Heyman, W.D.

Rooker, J.R.

Wetmore, L.S.

English

Aquatic Commons

 Proceedings of the 65th Gulf and Caribbean Fisheries Institute                  November 5 – 9, 2012   Santa Marta, Colombia 
Stable Isotopes as Natural Markers of Nursery Productivity for  
Juvenile Snappers Across Back-reef Habitats in Belize 
 
Isótopos Estables como Marcadores Naturales de Productividad de Vivero para  
Pargos Juveniles Ocupan Hábitats de Arrecife Posterior de Belice  
 
Isotopes Stables comme Marqueurs Naturels de la Productivité de la Pépinière  
pour Jeunes Vivaneaux Partout au dos des Récifs des Habitats au Belize 
 
LYNNE S. WETMORE, WILLIAM D. HEYMAN, and JAY R. ROOKER 
Texas A&M University, P.O. Box 1675, Galveston, Texas 77553 USA 
 
EXTENDED ABSTRACT 
In recent years, marine conservation efforts have focused increasingly on the preservation of productive nursery 
habitats for juvenile fishes (Beck et al. 2001), and characterizing nursery productivity is particularly critical in the tropics, 
where many important fisheries species require multiple back-reef nurseries during juvenile development (e.g. Adams et al. 
2006). The dependence of these fishes on multiple habitat types makes them particularly vulnerable to environmental 
degradation and habitat loss, and studies have shown that the removal or modification of one or more required nursery 
habitats can adversely affect recruitment success and year class strength (Mumby 2006). Because of this, identifying 
productive back-reef nurseries and characterizing the sources of primary production (i.e. organic matter) that support 
consumers within these early life habitats is critical to the effective management of many tropical fisheries.  
In the present study, we used d13C and d15N stable isotopes as dietary tracers to evaluate back-reef nursery productivity 
across the Belize coastal lagoon. The objectives of this study were to (a) identify the main sources of primary production 
supporting three species of juvenile snappers (family Lutjanidae), and (b) model the effects of seasonal runoff and water-
shed impact (i.e. anthropogenic influence) on primary productivity within these nurseries. To accomplish this, four primary 
producers (phytoplankton, benthic diatoms, mangroves, and seagrass) as well as three species of juvenile snappers (dog 
snapper Lutjanus jocu, gray snapper L. griseus, and schoolmaster L. apodus) were collected from two latitudinal regions 
(i.e. North, South) in southern Belize (Figure 1). Samples were taken from an inner-shelf and an outer-shelf site within each 
region, and each site was sampled separately during the dry and the rainy season. For each site and season, estimates of 
organic matter contribution from the four primary producers were modeled using the Isosource program, based on isotopic 
d13C and d15N values of producers and juvenile snappers (Table 1). 
Of our four producers, only phytoplankton showed a significant seasonal shift in isotopic signature, with more enriched 
d15N values (0.9 – 5.2 [dry season]; 2.0 – 4.6 [rainy season]) and more depleted d13C values (≈ -17-27 [dry]; ≈ -20-32 
[rainy]) during the rainy season.  Juvenile snappers at the north inner and south outer sites also showed significant seasonal 
differences in isotopic signature; however, these shifts occurred in the opposite direction from the observed shifts in 
phytoplankton, which indicates a seasonal shift in the source of primary production supporting juvenile snappers at these 
two sites, rather than a case of snapper signatures simply tracking seasonal changes in producers (Table 1). No significant 
differences in snapper isotopic signatures were observed at the north outer or south inner study sites, and this latter result 
was somewhat unexpected, given the strong riverine influence in our southern sampling region. 
Isosource results for the two study sites with no seasonal differences in consumer signatures (i.e. north outer, south 
inner) indicated that seagrass and benthic diatoms were the largest contributors of organic matter supporting juvenile 
snappers during both sampling seasons (42-62% [dry]; 47-72% [rainy]). At the north outer site, estimated contribution from 
phytoplankton production was also significant (median 32% contribution [dry]; 28% [rainy]), reflecting the strong marine 
influence and regional upwelling along this area of the shelf edge. Meanwhile, Isosource results for the two sites where 
strong seasonal shifts in consumer signatures were observed (i.e. north inner, south outer) revealed that isotopic differences 
in juvenile snappers at both locations were driven primarily by a marked decrease in phytoplankton contribution from the 
dry to the rainy season, with mean estimated contribution at the north inner site decreasing from 25% to 12% in the rainy 
season, and median phytoplankton contribution at the south outer site decreasing from 40% in the dry season to 15% in the 
rainy season. Declines in contribution rates of phytoplankton during the rainy season were countered by increases in 
seagrass/benthic diatom contribution (21 – 43% [dry season] to 56 – 73% [rainy]).  These results indicate that the main 
source of organic matter supporting upper-level consumers may change significantly between the dry and rainy seasons at 
certain back-reef nurseries. 
Trophic productivity of back-reef nurseries across the Belize coastal lagoon may be strongly related to land use and 
management policies within adjacent watersheds. The two study sites (i.e. north inner, south outer) where we observed 
seasonal shifts in juvenile snapper signatures are both impacted by watersheds with extensive aquaculture and agricultural 
development. Decreased water quality due to anthropogenic runoff during the rainy season may have influenced the 
Page 112  65th Gulf and Caribbean Fisheries Institute  
 
