Final – 2 October 2006



### Seagrass Monitoring in the Florida Keys National Marine Sanctuary

Jam e s W

. Fourqurean (Southeast Environm ental Research Center and Departm e nt of Biology, Florida Inte r n ationa l University, Mia m i, FL) Michael J. Durako (Center for Ma rine Science and Departm e nt of Biology, University of North Carolina at W i l m ington, W ilm ington, NC) Joseph C. Ziem an (Departm ent of Environm ental Science, Univers ity o f Virginia, Charlottesville, VA) Susie P. Escorcia and Leanne M. Rutten (S outheast Environm ental Research Center and Departm e nt of Biology, Florida Inte rnational University, Miam i, FL)

Goals The general objective of seagrass monitoring in th e Flo r ida Ke ys National Mari ne Sanctuary (FKNMS) (part of the W a ter Qualit y Protection Program ) is to m eas ure the status and trends of
seagras s co mmunities to evalua te progress to wa rd protec ting and r e s t oring the living m a rine resources of the Sanctuary. The scope and depth of this m o nitor i ng ef f o rt a r e witho u t pr eceden t or peer for seagrass ecosystem s throughout the worl d. Specific objectives are: 1) To provide data needed to m a ke unbiased, statistica lly rigorous statem ents about the status and tem poral trends of
seagras s co mmunities in the Sanctuary as a w hole and within def i ned stra ta; 2 ) To help def i n e reference conditions in order to develop resour ce-based water quality standards; and 3) To provide a fram e work for testing hypothesized pollu tant f a te /e f f ect rela tio nships through processoriented research and m onitoring. In order to m eet these object ives, we have developed these goals for the project:

• Define the present distribution of seagrasses within the FKNMS

• Provide high-quality, quantitative data on th e status of the seagrasses with in the FKNMS

• Quantify the im portance of seagra ss prim ary production in the FKNMS

• Def i ne the b a seline cond itions f o r the seagras s co mmunities

• Determ ine relationship s between wa ter quality and seagrass status

• Detect trend s in the d i str i bution a nd status of th e seagras s co mmunities

Methods To reach th ese goals, fou r kinds of data are be ing collected in seagras s bed s in the FK NMS:

• Distribution and abundance of seagrasses using rapid assess ment Braun-B l anquet surveys

• Dem ographics of the seagras s co mmunitie s using leaf -s car coun tin g and pop ulation dem ographics techniques

• Seagrass productivity of the dominant species of seagrass in the FKNMS ( Thalassia testudinum ) using the leaf- m ark and harvest m e thod

• Seagrass nu trien t availab ility us ing tissue concen tration assay s

These data are being collected at th ree di fferent types of sites within the FKNMS:

Level 1 Stations: Sam p led quarterly for seagrass abunda nce, dem ographics, productivity, and nutrient availability. These stations are all co -l oc ated with W a ter Quality Monitor i ng Projec t stations (F ig. 1).

57Final – 2 October 2006

Level 2 Stations: Random l y selected locations within the FKNMS, sampled annu ally f o r seagrass abundance, dem ographics, and nutrient av ailab i lity. Each year, new locations for Level 2 stations are chosen. Level 3 Stations: Random l y selected locations within the FKNMS, sampled annu ally f o r seagrass abundance. Each year, new locations for Level 3 stations are chosen.

We are assessing both interannual and seasonal trends in seagrass comm unities. The

m i x of site types is in te nded to m o nitor trends through quarterly sam p ling at a few perm anent locations (Level 1 sites) and to annually characterize the broa der seagrass population through less intensive, one-tim e sam p ling at more locations (Level 2 and 3 sites). Figure 1. Map of the Fl ori d a Key s sho w ing the location of Level 1 seag rass status and trends

m onitoring sites. Site num b e rs correspond to stations of the Water Quality Monitoring

Findings to Date The signif i c a nt changes in seagrass comm unities at the p e rm anent Level 1 sta t ions that we reported las t fiscal year conti nue to be pres en t after an a ddition a l year of sam p ling. These changes are consistent with m ode l predictions of nutrient-induced ch anges of th ese sys t em s. There m a y be reasons for these observations that are unrelated to hum an activities in the region, but the spatial pattern of changes and the agreem ent of the changes with m odels of the system suggest that there is regional-scale change in nutrient availability that is causing changes in seagrass beds over a wide portion of the FKNMS.

