Seasonality in nutrient concentrations in Galveston Bay

Marine Environmental

Research, Vol. 40, No. 4, pp. 331-362, 1995 Copyright 01995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0141-1136/95 %9.50+0.00

0141-1136(94)00005-4

ELSEVIER

Seasonal@ in Nutrient Concentrations in Galveston Bay Peter H. Santschi Department

of Oceanography, Texas A & M University, Galveston, Texas 77553-1675, USA

(Received 28 August 1994; revised version received 19 January 1995; accepted 21 January 1995)

ABSTRACT In order to investigate factors controlling nutrient cycling in the shallow and turbid coastal ecosystem of Galveston Bay, data from: (1) the Texas Water Commission (TWC) database 1980-1989, and (2) salinity transects in 1989 and 1993 are presented and analyzed. Statistical regression and time-series analysis were carried out on data acquired by TWC between 1980 and 1989, in an attempt to establish seasonality of nutrient and chlorophyll-a (chl.-a) concentrations in the bay and to determine factors which regulate these concentrations. A strong seasonality was found for phosphorus and chl.-a in the upper and mid-bay stations. A recurring maximum for phosphate occurred in September and a chl.-a maximum occurred regularly in March-April. It is hypothesized that benthic regeneration of phosphorus at the end of summer is responsible for the phosphate maximum. The inverse correlation of the partition coeficient (KJ for phosphate with the concentration of suspended particulate matter (SPM), coupled to a strong enrichment of phosphate in suspended particles at low SPM concentrations, indicates additional control by geochemical and physical processes such as particle sorting and/or particlecolloid interactions. Nitrate is inversely correlated with salinity at the upper and mid-bay stations, indicating the Trinity River is a major source. Nutrient concentrations in the lower bay (East and West Bay stations) are considerably lower and less predictable, as they are not correlated with salinity or temperature. Data from the 1989 and 1993 transects confirm the yearly maximum in phosphate concentration in late summer months, with peak concentrations in the upper Trinity Bay. It is concluded that despite possible phosphate buffering by physical and geochemical mechanisms, relatively large concentration maxima recur regularly every year during the summer, possibly caused by a benthic source of phosphate. 337

338

P. H. Santschi

INTRODUCTION Since the majority of the US population lives within 50 miles of the coastline, estuaries receive large anthropogenic inputs of nutrients and trace contaminants. Excessive nutrient inputs to coastal areas can cause hypoxic events and nuisance or toxic algae blooms which cause fish kills. Galveston Bay is one of the largest industrialized coastal embayments along the US coastline. Substantial inputs of urban and industrialized waste waters from the Houston and Dallas-Fort Worth metropolitan areas lead to relatively high concentrations of phosphorus and nitrate in the bay. These large nutrient inputs help to support a productive fisheries industry. Eutrophication of coastal areas is also responsible for hypoxic events, toxic and nuisance algae blooms. Galveston Bay receives nutrients mainly from two major rivers the Trinity and San Jacinto Rivers. Treated and untreated domestic sewage is also released into the water from surrounding areas and, with atmospheric inputs, contributes to the eutrophic state of Galveston Bay (Armstrong, 1982). Nutrient release from sediments is also a likely source, particularly during parts of the year when sediment-water interactions are most intense in shallow aquatic systems (Santschi et al., 1990). Galveston Bay receives annual amounts of anthropogenic nutrients ranging from 3.7 to 12.5 x lo6 kg of phosphorus, and 23-50.5 x lo6 kg of total Kjeldahl nitrogen, depending on the source of data (NOAA/EPA, 1989; Newell et al., 1992; Armstrong, 1987). From these inputs, one can calculate area1 loadings of 2-9 g P rnp2 of bay area per year and 1645 g N rnp2 year-‘. Atmospheric inputs, by comparison, are considerably smaller. For example, using our unpublished data on atmospheric inputs which show average rain water concentrations of 1.1 ?L 0.7 (la) mg NO3 N l-l, we estimate atmospheric area1 loadings of only 1.7 g N rnp2 year-’ from this source, which is only 410% of that from all rivers. We estimate NH4 and DON fluxes from the atmosphere to be lower than that. This assessment supports the conclusions of previous studies (Armstrong, 1982), about the relatively low percentage of atmospheric nutrient inputs to the bay, when compared to those from rivers. The major sources of these river inputs are waste water treatment plants (35% for P, 50% for N), industrial facilities (40% for P, 30% for N) and agriculture (10% each for P and N). Other upstream sources (10% each for P and N) are of minor importance. Relative to other estuaries, these overall nutrient inputs are large (e.g. Nixon et al., 1984; Armstrong, 1987; NOAA/EPA, 1989). Galveston Bay is a turbid estuary, with suspended particulate matter (SPM) concentrations ranging from 10 to 500 mg liter-’ (Texas Water Commission database; Guo et al., 1994; Wen & Santschi, unpubl. data).