consistent seasonal decreases in phytoplankton contribu-
tion supporting nursery food webs at these sites. In 
comparison, despite similarly strong riverine input during 
the rainy season, no significant seasonal effects on primary 
production were observed at the south inner site, where the 
majority of land within the watershed is managed under the 
Maya Mountain Marine Corridor and coastal development 
is strictly regulated. Land-sea connectivity is well estab-
lished in Belize (Heyman and Kjerfve 1999), and our 
results appear to indicate that terrestrial runoff from areas 
of extensive aquaculture and agriculture may negatively 
impact the productivity of adjacent coastal nurseries for 
upper-level consumers (i.e. juvenile snappers). Thus, 
preservation of ecological functioning within back-reef 
nurseries in Belize may require consideration of regional 
watershed influence, in addition to local habitat conserva-
tion. 
LITERATURE CITED 
Adams, A.J., C.P. Dahlgren, G.T. Kellison, M.S. Kendall, C.A. Layman, 
J.A. Ley, I. Nagelkerken, and J.E. Serafy. 2006. Nursery function of 
tropical back-reef systems. Marine Ecology Progress Series 318:287
-301. 
Barry, J.P. and P.K. Dayton. 1991. Physical heterogeneity and the 
organization of marine communities. Pages 270-320 in J. Kolasa and 
S.T.Pickett (eds.) Ecological Heterogeneity. Springer Verlag, New 
York, New York USA.  
Beck, M.W., K.L. Heck, Jr.,  K.W. Able, D.L. Childers,  D.B. Eggleston, 
B.M. Gillanders, B. Halpern, C.G Hays, K. Hoshino, T.J. Minello, 
R.J. Orth, P.F. Sheridan, and M.P. Weinstein. 2001. The identifica-
tion, conservation, and management of estuarine and marine 
nurseries for fish and invertebrates. BioScience 51(8):633-641. 
Heyman, W.D. and B. Kjerfve. 1999. Hydrological and oceanographic 
considerations for integrated coastal zone management in southern 
Belize. Environmental Management 24(2):229-245. 
Mumby, P.J. 2006. Connectivity of reef fish between mangroves and coral 
reefs: algorithms for the design of marine reserves at seascape 
scales. Biological Conservation 128:215-222. 
Polis, G.A., R.D. Holt, B.A. Menge, and K.O. Winemiller. 1996. 
Temporal and spatial components of food webs. Pages 435-460 in: 
G.A. Polis and K.O. Winemiller (eds.) Food Webs: Integration of 
Patterns and Dynamics. Chapman and Hall, New York, New York 
USA.  
Figure 1. Regional context of study sites in southern Belize. Northern and southern sam-
pling regions are labeled, and black triangles indicated the location of the inner-shelf and 
outer-shelf study site within each region. Shaded areas denote habitat distribution within 
the coastal lagoon.  
    Wetmore, L.S.  et al.     GCFI:65   (2013)        Page 113 
 