In 2003, we resurveyed 202 Level 2 and Level 3 stat ions that were last visited during the summer of 1996. The data collected during th is resurvey are stil l being assessed, but prelim inary analyses indicate that there are no large-spatial-scale trends in the abundance of the dom ina n t benthic plant types over these seven years.

In genera l, nutrient add ition to aqu a tic environ m ents shif ts the com p etitiv e balan c e to f a ster

growing pr im ary producers. The consequen ce of this genera lity in seagr a ss-dom inate d environm ents is that seagrasses are the dom i na nt prim ary producers in oligotrophic conditions. As nutrien t availability increas es, th ere is an in crease in the importance o f m acroalgae, both freeliving and epiphytic, with a con c om i t ant decreas e in seag ras s es because of com p etition for light. Macroalgae lose out to even faster-growing m i croalgae as nutrient avai lability continues to increase. F i rst, epiphytic m i croa lgae replace epip hytic m acroalgae on seagrasses; then planktonic

m i croalgae bloom and deprive all benthic plants of light under the m o st eutrophic conditions. The South F l orida case is m o re complicated than the general case descri bed above because there are six com m on seagras s species in South Florid a. These species have different nutrient and light requirem e nts, and have differing responses to eutrophication. Large expa nses of the shallow

m a rine environm ents in South Florida are so oli gotrophic that biom ass and growth of even the slowest-g r o w ing local s eagrass species, Thalassia testudinum , are nutrient- lim ited; at this very oligotrophic end of the spectrum

, increases in nutr ient availability actua lly cause increases in

58Final – 2 October 2006

seagrass biom ass and gr owth rate. A s nutrient av ailability increases beyond what is required by a dense stand of T. testudinum , there are oth e r s e agrass sp ecies th at will out-com p ete it (Fig. 2). The relative im portance of the va rious prim ary producers, then, can be u s ed to asses s the troph ic state of the comm unity.

Eutrophication model

Trophic state R e l a t i v e I m p o r t a n c e

Thalassia testudinum Syringodium filiforme Halodule wrightii Ruppia maritima Macroalgae Microalgae Oligotrophic Eutrophic

Figure 2. Conceptual m odel showing the change in i m portance of primary pr oducers as n u trient availabilit y increases from low (oligotrophic) to high (eutrophic). Each species in the species dom i nance-eutrophication gradient model (Fig. 2) can potentially dom i nate over a range of nutrient availability and the m o del predicts a change in species d o m i nance as nutrient ava ilab ility cha nges. These changes are not instantaneous, however. Field evidence suggests that spe c ie s repla cem ents m a y take place on a tim e scale of a decade or more. It is desirable that we be able to predict the tendency of the system to undergo these changes in species dom i nance before they occur, so th at

m a nage m e nt actions can be taken. Tissue nutrient concen trations can b e monitored to asse ss the re la tive availability of nutrients to the plan ts

. For phytoplankton communities, th is idea is captured in th e inte rpre tatio n of elem ental ratios compared to th e f a m iliar “Redf i eld ratio” of 106C:16N:1P. For the seagrass T. testudinum , the critical ratio of N:P in green leaves that indicates a balance in the availability of N and P is Figure 2. Conceptual m odel showing the ch ange in importance of prim ary producers as nutrient availab ility increases from low (oligotrophic) to high (eutrophic).approximately 30:1, and m onitoring deviations from this ratio can be used to in f e r whether N or P availabilities are lim iting this species ’ gr owth. Hence, T. testudinum is likely to be replaced by faster-growing

com p etitors if nutrient a v ailab i lity is such that th e N:P of its leaves is c a

. 30:1. A change in the N:P in tim e to a value closer to 30:1 is indicative of eutrophication (Fig. 3).