Seasonality of nutrients in Galveston Bay

339

One could therefore expect that water column cycling of phosphate and other nutrients would be significantly controlled by geochemical reactions, in addition to phytoplankton uptake and release. Little is known about the variability and controls of nutrient concentrations in shallow bays and estuaries such as Galveston Bay. Literature on phosphorus, for example, mostly concerns solubility control of phosphate by apatite and iron oxyhydroxides (e.g. Fox et al., 1985, 1986; Fox, 1989; Huanxin et al., 1994), or by microorganisms (e.g. Gachter, 1985; Laczko, 1988; Lebo, 1990). Kinetic control by a hypothetical buffer mechanism has also been proposed (Froelich, 1988). In the latter case, resuspension of surface sediments in estuaries would tend to release phosphorus and thus help to establish rapid equilibrium. Large variations in concentrations of different nitrogen species are observed in Galveston Bay as well. Little is known about the causes of these variations. Factors controlling dissolved nitrogen species include nitrificatiomdenitrification reactions (e.g. Blackburn & Sorensen, 1988; Seitzinger, 1988, and references therein), nitrate and ammonia regeneration from dissolved and particulate organic forms of nitrogen, processes occurring in freshwater reservoirs in the watersheds, and overflows of storm sewers after heavy rainfalls. This paper documents large seasonal variations of nutrient and chlorophyll-a (chl.-a) concentrations in Galveston Bay and discusses their possible causes. It is a companion study to the first systematic study of trace element cycling in Galveston Bay (Benoit et al., 1994).

EXPERIMENTAL The approach to investigate factors controlling concentrations in Galveston Bay was as follows:

of nutrients

TWC sampling and analysis The former Texas Water Commission (TWC, now called TNRCC) measures nutrients, chl.-a, temperature, salinity, and other related parameters (i.e. SPM and TOC, where available) in surface water (at 30 cm depths) at the mid-bay station (Smith Point/Eagle Point, TWC Station 2439.0025; 29’30’50”N, 94’52’45”W) (Fig. 1) on a monthly basis. This station is the only one which has been sampled monthly since 1980. Another station, Houston Ship Channel/Morgan’s Point (TWC Station 1005.0100; 29”40’58”N, 94’58’55”W) has been sampled monthly since 1985. Stations which are sampled four times a year include Trinity Bay (Exxon Cl

340

P. H. Santschi

Buffalo

5X””

(Hanna West Bay (Carancahua

Reef)

Reef

“3 Texas

$k

TWC Stations

0

Oct.Zl-23,

0

Aug.1 6, 1989,

0

July

1993

1989,

transec

transect

transect

(Sinbj

Fig. 1. Map of Galveston Bay with our sampling stations and those included in the Texas

Water Commission (TWC) database.

Platform, TWC Station 2422.0200; 29O39’54”N, 94”47’12”W), East Bay (Hanna Reef, Station 2438.0150; 29’28’12”N, 94”42’38”W; sampled by the US Army Corps of Engineers) and West Bay (Carancahua Reef, TWC Station 2424.0100; 29’12’45”N, 95”OO’OO”W).For locations, see Fig. 1. Sampling of all these stations was, however, often not carried out synoptically, i.e. only during the same month, not the same week, or, better yet, the same day. Methods used for nutrient and chl.-a analysis employed for this data bank are procedures recommended by EPA and the American Public Health Association for water and waste water analysis. Total P and ortho-P analysis is carried out for the TWC using USEPA methods 365.4

Seasonality of nutrients in Galveston Bay

341

and 365.2, respectively. Nitrate and nitrite were analyzed using method 353.2 (USEPA, 1979) based on reduction by Cd, and chl.-a analysis is by method Chl.a/10200H (American Public Health Association, 1987). TAMU sampling and analysis Samples for nutrient analyses from Galveston Bay were collected along salinity transects in August and October of 1989, and July 1993 (Fig. 1). The 1989 samples were collected concurrently with samples for trace metal analysis (Benoit et al., 1994). Samples, prefiltered in the field through prewashed 0.4 pm nuclepore filters, were stored refrigerated in acid-leached polyethylene containers until analyzed. Autoanalyzer methods used for ortho-phosphate, nitrate, nitrite, ammonia and silicate have been described elsewhere (Strickland & Parsons, 1972; Biggs, 1992). They are based on Cd reduction for nitrate, and a molybdate blue reaction for phosphate and silicate. Salinity was calculated from chlorinity titrations (American Public Health Association, 1987). Statistical analysis Statistical treatments of time-series data gathered and/or archived by the TWC are presented in order to identify relationships between variables. Regression and time-series analysis were carried out in order to establish seasonality and infer regulating factors controlling nutrient concentrations in the bay. As it turned out, the strength of the correlations was not very strong (i.e. r2 usually ~0.3). Thus, the level of significance of the linear correlation became of primary concern. The statistical analysis was thus carried out mostly to establish if the variables in question were correlated or not. In addition, time lags of nutrient maxima or minima occurring after temperature and salinity maxima and minima were considered. Time-series analysis consisted of: (1) correlation analysis of each parameter with temperature; (2) after smoothing of the data by taking the 5-point moving average, the regression coefficient, r, was optimized by shifting the smoothed or unsmoothed time-series data by l-4 months in either direction; and (3) for phosphate, fitting of the smoothed or unsmoothed data to a sine function. This method was chosen after Fourier analysis resulted in only one frequency, 1 year. Multiple-point averaging of data points removes some of the scatter, and allows one to better see trends and cycles in the data. Both 3-point and 5-point averaging gave satisfactory results. The statistical analysis was mostly focused on the data from the mid-bay station. The results were then compared with the analysis from the less frequently sampled TWC stations of the bay.

