¹³C

MANGROVE LEAVES.— Carbon. Mangrove leaves showed relatively constant C compositions. Elemental C compositions were near 45% for green and yellow leaves, with detrital leaves increasing to 55% at one station (Table 2, Fig. 2, top). Carbon isotopic compositions of the three age classes of leaves generally had quite similar values at each station, with a 1-2 increase in mean values in mid-estuary (Fig. 2, bottom).

Nitrogen. Changes in leaf nitrogen chemistry were larger than changes in plant carbon chemistry. Increases in N/C ratios and decreases in N isotopic compositions occurred for all three leaf color classes at the two most landward stations (Fig. 3). Withdrawal of N in senescing yellow leaves still on trees, followed by N enrichment of decaying orange and black leaves in water was evident at all stations where yellow leaves had lowest N/C ratios (Fig. 3 top). Isotopic changes accompanied these changes in leaf N status, with N withdrawal leading to highest ¹⁵N values in yellow leaves (Fig. 3 bottom).

Figure 2. C concentrations (top) and isotopic compositions (bottom) of three color classes of mangrove leaves in the Shark River estuary. Symbols represent green leaves taken from trees (open diamonds), yellow leaves taken from trees (open squares) and detrital orange and black mangrove leaves taken from shallow water below the channelside mangroves (open triangles). Large X shows values for a sample of mangrove wood taken at the 9 km station (Table 2). [click on images above for larger versions]

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Figure 3. N/C molar ratios (x1000; top) and N isotopic compositions of three color classes of mangrove leaves in the Shark River estuary. Symbols as in Figure 2. [click on images above for larger versions]

SULFUR.— Large sulfur changes were evident in mangrove leaves collected across the transect. Sulfur content of green leaves varied by 4x across the transect and peaked in mid-estuary (Fig. 4 top). Yellow leaves had very high S contents (and N/S molar ratios near 1; Fig. 5), while detrital leaves had much lower S contents that were comparable to those of green leaves. The S isotopic analyses showed strong mid-estuary minima for all three classes of leaves, and yellow and green mangrove leaves had similar isotopic values (Fig. 4, bottom). The net loss of S in the transition from yellow to detrital leaves (Fig. 4, top) was accompanied by a net increase in isotopic compositions (Fig. 4, bottom), i.e., detrital leaves had higher S isotopic compositions than yellow leaves. Strong changes in S chemistries were also evident in N/S ratios (Fig. 5), with values for all samples falling below the 30-40 values typical of plant protein (Dijkshoorn and Van Wijk, 1967).

Figure 4. S/C molar ratios (x1000; top) and S isotopic compositions of three color classes of mangrove leaves in the Shark River estuary. Symbols as in Figure 2. [click on images above for larger versions]

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Figure 5. N/S molar ratios of three color classes of mangrove leaves in the Shark River estuary. Symbols as in Figure 2. [larger image]

Figure 6. Isotopic compositions of filter-feeding barnacles (diamonds), mussels (squares), and mangrove leaves (symbols as in Fig. 2) across the Shark River estuary. Large + symbol shows values for wood-boring isopods taken from mangrove wood at the 9 km station. [larger image]

FILTER FEEDERS.— We collected filter feeding barnacles and mussels along with the three classes of mangrove leaves, and compared isotopic compositions of barnacles and potential mangrove foods to study possible trophic connections. Isotope results for the two types of filter feeders were generally similar (Fig. 6), although mussels had significantly lower values than barnacles for N and S isotopes (mean isotopic differences were -3.7 ± 2.7 and -1.8 ± 0.6 for N and S respectively, with error terms representing 95% confidence limits).

In a qualitative sense, if these filter feeders were consuming mangrove detritus as the dominant food resource, one could expect their isotope values to track mangrove isotope values throughout the estuary. This expectation was met in the S isotope results where consumer isotope values generally tracked those of mangroves (Fig. 6), but C and N results showed a different, divergent pattern between mangrove and consumer isotope values, especially at the seaward (C results) and landward (N results) ends of the estuary. In this overall qualitative evaluation, the case for dominant mangrove inputs, based on the combined C, N and S isotopes, appeared strongest in mid-estuary.

We used a two-source mixing model based on S isotopes to quantify these estimates, with mixing equations of the general form:

P_i = 100

P_i*S_i = 100*S_CONSUMER

where P_i values are percent contributions to the diet, and S_i and S_CONSUMER are the sulfur isotope values of respectively organic matter sources (foods) and consumers. Consumers were assumed to have the same S isotope values as their foods (Peterson and Fry, 1987). The two-source mixing model considered only phytoplankton and green mangrove leaves as possible food resources, and for barnacles, showed strongest mangrove use in mid-estuary and strong importance (>40%) of phytoplankton throughout the system (Table 3, Fig. 7).