These m ode ls lead directly to a definition of tr ends likely to be encountered in the seagrass communitie s of South Florida if h u m a ns are c a usi ng regio n al ch anges in nu trient availability because of alterations to quantit y and quality of freshwater inp u ts to th e marine ecosys tem :

1) regional eutrophication will cause N : P ratio s of seagras s es to approach 30:1 from higher or lower values indicative of oligotrophic conditions; and

59Final – 2 October 2006

3.0

0.05 0.10 0.15 0.20 0.25 0.5 1.0 1.5 2.0 2.5

N u t r i e n t

- r e p l e t e P-limited

Oligotrophic N-limited E u t r o p h i c a t i o n

E u t r o p h i c a t i o n N

: P

= 3 0

: 1

Leaf tissue P content (% of dry weight) L e a f t i s s Oligotrophic

o n t e n t ( f d r y w e i g h t ) 2) regional eutrophication will cau se a shift in species dom inance in So uth Florida seagras s beds.

u e N c % o The first responses to eutrophication will be evidenced by an increase in the relative abundance of fast-grow i ng seagrass species ( H. wrightii an d S. filiforme ) at the expense of th e now dom inant, slo w

growing T. testud inu m . At later stages of eutroph ication, m acroalgae and m i croalgae will becom e the dom i nant prim ary producers.

At four nearshore Level 1 sites in the Florida Keys, there has been an increase in the relative abundance
of m acroalgae over the period 1995-2003 that is consistent with an inc r ea se in nutr i en t availability

. At none of t h ese sites has there yet been a decrease in seagrass abundance, but our conceptual m o del predicts that increases in fast-growing m acroa lgae sho u ld preced e decreas e s in seagras s abundance (Fig. 2). One exam ple, from site 235 offshore of Lower Mate cum b e Key, shows how

m acroalgae have steadily increased in abundance ove r the m o nitoring period (Fig. 4). In addition to these sites where relative a bundance of primary producers has changed, at four more Level 1 sites there have been long-term shifts in the ratio of nitrogen to phosphorus in seagrass leaves that a r e cons isten t with increas es in nutrient ava i lability (Fig. 5). Figure 3. Conceptual m odel indicating how elemental rat i os of seagrasses r esponds to increasin g nutrient availabilit y

.

1995 1996 1997 1998 1999 2000 2001 2002 A b u n d a n c e

0 1 2 3 4 5

Figure 4. At Level 1 station 235 (see Fig. 1 for location) there has been a slow and consistent shift in s p ecies abunda nce, with faster-growing macroalgae beco m i ng m o re abundant o v er the seven-y ear period. This change is consistent with m odel predictions

60Final – 2 October 2006

The sites that showed changes consistent with increased nutrient availab ility were not random ly distributed across the Sanctuary. All of these site s were relatively close to shore in the Middle and Lower Keys (Fig. 6). The lack of any such changes in the Upper Keys suggests that the factor drivin g the ob serv ed cha nges is not p r esen t acros s th e entire Sanctu ary, so facto r s acting at the global s cale (lik e gl obal warm ing or coastal overfishing) are not likely responsible for the observations . In additio n to Level 1 sites th at are exhib i ting changes that ar e con s isten t with long-term increase in nutrient s upply, two additional sites w e re severely im pacted by hurricanes over the course of the monitoring period.

Change category, 1995-2003 None Severe Hurricane Impact Increase in Macroalgae Shift in N:P

Figure 6. Long-term changes in seagras s beds at Level 1 sites.

Resurveying the Level 2 and Level 3 sites reve aled no spatia lly consis ten t patte rns in changes in relative abu ndance of s eagrass com m unities th roughout th e Sanctuary. The m ean changes in Braun-Blanquet density for the m a jor taxa for the period 1996 and 2003 were not significantly different from zero (Fig. 7), but the r e were so m e location s that had large dif f e re nces betwe e n 1996 and 20 03. W h ether these chang e s were real ch anges in the benth i c comm unities or artifacts caused by s m all scale s p atia l he ter ogeneity is cu rrently being investigat ed. There were som e areas, such as around Islam o rada , that showed declines in Thalassia in a large area contiguous with Level 1 perm anent sites that exhibited chan ges consistent with eu trophication (see Fig. 8 for spatial pattern of change and Fig. 6 for Level 1 site summ ary). However, other regions of apparent change were not consistent with the pa tterns seen at the perm anent sites. In FY 2004, we will res a mple an add ition a l 200 s ites that were surveyed in 1997.