342

P. H. Suntschi

Correlations are termed ‘significant’ when correlation coefficients indicate p ~0.05, and ‘highly significant’ when p< 0.01; where p is the probability that any random sample of uncorrelated experimental data points would yield an experimental linear correlation coefficient as large as or larger than the observed value of Y. A small value of p implies that the observed variables are probably correlated (Bevington, 1969). Other statistical tests were sometimes needed to compare different data sets, which had data sampled at different frequencies and at different dates, for example correlation analysis to common physical parameters, such as temperature or salinity, and the comparison of the slopes of the best-fit curves.

DESCRIPTION

OF GALVESTON

BAY

Galveston Bay has a surface area of 1432 km2 (Table 1) and is one of the largest embayments on the US coastline. Because of its shallowness (average depth of 2 m), restricted water exchange with the Gulf of Mexico, and the relatively small tidal range (average of 40 cm), winds are often more important than tides in bay circulation and water exchange (NOAA, 1989). Exchange with Gulf waters is greatly facilitated in the 13 m deep ship channel. Facilitation of density circulation in the 13 m deep ship channel crossing the bay (Fig. l), together with high evaporation rates (Table 1) and frequent on-shore winds in the summer and off-shore winds in the winter introduce enough uncertainty into circulation models

Characteristics

of Galveston

TABLE 1 Bay (Modified

Drainage basin (IO4 km2) Long-term mean annual river flow (m3 set-‘) Trinity River San Jacinto River Ungaged flow (including local drainage) Combined Estuarine surface area (km*) Area of drainage basin (km*) Estuarine volume (km3) Mean depth (m) Mean precipitation rate (cm year-‘) Mean evaporation rate (cm year-r) Hydraulic residence time (days) Population in drainage basins (millions)

from Armstrong,

1982) 4.6 235 63 91 389 1432 62 000 2.91 2.1 135 119 40-88 7

Seasonality of nutrients in Galveston Bay

343

such that direct relationships between freshwater inputs and salinity distributions cannot be accurately predicted as yet (Ward, 1992). Threedimensional models of the circulation in Galveston Bay exist (e.g. Martin, 1993) but they only predict average salinity distributions during a particular season. Resuspension of sediment from the inflowing rivers provides most of the suspended matter to the water column (e.g. Morse et al., 1993; Benoit et al., 1994; Baskaran & Santschi, 1993). The Trinity River supplies about 78% of the total river input, and about 60% of the total freshwater which enters the bay annually (i.e. 389 m3 s-l, Table l), with the remainder coming from the San Jacinto River and from ungauged flows (including smaller streams, groundwater discharge, and direct precipitation) from local drainage (NOAA, 1989; Table 1). The Dallas-Fort Worth Metroplex (4 million people) and the Houston metropolitan area (3 million people) are in the Trinity and San Jacinto River drainage basins, respectively (Armstrong, 1987). The western shores of Galveston Bay are heavily industrialized, especially by the petroleum and chemical industries. For example, 3&50% of US chemical production facilities and oil refineries are situated around the bay, adding not only pollutants but also unknown amounts of nutrients. Galveston Bay receives more than 5 km3 year’ of waste water input, half of the total permitted waste water discharges for the state of Texas.

RESULTS

AND DISCUSSION

Ten-year averaged concentrations of nutrient species at different locations in Galveston Bay are displayed in Table 2, for illustrative purposes. As becomes obvious, this documents the general pattern of decreasing nutrient concentrations from the upper bay to the lower bay. However, highest concentrations are often observed in upper Trinity Bay at low salinity (but not zero salinity), and at Morgan’s Point at relatively high salinity. The often higher salinity at Morgan’s Point is due to the enhanced transport of seawater through the Ship Channel, and the relatively low freshwater input from the San Jacinto River. Higher sewage effluent inputs from the City of Houston are indicated by the often high concentrations of nitrite. The time-series analysis of the ten-year TWC data set from the mid-bay (Smith Point/Eagle Point) reveals strong seasonal cycles in the annual pattern of phosphorus and nitrate (Fig. 2). As will be shown later, these variations appear to be related to the seasonal variations in temperature and salinity.

Point

15 f

bg I-‘)

Chlorophyll-a

(mg C 1-l)

4.4 f 0.7

7.4 f 0.5 8fl

14.6 f 2.4

27.7 f 5.7

20 f

1.5

17.3 f 0.6

5.0 f

1.4

15.9 f 0.8

2.9 f 0.5

24.8 f 0.8

LICalled ‘total residue concentration’ in TWC database. The arithmetic averages are for illustrative purposes only, and do not claim true statistical meaning; Morgan’s Point has higher salinity due to the fast transport of seawater in the ship channel, and lower freshwater input from the San Jacinto River.

carbon

Total organic

3.8 f 0.9

1.2

1.7

9.1 f

solids (mg I-‘)

Salinity (0%)

nitrogen

Total suspended

Total Kjeldahl (mg N I-‘)

(mg N I-‘)

1.1

0.13 f 0.02

0.035 f 0.01

Nitrate

(mg N 1-l)

