al., 2014) (Fig.2).

Coral growth and sample collection at Masirah Island
Coral growth off Oman occurs within six distinct provinces, including Marbat, the Kuria Muria Islands, Masirah Island and Barr al Hikman, the Capital Area, the Daymaniyat Islands and Musandam ( Fig. 1; Burt et al., 2016;Salm et al., 1993). The occurrence of corals at Masirah Island is limited to very shallow water depths of 1-4 metres (Glynn, 1993). Fast growing cabbage and brain corals such as Montipora and Platygyra are the dominant genera, but massive Porites are also present 120 (Coles, 1996).

Methodology 125
Coral samples were cut to slices of 6 mm thickness parallel to the axis of maximum growth using a rock saw at lowest rotation speed and equipped with a water-cooled diamond blade. Subsequent usage of a CNC mill ensured co-planarity with maximum deviations of 1-3 % over the entire slab. The slabs were ultrasonically cleaned in deionized water and dried at 40 °C. Coral slabs were X-rayed using a digital X-ray cabinet (SHR 50 V) to document alternating growth bands of high (HDB) and low density (LDB) (Knutson et al., 1972), biogenic borings, encrustations, and cementation. Sampling transects for all further 130 analyses were carefully selected so as not to be affected by bioerosion and encrustations but normal to HDBs and LDBs following trajectories of maximum linear extension (for the positioning of individual sampling transects see supplementary material, Fig. S1). Density measurements were performed using X-ray densitometry based on CoralXDS software (Helmle et al., 2002(Helmle et al., , 2011. Grey-scale -density calibrations were verified by measurements of standards for zero density (air,  = 0 g/cm -deviations were 0.02 ± 0.01 g cm -3 for zero density (air) and 0.03 ± 0.06 g cm -3 for massive aragonite (Tridacna shell). Width of density measurement transects were set to 4 mm, including a representative mixture of approximately 12 corallites (4 x 6 mm). Annual extension rate was estimated from the distance between two HDBs of maximum grey scale intensity on the radiographs. The mean annual skeletal density was calculated from the mean of all individual measurements along the transect and within one annual growth increment. 140 Corals 5.10, 5.13 and 5.21 were selected for Li/Mg and Ba/Ca geochemical analysis based on optimal orientation and traceability of the corallites adjacent to the density measurement transects (Fig. S1). Element concentrations were determined at the Institute for Geosciences, Johannes Gutenberg University Mainz (Germany), using an Agilent 7500ce inductively coupled plasma-mass spectrometer (ICP-MS) coupled to an ESI NWR193 ArF excimer laser ablation (LA) system equipped with a TwoVol2 ablation cell. The ArF LA system was operated at a pulse repetition rate of 10 Hz and an energy density of 145 ca. 3 J cm -². Ablation was carried out under a He atmosphere and the sample gas was mixed with Ar before entering the plasma.
Measurement spots with a beam diameter of 120 µm were aligned along transects in spot mode with a midpoint distance of 250 µm following discrete skeletal elements. Backgrounds were measured for 15 s prior to each ablation. Ablation time was 30 s, followed by 20 s of wash out. The isotopes monitored were 7 Li, 25 Mg, 43 Ca and 138 Ba. Signals were monitored in timeresolved mode and processed using an in-house Excel spreadsheet (Jochum et al., 2007).  (Table S1). Element concentrations for the samples are converted into molar ratios of Ca, i.e., Li/Ca, 160 Mg/Ca, Ba/Ca as well as Li/Mg. Li/Mg thermometry was used for estimating absolute growth temperatures (Montagna et al., 2014;Cuny-Guirriec et al., 2019, Ross et al., 2019b, Harthone et al., 2013Fowell et al., 2016;D'Olivo et al., 2018;Zinke et al., 2019). Li/Mg -SST relationships were shown to be site dependent, however, and similar Li/Mg-ratios produce differences in SST estimations of ~2 °C between inter-reef (Hathorne et al., 2013) and intra-reef settings (Fowell et al., 2016). Such spatial variability in Li/Mg -SST 165 relationships is likely due to poorly constrained effects of extension rate and seawater pH on skeletal Li/Mg ratios (Fowell et al., 2016;Fowell, 2017;Inoue et al., 2007;Tanaka et al., 2015). For this reason, we use a separate calibration of the Li/Mg thermometer for Masirah Island corals in order to overcome misleading SST estimates resulting from local seawater pH and extension rate effects associated with upwelling.
