c b a Meteor stations 1999 Northern stations 1988-present Figure 1: Location map of hydrographic stations. a- DCPE project stations; b- Meteor stations (1999); c- Northern stations (1988-present) and coastal sampling stations.
N:P=18 b Station H (Red Sea), 30.9.75 a Figure 2: Example for the two criteria for data selection: 1. Compliance with the overall consistency of the water column profile (a, b), the data points excluded are circled with red for phosphate and blue for nitrate, note the good correlation of the N vs. P plot after the exclusion; 2. Compliance with the data of cruises from close dates (C), the nitrate profile of station A1 during 12.2.91 (red squares) looks coherent, but its concentrations are ca. 1.5 times higher (at deep and shallow water) than the nitrate profiles before and after this cruise that show the same deep water concentration. This is most probably a calibration (standard factorization) problem and the whole profile was discarded even that apparently dividing the data by 1.5 would produce a nice profile. c
Nitrate (µmol/L.) 0 1 2 3 4 5 6 7 8 0 100 a c b L50 200 300 L100 400 Depth (dbar) 500 L200 A1 600 OS 700 L400 B1 FF 800 C50 10 / 08 / 03 900 Depth (dbar) C100 C200 16.5.2000 16.5.2000 C400 Nitrate (µmol/L.) 0 1 2 3 4 5 6 7 8 0 Distance from shore (km) Distance from shore (km) 100 A 200 300 400 Depth (dbar) 500 A1 OS 600 B1 E1 700 W1 FF 800 22 / 02 / 04 900 Figure 3: Nitrate concentrations in the northern Gulf of Aqaba. a- Nitrate east-west and north-south contour profile of during 16.5.2000 (adopted from David, 2002). b- Nitrate profiles along a north-south line (see Fig. 1 for station location). c- Nitrate profiles along a north-south line (FF, OS, A1, B) and east-west line (E1, W1). Note that during the last four years (represent by the data in this figure) no nutrient gradients were observed from the northern tip to station B1, some 20 km southward along the main axis of the Gulf. This indicates homogenous horizontal distribution within the northern gyre of the Gulf of Aqaba..
oxygen “gained” in samples above this line (see caption) b a deep mixing zone ? ? line of minimum O2 line of minimum O2 c d ? ? line of minimum O2 line of minimum O2 Figure 4:Dissolved oxygen (DO) concentrations in deep water of station A versus time. All available data were plotted without applying data filtration criterion because after the fixation of the oxygen samples they never loose oxygen. The samples may “gain” oxygen (values above the black solid line) if not stored in a container under water for more than seven hours especially on hot days. This effect is probably responsible for several very high oxygen values measured in the 70’s and early 90’s. a - water sampled below 500 m; b - water sampled between 500m and 600 m; c – water sampled between 450m and 550 m. d – water sampled between at 600±25 m. Note that high oxygen values occur when mixing depth passed the limits of the plotted depths and ventilated the deep water (deep mixing zone). Therefore these high values are irrelevant for estimating long term changes in deep water organic load. Two features are evident from the plot: 1. The minimum oxygen concentration in deep water decreased with time by ca. 17 mmol·L-1,no matter what depth range is selected there is an DO decrease in deep water compared to the mid 70’s and end of 80’s; and 2. The rate of this decrease is higher in the end of the 90’s. The estimated nitrate increase (from Redfield ratios) is close to their observed increase (see Fig. 5), while phosphate (Fig. 6) is too noisy to be able to draw a definite conclusion. It should be noted that this the apparent DO decrease is most significant because DO is the most reliable relevant measurement.
? deep mixing zone Figure 5: Nitrate concentrations in deep water of station A versus time. Data presented are those that comply with overall consistency of the water column profile, and not contradicting with the data of cruises from close dates. Upper left – water sampled below 500 m; Upper right - water sampled between 500m and 600 m; Bottom – water sampled between 450m and 550 m. Note that low nitrate values occur when mixing depth passed the limits of the plotted depths inducing phytoplankton growth that lowered water column nitrate below the conservative mixing value (deep mixing zone). Therefore these low values are irrelevant for estimating long term changes in deep water organic load. The plot shows that the maximum nitrate concentration in deep water increased with time, especially during the end of the 90’s. Deep water nitrate increased by about 1.5 mmol·L-1and for the end of the 90’s the increase is even close to 2 mmol·L-1 even though the during deep mixing of the year 2000 nitrate concentration decreased to about 1.5 mmol·L-1.