15.3 f

0.05 f 0.01

0.20 f 0.02

0.08 f 0.01

0.06 f 0.01

Ammonia

-

0.04 f 0.01 0.08 f 0.02

0.43 f 0.05

0.08 f 0.01

0.10 f 0.025

(mg N 1-l)

1.4 f 0.06

0.10 f 0.009

0.03 f 0.007

0.51 f 0.02

Nitrate

0.07 f 0.007

0.07 f 0.008 0.105 f 0.013

0.42 f 0.02

0.22 f 0.01 0.27 f 0.01

(mg P I-‘)

0.26 f 0.02

(n = 87)

West Bay

(n = 34) (St.2424.01)

East Bay

(n = 57) (St.2423.01)

Morgan b Point (St.2421.0075)

0.21 f 0.001

(n = 104)

Smith Point/Eagle

(n = 35) (St.2439.025)

Total phosphate

(St.2422.02)

Trinity Bay

TABLE 2 of Nutrients and Other Chemical Parameters in Galveston Bay, Calculated from the TWC Database ( f la)

Ortho-phosphate (mg P I-‘)

Parameter

Average Concentrations

345

Seasonality of nutrients in Galveston Bay

G *na

30 25

c 3

0 $ g 2

20 15 10

6

-1

0.7

E

0.6

2

0.5

:

0.4

Tt

0.3

c"

0.2

A

0.1

5

(cl

30 25 h .Z .s d

20 15 10 5 0

(b) Fig. 2.

1983

1961 1960

1962

1965 1964

1967 1966

1969 1966

(4

Time series of: (a) temperature, (b) salinity, (c) ortho-phosphate perature at the Smith Point/Eagle Point Station.

and (d) tem-

TWC data from the mid-bay station at Smith Point/Eagle Point Chlorophyll-a

The chl.-a data measured from 1986-1989 are shown in Fig. 3. It is hard to see any seasonality, as the data are noisy and the &l-a maxima generally occurred anytime from January to June. After smoothing the data (by a Spoint moving average), however, a seasonal pattern emerges, with maxima occurring in the spring, during the months of March-April. Chl.-a concentrations are not correlated in any way to ortho-phosphate or nitrate concentrations (e.g. r = 0.015 for the correlation of orthoPOa3- with chl.-a, and 0.18 for the correlation of NOs- with chl.-a at it = 40). This behavior has been previously described (Pilson, 1984) for Narragansett Bay. Though no direct measurements exist for Galveston Bay, one could hypothesize that chl.-a concentrations and nutrient uptake

346

P. H. Santschi

1985

1986

1987

1988

1989

Fig. 3. Comparison of time-series of chlorophyll-a concentrations with temperature, measured at the Smith Point/Eagle Point Station. Smooth Iine = smoothed (i.e. 5-pt average) data; dotted line = raw data.

related to biomass-specific productivity are not directly related because of primary control by light availability, similar to that in other turbid estuaries (e.g. Cloern, 1987). Phosphorus

Ortho-phosphate concentrations co-vary significantly with total phosphorus concentrations, with a small but significant value for the intercept (Fig. 4(a)), suggesting that species other than ortho-phosphate (e.g. particulate and organic P) are minor fractions of the total phosphorus in the water of Galveston Bay. Calculated phosphorus concentrations in suspended particulate matter (in ,ug P/g sediment) in the bay water appear to be higher when suspended particulate matter (SPM) concentrations are low (Fig. 4(b)), and decrease to concentrations close to those in surface sediments (i.e. lo3 pg P/g or less) when suspended matter concentrations increase above 20 mg liter-’ (Fig. 4(b)). A similar elemental enrichment when suspended matter is present at low concentrations has been also described for Fe, Al, Pb and other trace metals in Galveston Bay (Morse et al., 1993; Benoit et al., 1994). The major factors appear to be particle sorting and the presence of filter-passing colloidal particles rich in trace elements and phosphorus, which would appear in the ‘dissolved’ fraction. These observations support a phosphate buffer mechanism unlike the one suggested previously (Fox et al., 1985, 1986; Froelich, 1988; Fox,

341

Seasonality of nutrients in Galveston Bay

y =0.037 + 1.06x;R= 0.88;P
i

0.6

0

0.1

0.2

0.3

0.4

Ortho-Phosphate

0.5

0.6

0.7

0.8

(mg P/L)

1

10

SPM (m&L)

100' 0.1

"111111' 1

"'I,',' 10

100

SPM (mg/L) Fig. 4. For the Smith Point/Eagle Point Station, the following correlations are shown: (a) ortho-phosphate (o-Pod) concentrations are correlated with total phosphate (TP04) (the value of intercept is significant, as the error of intercept is less than 30% of the value; the slope is 1, within the error); (b) phosphorus concentrations in suspended particles (Pp) are correlated with concentration of suspended particulate matter, SPM; (c) values of the partition coefficients, Kd, are correlated with SPM concentration. Note that removing the three high values in the left hand corner does not significantly change the regression line.