Ba/Ca ratios were used to identify skeletal portions calcified under upwelling conditions (Lea et al., 1989;Tudhope et al., 170 1996). Elevated Ba/Ca ratios in coral skeletons reflect high seawater Ba concentrations associated with upwelling deep waters, which are also nutrient-rich and acidic (Fallon et al., 1999;Montaggioni et al., 2006).
Daily Sea surface temperatures (SSTrem) were extracted from JPL MUR (v4.1) available from https://podaac.jpl.nasa.gov. The JPL MUR data range used for this study covers the period 2003-2018 and has a spatial resolution of 0.01° degrees (Grid cell: N19.90, E58.60). SSTs of equal calendar dates of consecutive years were averaged receiving one generalized annual record of 175 mean daily SSTs for the period 2003-2018 (Fig. 2). Reliability of the remote sensed data was confirmed by daily in-situ observed SSTs (SSTin-situ) recorded at the southern tip of Masirah Island (water depth: 5 m) between October 2001 and September 2002 (Wilson, 2007). Annual mean SSTs were in excellent agreement to each other (SSTrem = 25.79 °C ; SSTin-situ = 25.54 °C) and daily SSTs were strongly correlated (r 2 = 0.79 , p < 0.0001).

Data matching and age model development 
Records of skeletal density and element concentrations of corals 5.10, 5.13 and 5.21 were matched with optical microscope images allowing for the correlation of ablated spots from LA-ICP-MS with distinctive features on X-radiographs. Slight offsets between the x-axis of the LA-ICP-MS record and the density records can occur, because to some extent the LA-ICP-MS sampling paths were not ultimately straight due to following discrete corallites and avoiding bioerosion traces and 190 incrustations. To overcome this, the chronologies of the density records inferred from straight transects orientated parallel to the direction of growth were applied to the LA-ICP-MS records using AnalySeries software (Paillard et al., 1996).
Age models are based on Li/Mg-ratios in combination with Ba/Ca-ratios. Li/Mg is inversely related to temperature, which allows to identify the two warm (intermonsoon) and two cool (monsoon) seasons (Harthone et al., 2013). In order to identify the upwelling season among the two cool monsoons seasons (southwest monsoon, northeast monsoon), we use Ba/Ca ratios 195 as proxy (Tudhope et al., 1996) (Fig. 3). A detailed chronological frame for the Li/Mg records was established with the aid of the generalized annual record of remote sensing SST data (JPL MUR, daily averaged 2003 -2018) (Fig. 2). Dates of seasonal SST extremes as well as dates of inflection points between consecutive seasons were assigned to corresponding data points of the Li/Mg records (see supplementary material, Fig. S2). This methodology allows tuning the age models to a total of eight tie-points per year (two per season). Dates between tie-points were interpolated linearly and the entire time axis was resampled 200 to monthly intervals using AnalySeries software (Paillard et al., 1996). Accuracy of the age model was checked by comparing the timing of seasonal remote sensing SST maxima and minima of individual years with dates of the generalized annual record (daily averages 2003-2018) and was found to be ± 4 weeks during northeast monsoon, ± 3 weeks during spring intermonsoon and southwest monsoon, and ± 1.5 weeks during autumn intermonsoon.