61Final – 2 October 2006

Our surveys have provided clear docum entation of the distribution and im portance of seagrasse s in th e FKNMS. The seag ras s bed that carpets 80% of the FKNMS is part of the largest docum ented contiguous seagrass bed on earth. These
extens ive m eadows are vital fo r the ecological h ealth of the FKNMS and th e

m a rine ecosystem s of all of South F l orida. Maps of spatial distributions can be found at ( http://www. fiu.edu/~seagrass/ ) or DVD.

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 

### Change in Thalassia density 1996-2003 Our perm a n ent m onitoring sites have provided valuable data on the interannual and seasonal variability of seagrass cover and abundance. Tim e series of s p ecies

com position, seagrass productivity, nutrient a v ailab i lity, and physica l
param e ters can be found for each
perm anent monitoring site on the web site or the DVD. There have been som e strik i ng tren ds in the sea g rass communitie s at these perm anent sites: seag rass es were lost com p letely a t 3 of the 30 sites during hurricanes o v er the last four years. At the

rem a ining 27 sites, the benthic communitie s are relatively stab le. There are no common trends across th e sites in
seagras s cover or community com p osition

.

This can b e interpr e te d
to m e an that there are n o regional trends in the health of the seagrass beds represented by the
perm anent m onitoring sites tha t c a n be de te cted with the six y ears of m onitoring data. However, m a nipulative experim e nts in seagrass beds in south Florida demonstrat e that the tim e course of the response of Change from 1996-2003 -5 -4 -3 -2 -1 0 1 2 3 4 5 N u m b e r o f S t a t i o n s

0 20 40 60 80 100 120 140

Change from 1996-2003 -5 -4 -3 -2 -1 0 1 2 3 4 5 N u m b e r o a t i o n s

f S t

0 20 40 60 80 100 120 140

Chan g e from 1996-2003 -5 -4 -3 -2 -1 0 1 2 3 4 5 N u m b a t i o n s

e r o f S t

0 20 40 60 80 100 120 140 Thalassia testudinum

Calcareous Green Algae

Syringodium filiforme

Figure 8. Spatial pattern in the changes in the Braun

- Blanquet density of Thalassia testudinum at Level 2 and 3 sites survey ed in 1996 and revisited in 2003

. Figure 7. Frequency dis t ributio ns of changes in Braun-Blanquet densities for the three m o st co mm o n taxa based o n revisiting 202 sites in 2003 that were originall y su rvey ed in 1 996. The mean change in density for all three taxa i s not significantly different from zero.

62Final – 2 October 2006

seagrass bed s to eutrophication is on the o r der of decades, an d we do no t understand com p letely the interaction hum a ns have with the natural d ynam i cs of these system s. These 30 sites should continue to be m onitored on a quarterly basis.

Detailed analyses of the monitoring data have led to 17 peer-reviewed publications in the peerreviewed s c ientif ic lite r a ture. The s e publica tion s address as pects of the functioning, status and trends of benthic co mmunities as well as lay the groundwork for forecasting futur e anthropogenic im pacts on this ecosystem

. Acknowledgments This data re port r e pres e n ts the dedicated work of m a ny people. Cra i g Rose, Alan W illsie, Bra d Peterson, and Leanne R u tten led the field collection efforts and spea rhe a ded the co mpilation of the data re port; Me red ith Ferdie, Dottie Byro n, Virginia Cornett, Se an Meehan, and Kevin Cunniff collectively put in the thou sands of hours in the field and laboratory that went into the collection of these data. The fieldwork was conducted under perm its FKNMS-2003-036 from the FKNMS, FDEP per m it num b er 1587, FDEP Park s and Recreation Perm it number 5-04-09, and
perm it number DRTO-2003-SCI-0009 from the National Park Service

. Publications Resulting from the Pr oj ect to Date (Chronological Order ) Rose, C.D., W.C. Sharp, W