348

P. H. Santschi

1989). While these authors suggested that dissolved ortho-phosphate concentrations in the water column could be regulated to a large extent by mineral phase equilibria, the Galveston Bay data suggest a more physical mechanism of particle and colloid sorting, as discussed below. This mechanism, however, does not maintain constant P concentrations throughout the bay. Phosphorus concentrations in Galveston Bay waters are about an order of magnitude higher than can be calculated from apatite and iron phosphate solubility, and vary over an order of magnitude with season. In addition, annual variations of Ca2+ and HCOs- concentrations in the Trinity and San Jacinto River, which could regulate apatite solubility, are small (i.e. &30%; e.g. USGS, 1983) and not of sufficient size to explain the order of magnitude variation in phosphorus concentrations in Galveston Bay waters over an annual cycle. Phosphorus concentrations could, however, also be regulated by the transformation of iron oxyhydroxides which can carry large amounts of adsorbed phosphate, to Fe sulfides, during iron and sulfate reduction in sediments (e.g. Caraco et al., 1989; Caraco et al., 1990). During this process, adsorbed phosphate could be released to the pore waters, and become available for diffusion into the water column. Even though particulate phosphorus concentrations were not measured by the TWC, its concentrations can be estimated as total P - total dissolved P concentrations. Calculated partition coefficients, & (= (particulate P (mg kg-‘))/(ortho-phosphate (mg I-‘))), of phosphate are negatively correlated with suspended particulate matter concentration (SPM, the so-called ‘residue’ concentration by TWC), as shown in Fig. 4(c). When Kd values at the same SPM concentration of 100 mg liter-’ are compared, those from Galveston Bay are one order of magnitude below those calculated from data (Froelich, 1988) for the Amazon estuary (i.e. & values are l-2 x lo4 cm3 gg’ for the Amazon estuary, while for Galveston Bay, Kd values are l-2 x lo3 cm3 g-r). The & values calculated for Galveston Bay are near the lower end of the global range calculated from data given elsewhere (Froelich, 1988) and closer to the values which can be calculated for sediment-pore water systems. For example, from the recently published data for the Gulf of Mexico estuaries (Huanxin et al., 1994), one can calculate I& values of 102- lo3 cm3 g-’ for sedimentary phosphorus. Because high particle concentration regimes such as sediments and nepheloid layers have lower I& values, phosphorus would be less efficiently retained in these environments. Thus, P remobilization would become favorable. The negative correlation of & values with particle concentration found in Galveston Bay, often called ‘the particle concentration effect’, agrees

Seasonality of nutrients in Galveston Bay

349

with earlier observations on trace element and radionuclide partitioning made in the laboratory and in the field (e.g. Honeyman & Santschi, 1988, and references therein), including Galveston Bay (Baskaran & San&hi, 1993; Benoit et al., 1994). A strong particle concentration effect was found for Fe, Al, Pb, Zn, Cu and Ag in Galveston Bay waters (Benoit et al., 1994). This effect is to be expected if a fraction of trace elements and phosphorus is associated with colloids which are passing the 0.4 pm filter. Colloid mass needs to be related to that of suspended matter (Benoit et al., 1994). Indeed, we have found a substantial concentration of colloidal phosphorus > 1000 Dalton, isolated by cross-flow ultrafiltration, in Galveston Bay waters (Guo & Santschi, unpubl. results). The colloidal P concentrations were 3&80% of the filter-passing organic phosphorus concentration, sufficient to account for this effect. Colloidal phosphorus occurs in both fresh and salt water, associated with Fe, humic acids and other organic compounds (e.g. Carpenter & Smith, 1984; Young & Comstock, 1986; Ridal & Moore, 1990; Hollibaugh et al., 1991). Alternatively, resuspended coarser grained quartz-rich particles, generally low in trace element and phosphorus concentrations, could make up a larger percentage of SPM at higher concentration, thus diluting the fine particulate phosphorus concentration at higher suspended particle concentrations in the water. In addition, phytoplankton could be found in the fine fraction which likely dominates at low SPM concentration. The uptake of P by phytoplankton is higher than that caused by adsorption to detrital particles, on a per mass basis (Lebo, 1990). Ortho-phosphate is significantly correlated with nitrate concentrations (NO3 (mg N liter-‘) = -0.01 + 0.38(P04 (mg P liter-‘)); Y = 0.45, p < 0.001; it = 104, data not shown), indicating some linkage between the two, either by similar inputs or removal pathways and rates. Orthophosphate concentrations are also significantly correlated with temperature (Y = 0.33; p < 0.001; n = 104; Fig. 5(a)), and with salinity, S (Y = 0.36; p < 0.001; y1 = 104; Fig. 5(b)), but even better with both of these parameters simultaneously (i.e. Y = 0.504, p < 0.0001). Since salinity and temperature are not correlated (r = 0.12, n = 104) the negative correlation with salinity therefore likely indicates that the major source of phosphorus to the mid-bay is, as expected, from the river water end member (i.e. S = 0). On the other hand, the positive correlation with temperature suggests an external source of phosphorus during the summer months, such as that originating from benthic remobilization. The correlation of phosphate with temperature alone can be slightly improved (Y = 0.37, n = 104, p < O.OOl), when all data are shifted backwards by one month (i.e. the ortho-phosphate maximum occurred one month after the temperature maximum, in September, not shown).

350

P. H. Suntschi

0.8

y = 0.101+ 0.0056x; R = 0.33;PcO.001 0

J

0

0.6

Temperature

0

(“C)

I 10

20

30

Salinity

Fig. 5.