Table 1. d13C and d
15N signatures (mean ± SD) of primary producers (phytoplankton [PMA], benthic microalgae [BMA], 
mangrove [MG], and seagrass [SG]) and juvenile fishes (dog snapper [DS], gray snapper [GS], and schoolmaster [SM]) 
collected at each region (North, South) and shelf position (Inner, Outer) during the Dry and Rainy sampling seasons in 
2009. MG leaves were sampled from each region during each sampling season, but were pooled across shelf positions.  
Region Site 
Sample 
Type 
  d13C   d15N 
  DRY RAINY   DRY RAINY 
Producers                 
North Inner PMA   -27.1 ± 4.7 -30.2 ± 4.1   1.5 ± 0.8 4.6 ± 0.6 
    BMA   -15.5 ± 2.9 -15.2 ± 5.7   1.3 ± 0.7 2.1 ± 0.3 
    MG   -28.4 ± 1.0 -28.5 ± 0.5   0.8 ± 2.0 -0.1 ± 2.5 
    SG   -7.5 -11.7   2.4 0.9 
                  
  Outer PMA   -23.8 ± 3.3 -23.8 ± 5.6   0.9 ± 0.7 1.9 ± 1.2 
    BMA     -8.7 ± 3.5 -11.6 ± 7.7   2.4 ± 0.5 2.4 ± 0.2 
    MG   -28.4 ± 1.0 -28.5 ± 0.5   0.8 ± 2.0 -0.1 ± 2.5 
    SG   -8.3 -8.3   3.3 1.4 
                  
South Inner PMA   -18.3 ± 1.7 -31.3 ± 2.1   5.2 ± 0.2 4.4 ± 0.8 
    BMA   -12.2 ± 7.0 -11.2 ± 5.3   2.3 ± 0.2 1.5 ± 1.9 
    MG   -29.6  ± 1.3 -28.5 ± 0.5   2.1 ± 0.5 0.8 ± 0.7 
    SG   -6.8 -7.6   2.3 3.0 
                  
  Outer PMA   -17.9 ± 3.8 -20.1 ± 7.9   1.0 ± 0.9 2.0 ± 2.1 
    BMA     -4.5 ± 0.6   -8.8 ± 2.3   0.8 ± 0.6 1.8 ± 1.7 
    MG   -29.6  ± 1.3 -28.5 ± 0.5   2.1 ± 0.5 0.8 ± 0.7 
    SG   -8.1 -8.3   1.3 2.4 
                  
 Juvenile Snappers               
North Inner DS   -16.5 ± 1.3 -15.9 ± 2.6   11.1 ± 0.6 10.0 ± 0.4 
    GS   -16.8 ± 2.6 -15.1 ± 1.0   10.9 ± 1.8 10.2 ± 0.4 
    SM   -14.4 ± 2.0 -14.9 ± 2.3   10.9 ± 0.8 10.3 ± 0.9 
                  
  Outer DS   -14.5 ± 0.9 -14.4 ± 1.4     9.6 ± 0.4 10.6 ± 1.1 
    GS   -15.1 ± 1.0 -15.0 ± 0.8     9.8 ± 0.5 10.0 ± 0.2 
    SM   -14.3 ± 1.3 -14.9 ± 1.0     9.4 ± 0.5   9.8 ± 0.5 
                  
South Inner DS   -16.8 ± 1.4 -16.8 ± 0.7   11.7 ± 0.4 10.8 ± 0.6 
    GS   -14.8 ± 2.0 -15.3 ± 1.6   11.2 ± 0.2 10.2 ± 0.4 
    SM   -15.1 ± 2.0 -14.9 ± 1.3   10.9 ± 0.4 10.4 ± 0.4 
                 
  Outer DS   -16.4 ± 0.1 -13.7 ± 1.7   11.0 ± 0.2   9.5 ± 1.0 
    GS   -16.5 ± 1.1 -11.4 ± 0.9   10.9 ± 0.5 10.0 ± 0.1 
    SM   -14.6 ± 0.7 -13.3 ± 1.7   10.4 ± 0.7   9.3 ± 0.4 
                  


Stable Isotopes as Natural Markers of Nursery Productivity for Juvenile Snappers Across Back-reef Habitats in Belize

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Stable Isotopes as Natural Markers of Nursery Productivity for Juvenile Snappers Across Back-reef Habitats in Belize

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