Multi coral monthly means in Li/Mg are used for the calibration of the Li/Mg thermometer with mean monthly SSTsrem (averages of 2003-2018, JPL MUR) (Fig. 4). 83 % of the intra-annual multi coral monthly Li/Mg variation is explained by 225 temperature and the resultant SST-calibration is estimated as:

Pattern of calcification
Mean annual skeletal density, extension rate and calcification rate as well as the number of record years investigated within 240 each of the six individual coral specimens is shown in Table 2. Sub-annually resolved patterns of calcification of coral 5.10, 5.13 and 5.21 are expressed as multi-year monthly means (Fig.5). The variability in monthly calcification rate within all three specimens is strongly determined by extension rate (5.10: r 2 = 0.94, p < 0.0001; 5.13: r 2 = 0.77, p < 0.0002; 5.21: r 2 = 0.81, p < 0.0001). Coral 5.13 and 5.21 show three distinct peaks of highest extension rate in March, June and October. Reduced linear growth occurs during southwest monsoon, northeast monsoon as well as between April and May. Coral 5.10 slightly deviates 245 from the pattern of the two other specimens by a lower growth rate during June, leading to two peaks of maximum extension in February/March and October. In this specimen, a decrease in linear growth starts in spring intermonsoon and reaches its minimum during southwest monsoon.
Sub-annual variability in skeletal bulk density of coral specimens 5.10 and 5.13 show a pattern of two distinct high density bands (HDBs) between two bands of low density (LDB) within one annual growth increment (Fig.5). Skeletal portions of low 250 density were deposited during spring intermonsoon and autumn intermonsoon, high density portions formed during southwest monsoon and northwest monsoon. This alternating pattern of HDBs and LDBs causes skeletal density variation in coral 5.10 and 5.13 to be significantly inversely related to monthly reconstructed Li/Mg-SSTs (5.10: r 2 = 0.87, p < 0.0001; 5.13: r 2 = 0.34, p < 0.05) (Fig.6c). This is not the case for coral 5.21, as a well expressed LDB equivalent with autumn intermonsoon is lacking. Rather, it shows one wide LDB with density increasing from southwest monsoon to northeast monsoon. 255

270
Calcification rate during southwest monsoon is only 51 %, 96 % and 78 % of that observed during northwest monsoon (for coral 5.10, 5.13 and 5.21, respectively) ( Table 3; Fig.6a). This difference in seasonal calcification rate is related to low extension rate during southwest monsoonal upwelling, which is 49 %, 71 % and 91 % relative to that observed during northeast monsoon (for coral 5.10, 5.13 and 5.21, respectively). Interestingly, monthly extension rate during southwest monsoon exhibit a strong negative correlation with skeletal density across all three specimen (r 2 = 0.88, p = 0.0002), which is not the case during 275 northeast monsoon (r 2 = 0.04, p = 0.60) (Fig.7). According to individual extension rates during southwest monsoon, this produces varying density patterns across the three specimens, with corals 5.13 and 5.10 having the lowest extension rate during southwest monsoon showing the highest intra-annual skeletal density during this season (Table 3; Fig.5). Coral 5.21 that shows relatively fast extension growth during southwest monsoon compared to all other specimens only reveals a moderate increase in skeletal density, which remains below those observed during the northeast monsoon. 280 Table 3: Mean seasonal skeletal bulk density, linear extension rate and calcification rate (± 1) of three corals from Masirah Island.

Ba/Ca records
Multi-year monthly means in skeletal Ba/Ca are in excellent agreement with WOA18 nutrient data. This confirms Ba/Ca to be an appropriate proxy for intra-annual variability in seawater nutrient concentrations (Fallon et al., 1999;Montaggioni et al., 2006). Effects of riverine input on the skeletal Ba/Ca record can be excluded due to the absence of rivers in the arid region of north-eastern Oman (Alibert et al., 2003;Jiang et al., 2017). This allows intra-annual variability in Ba/Ca to be fully attributed 295 to upwelling (Tudhope et al., 1996;Lea et al., 1989). Variable Ba/Ca ratios across specimens during southwest monsoonal upwelling season are likely to result from spatial heterogeneous seawater barium distribution within the reef.