Correlation

of ortho-phosphate, o-Pod, with (a) temperature, at the Smith Point/Eagle Point Station.

T, and (b) salinity,

A September maximum is also obtained when the smoothed (using a 5-point moving average of the data) ortho-phosphate concentrations are fitted to a sine function (Fig. 6). While initial data smoothing increases the correlation coefficient, the result is the same when P data are not smoothed. Similar correlations can be found for the Morgan’s Point Station. Mean monthly concentrations of P in the Trinity River vary only slightly (TDWR, 1981), and thus cannot be responsible for the P maxima in the summer. As a matter of fact, the Trinity River usually has the lowest P loadings during the month of August and the highest ones during the

351

Seasonality of nutrients in Galveston Bay

0.8 2 \ 9 3 r” 5: 2

0.7 0.6 0.5 0.4 0.3

%

0.2

5

0.1

&

0 1960

1961

1982

1963

1964

1965

1966

1967

1966

1969

Fig. 6. Comparison of time-series data of ortho-phosphate, o-P04, at Smith Point/Eagle Point Station with smoothed data, and with least-square fit of o-PO, to a sine function (o-PO4 (mg liter-‘) = 0.069sin(29.4 x m - 111) + 0.22, with m = month; r = 0.42, p < 0.001).

month of February (TDWR, 1981). This river is also responsible for the bulk of the nutrient loadings to the estuary (TDWR, 1981). Point sources such as power plant emissions could be responsible for some of the seasonality of the phosphorus concentrations, seen for the mid-bay and Trinity Bay stations. For example, a power plant situated at Moses Lake near Smith Point discharges cooling water into mid-bay, and an HLP power plant station draws water from Cedar Bayou near Morgan’s Point and discharges it into the Trinity Bay. As the emission rates of power plants and municipal and industrial plants are much lower than the riverine phosphorus loadings (TDWR, 1981), and would not be expected to strongly vary with season, this possibility is less likely. Nutrient concentrations in the water column are the net result of removal processes and supply (from rivers, municipal and industrial plant effluents, atmospheric deposition and sediment regeneration). Even though biotic and abiotic processes removing phosphorus from the water are related to chl.-a concentrations (which peak in spring) and the concentrations of Fe and Ca mineral phases (which do not vary much seasonally), the regular phosphorus maxima in September, coupled with observations of phosphorus maxima in Trinity Bay during the summer months, suggest that the conventional picture of purely biological control by water column processes needs to be modified to include sedimentwater interactions. In another estuary, Delaware Bay, maximum phosphorus regeneration in the summer months was linked to sewage treatment plant inputs (Lebo & Sharp, 1992). Like the Delaware River, the average Trinity River

352

P. H. Santschi

discharge rate also shows a seasonal pattern, with peak flow rates in May-June, and lows in August (Solis & Longley, 1992). However, this pattern is not reflected in the salinity record in Fig. 2. River input control should result in nutrient concentration maxima after peak flow rates from sewer overflows. With generally low river flows in the late summer and fall, this would result in peak concentrations of PO4 in spring/early summer, not in the early fall. Even though river discharge undoubtedly exerts some influence on nutrient concentrations, it is suggested that phosphorus regeneration from the Trinity Bay sediments is modifying inputs from the river (see later sections). The importance of phosphorus regeneration suggests a significant fraction of phosphorus reaching Galveston Bay is released into the Gulf of Mexico. This would be expected, as estuaries generally have a low capacity to retain phosphorus. For example, Narragansett Bay only retains 3% of all P inputs, Chesapeake Bay 1l-16% (Nixon, 1988), and Delaware Bay 16% (Lebo & Sharp, 1992). Inorganic nitrogen

Most of the dissolved nitrogen in the water is present in an organically bound form (Table 2), as total Kjeldahl nitrogen. However, since Kjeldahl nitrogen concentrations are erratic, they will not be considered for discussion here. Nitrate concentrations are significantly (r = 0.40; n = 104, p < 0.001) and inversely correlated with salinity (Fig. 7). Such a pattern could indicate nitrogen inputs in the initial phase of large fresh water inputs,

Salinity Fig. 7.

Correlation

of nitrate concentrations with salinity for the Smith Point/Eagle Station.

Point

Seasonality of nutrients in Galveston Bay

353

possibly from storm sewer overflows in the drainage basin of the Trinity River, or two end-member mixing and removal. On the other hand, frequent observations of close to zero concentrations of nitrate in the summer could indicate denitrification in bottom waters and sediments (Zimmerman & Benner, 1994).