? deep mixing zone Figure 6: Phosphate concentrations in deep water of station A versus time. Data presented are those that comply with overall consistency of the water column profile, and not contradicting with the data of cruises from close dates, hence, there are considerably less data points in the phosphate plot than in the nitrate plot (Fig.5). Upper left – water sampled below 500 m; Upper right - water sampled between 500m and 600 m; Bottom – water sampled between 450m and 550 m. Note that low phosphate values occur when mixing depth passed the limits of the plotted depths inducing phytoplankton growth that lowered water column phosphate below the conservative mixing value (deep mixing zone). Therefore these low values are irrelevant for estimating long term changes in deep water organic load. The plot suggests that the maximum phosphate concentration may have increased slightly, by ca. 0.05 mmol·L-1, since the late 90’s. Such increase is compatible with the measured nitrate increase during that preiod (Fig. 5).
8 N:P≈18 7 6 N:P=16 ) N:P≈12 1 - 5 mol L m ( 4 1975-1977 Nitrate 1989-1992 3 1997-1998 1999-2003 2 a b 1 0 0.0 0.1 0.2 0.3 0.4 0.5 - 1 m Phosphate ( mol L ) Figure 7: Nitrate versus phosphate concentrations in water deeper than the summer thermocline (depth > 290 m) of station A1 from 1975 to 2003. a – The screened P and N data; and for comparison plot b – All P and N data that fit in the in the concentration scale of a, there are ca. 20 data points that their P value is larger than the range of the graph (0.5 mmol·L-1), these point are not plotted here for the sake of scale similarity. The screened data revealed four time intervals: 1. The mid 70’s; 2. Beginning 90’s; 3. End of the 90’s before the large jump in the concentration of deep water nutrients; and 4. The beginning of the 2000. The average N:P ratio is about Redfield, however, it looks that the N:P ratio of the present decade is higher than that of the last decade suggesting input of material with N:P ratio higher than the original Gulf’s POM. The noise in the data is “real” because each sampling day shows N vs. P plot with very high linear correlation, but the N:P ratio of these plots varies seasonally (Al-Qutob et al., submitted). It is unlikely the slope change was due to analytical method (at present analyze P by the MAGIC pre-concentration method) because according to our test P values greater than 0.1 mmol·L-1 give similar results when analyzed directly or by the MAGIC pre-concentration.
Deep mixing Missing data Missing data Missing data Missing data DO2≈-13 mol m-2 Figure 8: Long term variations of water column dissolved oxygen inventory (integral) in station A1. Note that oxygen inventories decrease by ca. 13 mol·m-2 since the end of the 80’s despite the deep mixing of the year 2000 that replenished the water column oxygen.
DN≈0.5 mol m-2 DN≈0.8 mol m-2 ? Missing data Missing data Missing data Figure 9: Long term variations of water column nitrate inventory (integral) in station A1. Note the relatively high values during 1991-1993 that may suggest an increase during late 80’s and early 90’s and decrease during mid 90’s (dashed arrows) maybe also due the deep mixing of 1992. The rather complete data set since 1997 shows a steady increase in nitrate inventories by ca. 0.8 mol·m-2 and by 0.5 mol·m-2 from the mid 1999 just before the deep mixing event of 2000, even that the deep mixing event depleted the water column nitrate (see chapter “Clues from fish farms emissions data” for further discussion).
3.0 2.5 DIN 2.0 Nitrate Nitrate, DIN (mol·m-2) 1.5 1.0 0.5 0.0 01/00 07/00 01/01 07/01 01/02 Date (mm/yy) Figure 10: Seasonal variations of nitrate (solid circles) and nitrate+nitrite (DIN; open circles) water column inventory (integral) in station A1. Note that during an annual cycle, DIN changes by factor of 2, the net yearly change is very small. Therefore multi-annual observations are needed in order to establish net change in water column DIN inventory. Nitrate is the main DIN species and even during active mixing, nitrite comprises less than ca. 7% of the water column DIN.