Li/Mg records
SST reconstructions based upon the calibration of the Li/Mg thermometer described above (Eq. 1) reproduce the monthly curve of the SSTrem data (r 2 = 0.83, p < 0.0001) as well as observed SSTin-situ variations at Masirah Island (r 2 = 0.93, p < 0.0001) 300 (Wilson, 2007). Temperature sensitivity of the Li/Mg-thermometer deduced from reconstructed seasonality is in good agreement with the majority of proxy calibration studies from the literature (Hathorne et al., 2013;Ross et al., 2019b;Cuny-Guirriec et al., 2019;Montagna et al., 2014;Fowell et al., 2016). The intercept of the linear Li/Mg-SST calibration of the Masirah corals, however, is 4-5 °C higher than reported in the literature (Fig. 8). Analytical uncertainties that noticeably bias the Li/Mg ratio are unlikely as a source for high Li/Mg ratios, because systematic measurement discrepancies deduced from 305 the JCp-1 QCM for Li/Ca and Mg/Ca would rather tend to underestimate the Li/Mg ratios (Li/Ca: +4.44 % ; Mg/Ca: +7.37 %; Table S1). In addition to temperature, seawater pH also has shown to bias the skeletal Li/Mg ratio of corals (Fowell et al., 2016;Fowell, 2017;Tanaka et al., 2015). For identical SSTs, culture experiments on Sideastrea siderea show an increase of 0.325 mmol/mol in Li/Mg per decreasing pHsw unit (Fowell, 2017). However, comparatively high Li/Mg values found in the Masirah corals are present year-round, while exceptionally low pHsw is limited to the three monthly southwest monsoon 310 upwelling season (Omer, 2010;Takahashi et al., 2014). This suggests that skeletal Li/Mg is not directly sensitive to the external pHsw, but rather to the carbonate chemistry of the calcification fluid, which through modification is independent of external variations in pHsw . Conditions within the calcifying fluid must permanently alter skeletal Li/Mg of Porites at Masirah Island towards higher values compared to published SST-Li/Mg calibrations reported in the literature, which might imply year-round stable but low pHcf. 315

Monthly records of coral calcification
Monthly variability in calcification rate is largely driven by extension rate across all samples (Fig.5), similar to that reported for Porites from the Indo-Pacific region (Lough and Barnes, 2000). However, the typical positive correlation between monthly 325 extension rate and SST does not exist in our data (Fig.6b). All three specimens reveal noticeably small extension growth during April and May despite high temperatures during spring intermonsoon. Interestingly, coral spawning at Omani reef sites is reported to take place between March and May (Howells et al., 2014). Correspondingly, a reduction of extension rate during spring intermonsoon could be linked to high-energy expenditures required for reproduction (Cabral-Tena et al., 2013).