TWC DATA AT OTHER

STATIONS

IN GALVESTON

BAY

Since sampling of the different bay stations was in general not carried out synoptically (i.e. on the same day, or the same week), no direct comparison of the results is possible. However, correlations of nutrient concentrations with either salinity or temperature (as a surrogate for seasonal variations), or both, can still be compared for the different stations. Nutrient concentrations at the Morgan’s Point and Trinity Bay station, but not at East and West Bay stations, correlate with temperature and salinity in a similar fashion (i.e. same slope, but different intercepts) as is the case for the mid-bay station (not shown). The Morgan’s Point Station also shows a similar multiple correlation with temperature and salinity as the mid-bay station. The best correlation of phosphorus with temperature alone yields Y = 0.30 (p < 0.02) with salinity (or chlorinity) alone, Y = 0.21 (p < 0.1 at yt = 57). However, if both temperature and salinity are used as variables, the multiple stepwise regression becomes highly significant, with r = 0.363 @ < 0.01). The best fit through the data set (n = 57) is almost identical to that for the mid-bay station, within the errors, with the same values for the intercept and the slope of the temperature part of the regression. In addition, the Trinity Bay data set also produced a similar temperature dependence (slope) as the mid-bay and upper bay stations. However, the salinity parts of the multiple stepwise regressions of phosphorus concentrations are different at each station. Inverse correlations of phosphorus and nitrate concentrations with salinity at the Trinity Bay station are significant and almost identical to those at the mid-bay station, strongly suggesting that the mid-bay station is strongly influenced by water from Trinity Bay. In contrast, NO3 is not correlated with salt at the Morgans Point, East and West Bay stations (not shown). This likely implies different processes regulating their concentrations for these latter three stations. This comparison indicates that the waters at the mid-bay station are mostly affected by waters from the Trinity River, and less by those from the San Jacinto River (Morgan’s Point Station). Nutrient concentrations in East and West Bay are significantly lower than those in the upper bay,

354

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0

5

10

15

20

25

30

Salinity

Fig. 8.

Results from salinity transects of Galveston Bay in 1989. (a) ortho-phosphate, o-Pod, (b) N03, (c) Si02 (October only). Circles are August 1989 data, squares are October 1989 data, triangles are data from the TWC database, taken within a week from our sampling date. No Morgan’s Point data are displayed.

355

Seasonality of nutrients in Galveston Bay

and vary in a different fashion than those from the mid-bay station. This is likely due to more restricted mixing with the rest of Galveston Bay, and local factors (such as variable local nutrient inputs), or increased productivity in shoals. Better area1 and temporal data coverage would be needed to define the processes which control nutrient concentrations in East and West Bay. A close inspection of the spatial and temporal variability of nutrient concentrations suggest local input control as more important also for the Morgan’s Point Station, which is close to the outflow of the San Jacinto River and Buffalo Bayou, which contain water from many industrial and municipal sewer effluents.

NUTRIENT CONCENTRATIONS ALONG BAY TRANSECTS (1989, 1993) Results from the determination of nutrient concentrations in water, taken in August and October 1989, and in July 1993, during transects leading from the Trinity River to the Gulf of Mexico, are given in Figs 8 and 9. These data showed that phosphorus concentrations in the river water endmember and in the upper bay were considerably higher during the warm August month than during the colder October (Fig. 8(a)). The phosphorus transect from July 1993 shown in Fig. 9 clearly shows concentration maxima in upper Trinity Bay. Nitrate, phosphorus and silica also showed local concentration maxima in the intermediate salinity range located in upper Trinity Bay (Fig. 8(a)-(c)), indicating an extra source in this region, likely due to nutrients regenerated from sediments. These observations strongly support the long-term trends observed for P concentration maxima occurring during the late summer months seen in the TWC data set. 0.14 3 2

0.12

E

0.1

2 0.08

c 4

0.06

zc,

0.04

“r 0

0.02

!/! 0

-0e yy

- 0.0039

0 0.5

+ 0.075x; lb 0.72; P
1

1.5

SiO, (mg S/L) Fig. 9.

Correlation

between

Si02 and ortho-phosphate,

o-Pod,

of the October

1989 data.

P. H. Santschi

356

Other sources such as San Jacinto River water or power plant effluents are unlikely, as this is not reflected in the data, which clearly show P concentration maxima in the upper, not lower, Trinity Bay. Dissolved silica, which significantly correlates with phosphate (Fig. 9) has only benthic and no anthropogenic sources. Biogenic silica is regenerated from the sediments during the dissolution of diatom remains. Its flux can be greatly enhanced during low oxygen conditions (Hammond et al., 1984), because of remobilization of silica pre-adsorbed onto iron oxyhydroxides released during pyritization. Its concentration maxima is in the same region as that of phosphorus (Fig. 8(a) and (c)), suggesting a benthic source for phosphorus in Trinity Bay. A pronounced benthic flux of phosphorus in Trinity Bay is also suggested from a phosphorus transect taken in July 1993 (Fig. 10). Preliminary pore water measurements in Trinity Bay sediments indicate that the benthic flux of phosphorus could rival that from the river (Warnken et al., unpubl. data). More detailed benthic flux studies will be required to test the hypothesis of benthic regeneration as the dominant nutrient source in this region. Phosphorus release from estuarine sediments is not uncommon. For example, Liss (1976) and Pagnotta et al. (1989) described non-conservative behavior of phosphorus in estuaries, i.e. phosphate maxima at mid-salinities. Typical phosphorus fluxes from estuarine sediments are of the order of 3&60 mg P rnp2 day-’ or higher (up to 230 mg P rnp2 day-i), as was experimentally determined in Narragansett Bay (Elderfield et al., 1981) the Potomac River (Callender & Hammond, 1982), San Francisco Bay (Hammond et at., 1985) and Guadalupe Bay in South Texas (Montagna, 1989), using in-situ benthic flux chamber incubations. Benthic fluxes of this magnitude are high enough to produce phosphorus maxima as those seen 0.12 5‘ 'a ;. $ z o::

July

1993

:

25

30

35

01 0.08 0.06

E 2 g

0.04 0.02 0

5

10

15

20

Salinity Fig. 10.

phate,

Results from a salinity transect of Galveston Bay in July 1993 for ortho-phoso-Pod, showing a benthic source for phosphate in Trinity Bay. No Morgan’s Point data are displayed.