. J. Ke nworthy, J.H. Hunt, W

. G. Lyons, E.J. Prager, J.F. Valentine, M.O. Hall, P. W h itfield, and J.W

. Fourqurean. 1999. Sea urchin ov ergrazing of a large seagrass bed in outer Florida Bay. Marine Ecology Progress Series 190: 211-222. Fourqurean, J.W ., A. W i llsie, C.D. Rose, and L. M. Rutten. 2001. Spatial and tem poral pattern in seagrass community composition and productivity in south Florida. Marine Biology 138: 341-354

. Davis, B.C., and J.W

. Fourqurean. 2001. Deter m ining species in te ractions between the tropic a l alga, Halim eda incrass a ta , and the seagrass, T halassia testudinum

. Aquatic Botan y 71(3): 217-232. Peterson, B.J., and J.W

. Fourqur ean. 2001. Spatial variability of Thalassia testudinum population dem ographics in south F l orida. Limnology and Oceanography 46(5): 1077-1090. Fourqurean, J.W ., M.J. Durako, M.O. Hall, and L. N. Hefty. 2002. Seagrass distribution in south Florida: a multi-ag ency coordinated m onitori n g program . Pp. 497-522 in: Porter, J.W

. and K.G. Porter, eds. The Everglades, Florida Ba y, and the coral reefs of the Florida Keys. CRC Press LLC, Boca Raton, FL. 1000 pp. Peterson, B.J., C.D. Rose, L.M. Rutten, and J.W. Fourqurean. 2002. Disturbance and recovery following catastrophic grazing : stu d ies of a succe ssion al chronosequen ce in a seag rass bed. Oikos 97: 361-370. Fourqurean, J.W ., and J.C. Zie m an. 2002. Seagrass nutrient content reveal s regional patterns of relative availability of nitrogen and phos phorus in the Florida Keys, FL, USA. Biogeochem i stry 61: 229-245. Fourqurean, J.W ., and L.M. Rutten. 2003. M onito ring of soft-bottom m a rine habitat on the regional scale: the com p eting goals of spatia l and tem poral resolution. Pp. 257-288 in: Busch, D. and J.C. Trexler, eds. Ecological m onito r i ng of ecos y stem initia tives. Is lan d Press, Washington, D.C. Fourqurean, J.W ., J.N. Boyer, M. J. Durako, L.N. Hefty, and B.J. Peterson. 2003. The response of seagrass distribution to changi ng w a ter quality: predictive m odels from monitoring data. Ecological Applications 13(2): 474-489

.

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Anderson, W.T. and J.W. Fourqurean. 2003. Intra- and interannual variab ility in seagrass carbo n and nitrogen stable isotopes from south Florida, a prelim inary study. Organic Geochemistry 34: 185-194

. Fourqurean, J.W ., S.P. Escorcia, W.T. Ande rson, and J.C. Ziem an. Spatia l an d seasona l variab ility in elem ental content, δ 13 C and δ 15 N of Thalassia testudinum from south Florida. Subm itted to Estuaries

. Rutten, L.M. 2002. An assessment of nearshore benth i c communities of the Florida Key s

. Masters of Science Thesis, Florida Inte rnational University, Biology Departm e nt. Ferdie, M. 2002. Nutrient lim itation of benthi c prim ary pr oduction in the upper Florida Keys. Masters of Science Thesis, Florida Intern ational University, Biology Departm e nt. Fourqurean, J.W ., and L.M. Rutten. In press. Th e im pact of Hurricane Georges on soft-bottom

, backreef co mmunities: site - and sp ecies -spec i f i c ef f ects in South Flor ida se agras s beds. Bulletin of Marine Science

. Fourqurean, J.W ., N. M a rbà and C.M. Duarte . 2003. Elucidating seagrass population dyna m i cs: theory, constraints and p r actice. Lim nology and Oceanograp hy 48(5): 20 70-2074. Collado-Vides, L., L.M. Rutten, a nd J.W

. Fourqurean. In review. Sp atiotemporal variation of the abundance of calcareous green m acroalgae in th e Florida Keys: A study of synchrony within

a m acroalgal functional-for m /group. Journal of P h ycology. Ferdie, M. and J. W

. Fourqurean. In review. Re sponses of seagrass com m unities to fertilization along a gradient of relative availability of nitrog en an d phosphor us in a carbonate environm ent. Limnology and Oceanography

.

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