All specimens show lower calcification rate during southwest monsoon compared to that observed for the northeast monsoon, 330 which is attributed to smaller extension rate during the upwelling (Table 3). Declining extension rate during upwelling season is in agreement with growth studies on Pocillopora damicornis from the Gulf of Panama (Glynn, 1977). For corals from Masirah Island, at least part of the seasonal difference in extension rate between southwest monsoon and northeast monsoon might be related to slightly lower SSTs during the southwest monsoonal upwelling (Table 1). Given an increase in extension rate of 0.33 cm yr -1 per 1°C as suggested for Indo-Pacific Porites, differences in extension rate between southwest monsoon 335 and northeast monsoon could be fully explained by temperature for coral 5.21, but only to some extent for coral 5.10 and 5.13 (Lough and Barnes, 2000). Intense eutrophication might have additional impact on growth of Masirah corals, with detrimental effects on extension rate during the upwelling season (Tomascik, 1990). Assuming direct detrimental effects of the essential nutrients on coral growth, the inhibiting effect of NO3would have to outweigh the promoting effect of PO4 3on southwest monsoon extension rate of the Masirah corals (Koop et al., 2001;Dunn et al., 2012;Bucher and Harrison, 2001). However, 340 the northern Arabian Sea is characterized by a high PO4 3to NO3ratio, making this direct effect of nutrients on the extension rate unlikely (Kleypas, 1999). As a general consequence of the excessively high nutrient concentrations, primary productivity in the Arabian Sea increases rapidly during the upwelling season, leading to high levels of turbidity associated with low light transparency of the water column (Anderson et al., 1992;Bauer et al., 1991;Quinn and Johnson, 1996). Reduced photosynthetic efficiency of the micro-algae symbionts seems to be a potential factor for diminished extension rate in response 345 to reduced energy reserves during upwelling (Al Shehhi et al., 2014;Logan and Tomascik, 1991;Muscatin et al., 1981;Sun et al., 2008;Tomascik, 1990). Energy through heterotrophic feeding does not seem to be sufficient for Masirah corals to fully compensate for reduced levels of photosynthates (Tomascik and Sander, 1985). Interestingly, highest monthly extension rate occur immediately before and after the upwelling season during June and October, respectively (Fig.5). A decrease in the extension rate during the southwest monsoon sets on immediately at the beginning of the upwelling, only to recover rapidly 350 again afterwards. We therefore conclude that the coral's energy reserves available for skeletal upward growth response instantaneous to monthly environmental changes (i.e., turbidity), without delays or extended times for recovery.
In contrast to Porites from the Indo-Pacific region, which show only a moderate dependence of skeletal density on SST, Li/Mg-SSTs at Masirah Island are strongly correlated with the monthly variation in skeletal density in coral 5.10 and 5.13 (Lough and Barnes, 2000). This is however not observed for coral 5.21, as a weakly developed HDB during southwest monsoon 355 worsens a causal relationship with SST (Fig.6c). This comparatively low density during southwest monsoonal upwelling coincides with relatively high extension rate in coral 5.21 compared to all other samples (Table 3; Fig.6b). Excellent negative correlation between southwest monsoonal data of monthly density and extension rate across the three corals (n = 9, r 2 = 0.88, p = 0.0002) indicates density during upwelling to be substantially controlled by extension rate (Fig.7). A simple model in which the active calcification surface to which CaCO3 is accreted is relatively large at high extension rate, resulting in thin 360 skeletal elements and low bulk density (and vice versa for HDBs) might explain inter-specimen variability in skeletal density patterns of the Masirah corals during southwest monsoon (DeCarlo and Cohen, 2017). No evidence supports an immediate detrimental impact of upwelling, i.e., a temporally low sw, on sub-annual patterns in skeletal density. Given that density is likely controlled by the carbonate chemistry of the calcification fluid, we propose cf is kept relatively constant by modification independent of external variations in sw during upwelling and non-upwelling seasons (DeCarlo et al., 2018;McCulloch et 365 al., 2017;Mollica et al., 2018).

Annual records of coral calcification
The mean annual calcification rate at Masirah Island is indistinguishable from those of Porites from Indo-Pacific and Atlantic 370 reef sites, which are unaffected by upwelling (Fig.9a). A similar finding is reported from two sites located within the Galapagos upwelling zone (n = 7-8 cores per site) (Manzello et al., 2014). Poor replication of Porites calcification data from the upwelling areas of Panama and the South China Sea (n = 1, respectively) does not enable a proper comparison (Mollica et al., 2018).