Seasonality of nutrients in Galveston Bay

351

in the data. For example, a flux of 10 mg P rnp2 day-’ over 30 days in a 2 m water column would result in a concentration increase of 0.15 mg P liter-‘. If phosphorus maxima are indeed produced by regeneration in surface sediments, several mechanisms could be responsible: (1) Phosphorus release from sediments during the anaerobic diagenesis of iron minerals, as has previously been suggested (Caraco et al., 1989, 1990) for other estuarine systems. (2) Phosphorus release from sediments during organic carbon remineralization (e.g. Froelich et al., 1979; Baccini, 1985). (3) Phosphorus release from phosphorus stored in bacterial biomass residing in the surface sediments (e.g. Shapiro, 1967; Laczko, 1988). For cases (2) and (3), fluxes of phosphorus should be linked to those of nitrogen species. Furthermore, it has been reported (Baccini, 1985) that experimental results indicate that pathway (2) can be negligible. In addition, since there is no discernible correlation of phosphorus with any nitrogen species in the water column in the data set from these two transects, cases (2) and (3) are less likely. This can be expected when the same benthic redox conditions which promote phosphorus regeneration to the overlying water also promote denitrification and loss of nitrate from the water column. The pyritization of iron minerals and the rapid incorporation of various trace elements into pyrite in the surface sediments of Galveston Bay has been reported (Morse et al., 1993). It is therefore proposed that phosphorus is regenerated during iron and sulfate reduction processes, which are strongest at the end of the summer.

SUMMARY

AND CONCLUSIONS

The statistical and time-series analysis of the Galveston Bay data base of the TWC for 1980-1989, and the nutrient analysis from two bay transects in 1989, revealed the following: (1) A phosphate buffer mechanism appears to regulate phosphorus concentrations in the water unlike those previously described in the literature. As the analysis of data indicates, such a mechanism must include physical mechanisms such as colloid-particle interactions and particle sorting, as indicated from correlations of phosphorus concentrations in suspended particles, or the partition coefficient, &, of phosphorus between particles and water on one hand, and suspended particulate matter concentration, SPM, on the other hand. This effect has previously been termed the ‘particle concentration effect’ (e.g. Honeyman & Santschi, 1988). Phosphorus

358

(2)

(3)

(4)

(5)

(6)

P. H. Santschi

concentrations vary seasonally and are higher than in many other estuaries. The most important sources of phosphorus to the bay are from the Trinity River and the San Jacinto River, as can be seen from the inverse correlations between phosphorus and salinity at the stations close to the river mouths. Phosphorus maxima in the bay water occur regularly in September at the mid-bay station (Smith Point/Eagle Point), the Trinity Bay and Upper Bay/Buffalo Bayou station (Morgan’s Point), but not as much at the lower bay stations in East and West Bay. This is expressed by a strong correlation between phosphorus and temperature, which is at its optimum when phosphorus data sets are shifted by one month. Chlorophyll-a maxima occur on a regular basis in March-April at the mid-bay station, suggesting that in Galveston Bay, phytoplankton blooms in the spring, the same time as in most temperate estuaries and coastal embayments. Nitrate concentrations in the water appear to be mostly regulated by freshwater inputs, as seen by the significant negative correlation of nitrate concentrations with salinity, and by denitrification processes occurring mainly during the summer months at the stations located in or near the ship channel, frequently leading to close to zero concentrations during that time. Concentrations of nutrient elements in the East and West Bay are considerably lower, and do not significantly correlate with temperature and salinity as those in the mid-and upper regions of the bay. Nutrient data from transects along salinity gradients collected in 1989 and 1993 show mid-salinity maxima centered in upper Trinity Bay. Along with the significant correlations between dissolved silica and phosphorus concentrations, they support the hypothesis of significant benthic inputs of phosphorus during the summer/fall months, leading to recurrent concentration maxima during that time.

ACKNOWLEDGEMENTS The author thanks Patrick Roques and Bizzy Cubala from the Texas Water Commission for sending copies of the database. The help of Herold Stone and Jonathan O’Neill Samuel1 with the transfer of the Texas Water Commission data base to MS Excel on the Macintosh, as well as with the statistical analysis, is gratefully acknowledged. Nutrient analyses of samples collected in October 1989 along a salinity transect were carried out by the Marine Operations Group of the Department of Oceanography, Texas

Seasonality of nutrients in Galveston Bay

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A & M University, College Station, TX. Data were made available by courtesy of Dr Doug Biggs. Selected samples from an August 1989 transect were also analyzed by Rama Santschi. The critical comments by L. Cifuentes and anonymous reviewers, which helped to improve this manuscript, are gratefully acknowledged. The research reported here was supported in part by the Texas Institute of Oceanography and the Texas Seagrant Program.

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