Missing data Missing data Figure 11: Long term variations of water column phosphate inventory (integral) in station A1. Phosphate integrals are noisy and show features that resemble the nitrate inventories since the end of the 90’s. Due its relatively large sampling and analytical error, phosphate is particularly sensitive to integration (errors are additive) compare to nitrate that is ca. 20 time higher. P inventory estimates for the end of the 90’s are reliable due to high vertical resolution of the water samples and the use of MGIC pre-concentration method. Despite of the large noise, it seems the P inventory increased since the end of the 90’s by ca. 0.02 mol·m-2.
3.0 2.5 DDO/DN=-15 DDO/DNRedfield=-8.6 Gordon et al., 1994 3.0 2.0 DDO/DN=-15 2.5 Nitrate (mol·m-2) 1.5 a b 2.0 1.0 Nitrate (mol·m-2) Maximum limit of DO inventory DN/DPRedfield=16 1.5 0.5 DN/DP=20 1.0 0.0 0.00 0.05 0.10 0.15 0.20 0.5 Phosphate (mol·m-2) 0.0 120 130 140 150 160 DO (mol·m-2) 1999-2003 1997-1998 1999-2003 1997-1998 1988-1992 1989-1992 1975-1977 1975-1977 Figure 12:Property/property plots of water column inventories in station A1 during the years 1975 to 2003, plotted in the same time intervals of Fig. 7. a - Nitrate versus DO; b - nitrate versus phosphate. The average slopes (DDO:DN and DN:DP ratios) are about Redfield (slopes indicated on the lines), it looks however, that the DDO:DN ratio of the present decade (open squares, DDO:DN≈-15) is higher than that of the last decade, especially 1988-1992 (that was closer to Redfield). This may represent an excess “oxygen demand” as compared to the “oxygen demand” of the original Gulf’s POM. Plot (a) reveals another important feature: The intersection of the 1999-2003 data with the maximum limit of DO inventory (MDO; DO inventory of 700 m water column in atmospheric equilibrium when water is at average salinity and deep water temperature that could result of a long deep mixing event such as 1992) is higher than the intersection of the 1988-1992 line (dashed line with Redfield slope) with this DO limit. This clearly indicate that after a deep mixing event, present day water column will contain about twice more nitrate than after deep mixing during the early 90’s. Deep mixing shifts the phytoplankton community (PTC) from nutrient limitation regime to light limitation regime, hence even large PTC cannot assimilate all the water column nutrients. According to this rationale, the larger the DIN water column inventory, the larger the “leftover” nitrate that PTC cannot assimilate during a complete mixing event. As indicated above this “leftover” nitrate is estimated from extrapolating the trend line to MDO. It should be noted that even if we draw a line with the similar DDO:DN to the present (see dashed with slope of -15) that passes through the center of the 1975-1992 data its intersect with the MDO is below the intersection of the 1999-2003 line. The noise in the nitrate versus phosphate inventories plot (b) is large but seems to follow the temporal behavior of the P vs. N concentrations plot (Fig. 7a). It seems that before 1999 the DN:DP ratio of water column inventories was lower than at present with the except of the inventories calculated from Gordon et al. (1994) data for 1991, 1992 that clearly deviates from all data collected before 1999. The lines were drawn manually to emphasize trends.