Patterns of calcification in Porites from Masirah Island and Galapagos differ from those found in regions without upwelling by showing enhanced extension rate and lower skeletal density, similar to the pattern termed "stretching modulation of skeletal 375 growth" by Carricart-Ganivet (2004) (Fig.9b, 9c). High mean annual Porites extension rate at Galapagos was discussed as potential impact of the stimulating effects of nutrients on upward growth (Manzello et al., 2014). Nutrient-stimulated growth is also suggested for enhanced annual extension growth of Pocillopora damicornis and Pavona clavus from the Panama upwelling zone compared to data from regions unaffected by upwelling (Glynn, 1977;Wellington and Glynn, 1983). In fact, enhanced annual extension rate of Pavona clavus from Panama is the result of high extension growth during the non-upwelling 380 season, similar to the pattern seen in Porites from Masirah Island (Wellington and Glynn, 1983) (Table 3). Hence, a stimulating effect of nutrients on extension rate during the non-upwelling seasons is possible, since moderate nutrient concentrations with high PO4 3to NO3ratio exist year-round in the Arabian Sea (Dunn et al., 2012;Kleypas et al., 1999;Koop et al., 2001).
Low skeletal density of coral specimens from Masirah Island is consistent with low skeletal density reported in the literature for Porites from the upwelling regions of the eastern Pacific and the China Sea (Mollica et al., 2018;Manzello et al., 2014) 385 (Fig.9c). Boron isotope analyses on some of these samples have revealed the low skeletal density to be driven by a relatively low cf compared to corals from regions unaffected by upwelling (Mollica et al., 2018). Low cf in these specimens is maintained year-round, independent of external variations in sw (i.e., seasonal upwelling) (Fig. S3). Accordingly, we hypothesise the year-round relatively low skeletal density of the Masirah corals to be also related to a constantly low cf. This hypothesis is further supported by the Li/Mg ratios of the Masirah corals, which are offset to higher values than those expected 390 from the literature (Fig.8). As this offset is present throughout the year, it cannot be attributed to temporarily low pHsw associated to seasonal upwelling, but rather reflects year-round constant conditions within the calcifying fluid (Fowell, 2017;McCulloch et al., 2017). This finding implies that there is no intensified upregulation of internal cf relative to sw during the non-upwelling seasons DeCarlo et al., 2018;D'Olivo and McCulloch, 2017). As an explanation, we propose that internal upregulation processes of corals affected by seasonal upwelling are not capable to adapt completely to 395 ocean chemistry change on a quarterly scale. As a consequence, a relatively low cf could be maintained year-round so as to avoid high gradients to the external sw during southwest monsoonal upwelling.

Conclusion
This study investigates the effect of seasonal upwelling on sub-annual and annual patterns of calcification in reef corals (Porites) from the northern Arabian Sea (Masirah Island, Oman). On a sub-annual basis, calcification rate is lower during the upwelling season compared to periods of near identical SST in the non-upwelling season. This is attributed to a rapid decline 405 in extension rate with the onset of the upwelling season. We attribute this decline to a reduction of photosynthetic performance of the micro-algae symbionts potentially caused by enhanced turbidity of ambient seawater through elevated primary production. Patterns of skeletal density do not exhibit an instantaneous response to the external decline in sw during upwelling, which indicates stable levels of cf to be maintained throughout the year. Nonetheless, mean annual skeletal density is significantly lower than in Porites from typical reef environments of the Indo-Pacific and Atlantic Ocean. In contrast, mean 410 annual extension rate is high, likely due to the simulating effect of moderate seawater nutrient concentrations during the nonupwelling seasons. As high extension rate compensates for the deficit in skeletal density at Masirah Island, annual calcification rate is indistinguishable from Porites growing in regions unaffected by upwelling.
These results suggest that temporarily reduced sw (seasonal upwelling) has no instantaneous impact on sub-annual variability in skeletal density but could cause a permanent adaptation towards year-round unexpected low skeletal density. Unless the low 415 skeletal density is compensated through high extension rate, this will yield detrimental effects on the net carbonate accumulation in coral reefs. Furthermore, this study highlights variable effects of nutrients on extension rate, with negative effects at excessively high nutrient levels (i.e., upwelling season) and stimulatory effects at moderate nutrient levels (i.e., nonupwelling season).
Further research should include combined analyses of  11 B and B/Ca ratios in order to confirm the hypothesis of a year-round 420 relatively low cf in reef corals from sites affected by seasonal upwelling.