Missing data DSi≈0.42 mol m-2 Missing data Abrupt decrease due to high productivity induced by deep mixing Nov. 1992 Figure 13: Long term variations of water column silica inventory (integral) in station A1. here were very few reliable silica determinations during the 70’s. Apparently, part of the silica values for 1991, 1992 (circled in blue, all digitized from Gordon et al., 1994) are about factor of 2 too high when compared to the values measured at the IUI before and even one month after this data set (see blue arrow, for Nov. ‘92 compared to Oct. ‘92 in the Gordon et al., 1994 data set; both profiles were not conducted during a period of maximum mixing and “new production” that could explain such large abrupt difference. Abrupt decrease in silica was noticed during Apr. ‘98 and Mar. ’00, circled with red). Note the increase in silica inventories by ca. 0.42 mol·m-2 despite the deep mixing event of the year 2000 that depleted the water column silica. Note that due the geological setting of the Gulf of Aqaba, silica is not a limiting nutrient in the Gulf as indicated by the summer deep/surface water concentration ratio that is ca. 100 for nitrate and 4 to silica (see Appendix A).While silica is supplied by solution of quartz from the crystalline basement in surface water, nitrate is supplied by the Red Sea and all external sources that are mainly anthropogenic. It is expected that silica will follow nitrate and not vise versa because increasing the input of nitrogen will bound more silica in diatoms (and/or other siliceous plankton) that means converting dissolved silica to opal that is much soluble than quartz and has faster solution kinetics. Upon sinking below the thermocline the siliceous plankton will dissolve increasing the overall water column silica inventory. It is expected therefore that adding nutrients to a low nitrate (and phosphate)/high silica environment, will end up in increase in silica inventory due to increase in siliceous phytoplankton population. The documented increase in water column productivity (Iluz et al., submitted) may be due to such increase.
Figure 14: Dissolved oxygen profiles along the Gulf of Aqaba during project DCPE, the mid 70’s (upper row) and the Israeli-Egyptian joint cruise at the beginning of the 90’s (lower row). Stations location is presented in Fig. 1a. Note the distinct mid water oxygen minimum in all stations inside the Gulf of Aqaba during the mid 70’s. The Gulf of Aqaba minimum was much smaller than in the northern Red Sea (station H). The mid water oxygen minimum during the beginning of the 90’s was not as distinct as it was during the mid 70’s.
Figure 15: Dissolved oxygen profiles along the Gulf of Aqaba during the Meteor cruise 1999. Stations location presented in Fig. 1b, upper row- the Gulf of Aqaba; lower row- northern Red Sea. Note that compare to the marked mid water oxygen minimum in the northern Red Sea, it was quite small in the southern Gulf of Aqaba and was not present in the northern Gulf of Aqaba.
Figure 16: Nitrate profiles along the Gulf of Aqaba during project DCPE, the mid 70’s (upper row) and the Israeli-Egyptian joint cruise at the beginning of the 90’s (lower row). Stations location is presented in Fig. 1a. Note the distinct mid water nitrate maximum in all stations inside the Gulf of Aqaba during the mid 70’s even that it was much smaller than in the northern Red Sea (station H). The mid water nitrate maximum during the beginning of the 90’s was not as distinct as it was during the mid 70’s.
Figure 17: Nitrate profiles along the Gulf of Aqaba during the Meteor cruise 1999. Stations location presented in Fig. 1b, upper row- the Gulf of Aqaba; lower row- northern Red Sea. Note that compare to the marked mid water nitrate maximum in the northern Red Sea, it was quite small in the southern Gulf of Aqaba and very small in the northern Gulf of Aqaba.
1997 1998 2003 1999 2000 2004 Nitrate (mmol·L-1) 2002 0 1 2 3 4 5 6 7 0 11 Feb. 97 2003 1999 2000 2004 a 100 2001 2 Feb. 98 9 Feb. 98 200 1 Feb. 99 6 Feb. 00 300 11 Feb. 01 24 Feb. 02 400 16 Feb. 03 1999, 2000 2003, 2004 Depth (m) 22 Feb. 04 500 2001 600 2002 1997, 1998 700 bottom 800 b mmol·L-1 No cruises Figure 18:Nitrate water column temporal variations. a- profiles in station A1 for February cruises between 1997-2004. b- Time versus depth variations of nitrate in station A1 (solid points denote water samples) from Sep. 99 to Apr. 04. We chose to plot February profiles in (a) because this month integrates the deep water nitrate for a whole year, just before reaching the deepest annual mixing depth of March. March mixing may reach the bottom and reset nitrate concentrations of the whole water column, as happened during the year 2000 (see b). The lines in (a) are eyeballed fits to the depth profiles. Similar behavior of the depth profile over several years is represented by one line section that is labeled by the years it represents. Blue- profiles with low deep water nitrate (1997, 1998); Red- profiles with high deep water nitrate (1999, 2000, 2003, 2004); Purple- the two consecutive years (2001, 2002) that show the perturbation caused by the deep mixing event of winter 2000 that during March 2000 reset the nitrate concentration of the whole water column to ca. 1.5 mmol·L-1. Note that during the years 2001 and 2002 nitrate increased in deep water and decreased in shallow water even that February mixing depth was similar. During the year 2003 deep water nitrate returned to its 1999 high level, and stayed high also this year (2004), even that February mixing depth in 2004 was deeper than that of 2003 as reflected also by the lower surface water nitrate of 2003. The average change in deep water (z>500m) nitrate between 1998 and 2000 was about 1.5 mmol·L-1. Apparently most of this change occurred during several days between Jan. 1999 and Feb. 1999 (appendix A) when mixing depth increased by ca. 80 m after 2 consecutive very shallow winter mixing of 1997-1998 (see Fig. 20). Figure 18b clearly demonstrates the gradual increase of deep water nutrient reservoir to its pre-mixing values and demonstrates the absolute absence of mid water nitrate maximum observed many times during the mid 70’s (Fig. 16).
8 7 DN:DSi=1.5 DN:DSi=4.5 6 5 4 Nitrate (mmol·L-1) 3 1999-2003 2 1997-1998 1989-1992 1 1975-1977 0 0 1 2 3 4 5 Silica (mmol·L-1) Figure 19: Nitrate versus silica water column concentrations in station A1 during the years 1975 to 2003 plotted in the same time intervals of Fig. 7. The average DN:DSi is estimated to be 4.5 for surface to intermediate water and 1.5 for deep water. The plot reveals two main features: 1. During the 2000’s deep silica (and nitrate) concentrations are higher than before; and 2. DN:DSi of 1.5 characterizes mainly deep water of the last five years suggesting enhanced regeneration of siliceous phytoplankton (diatoms). The lines were drawn manually to describe the general trends.
0 100 200 300 Mixed Layer Depth (m) 400 500 600 Bottom at station A1 700 800 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 Year Figure 20: Depth of the mixed layer in station A1. The 1975-1977 data was estimated only from the Nansen bottles hydrocasts, therefore their accuracy is not more than ±50 m. The blue ellipses with the downward pointing arrows represent bottle hydrocasts and/or CTD profiles where the thermocline was not encountered, indicating that the mixed layer was deeper than the maximum depth of the particular profile.
5000 500 80 ∫ C=33,000 Ton ∫ PD=200 Ton ∫ NT=2,500 Ton 4000 400 60 P Fish Feed P Total Emission N Fish Feed 3000 300 N (Ton·y-1) N Total Emission C Emission (Ton·y-1) 40 P (Ton·y-1) C Total Emission b P Solid a 2000 200 N Dissolved 20 1000 100 P Dissolved N Solid 0 0 0 1990 1992 1994 1996 1998 2000 2002 2004 1990 1992 1994 1996 1998 2000 2002 2004 Year Year 1.0 N 0.9 0.8 P 0.7 0.6 Fraction of feed emitted to the marine environment 0.5 0.4 c 0.3 0.2 0.1 0.0 1990 1992 1994 1996 1998 2000 2002 2004 Year Figure 21: Nitrrogen and phosphorus budgets of the fish farms for the last decade (data provided by Dr. Yuval Cohen, IOLR). The total emitted C, N and P are indicated in the graphs assuming that the present values are equal to the 2002 values. The integrated total N and C (assumed to be available for oxidation in the marine environment) and dissolved P are indicated above the graphs. a- Nitrogen and carbon emission rates; carbon emissions were calculated by subtracting the amount of nitrogen and phosphorous in the harvested fish from the total feed weight assuming that the fish water content is 60% (if the water content is 80% as measured on cod (Martinez et al., 2003 [The Journal of Experimental Biology 206: 503-511] than the C estimate should be ca. 12% higher). b- Phosphorus emission rates. c- The Nemitted/Ntotal fish feed and Pemitted/Ptotal fish feed ratios that are the solid + dissolved fractions of N and P emitted into the marine environment by the fish farms.